BMW E39 Air Conditioning Evaporator Replacement

Sep 12 2015

If you have an Air Conditioner blowing warm, there's a number of things it could be.  It's typical for people to brush it off and say 'it just needs a recharge' - well, if that's the case you have to ask why it needs recharging.  Unlike older AC systems which had porous hose and inefficient joints, the AC in an E39 uses barrier hose, R134 refrigerant and high quality connections and o-rings.   In other words, they don't leak unless there is a problem.  You can certainly try refilling the system with gas - if you get cold air afterwards, you know that low gas was the problem - but the length of time the cold air lasts for might be measured in days rather than months or years.

Most AC systems have a 'dye' added to them when they were last refilled, which helps a technician trace a leak - wherever the gas is leaking out, AC oil is also leaking out with the gas, and that oil contains a fluorescent dye.

Presumably if you're reading this page you're already expecting to replace the evaporator - which is a big job.  You want to eliminate all other possibilities for leaking AC gas before starting down the road of evaporator replacement.

The best way to do this is by using a UV light.  You can purchase these or you can make your own.  The process for making a UV light is simple:

  1. Get a LED flashlight, some clear sticky tape, a blue 'sharpie' pen, and a purple 'sharpie' pen.
  2. Put a layer of tape over the LED lens, and colour it in blue pen.
  3. Put another layer of tape over the top, and colour it in with the purple pen
  4. Put another layer of tape over the top, and colour it in the blue pen

Once the light has been created, you can test by using a normal yellow highlighter - write a message on a piece of paper.  Take it somewhere dark and shine the light - it should clearly show the highlighter.

Put the car somewhere dark and then go over all the AC lines slowly with the light, looking for leaks.  If you don't find any, crawl underneath the car and look for dye around the area of the gearbox crossmember.  You don't need to jack the car up - just shine the light and snap some photos with a camera and check the results.  If the evaporator is leaking, there is a good chance that you'll see dye and oil in the evaporator drain tubes - there are two of these which run down either side of the transmission tunnel.

Oil on e39 transmission crossmember

Telltale green oil on the transmission crossmember of the E39.  Despite the proximity to the gearbox this is oil and dye from the Air Conditioning, leaking out the Evaporator drain tubes.

using UV light to detect dye bmw e39

Fluorescent dye in the oil detected by home made UV light in a dark garage – confirming an Evaporator leak.

Assuming you've confirmed that the Evaporator is to be replaced, then it's time to start the procedure.  You'll need something between 20 and 30 hours all up, depending on your speed and whether you run into any issues.

You'll need:

  • Screwdrivers of various sizes
  • Torx bit set and driver
  • 10/12/13/14 mm socket, extension bits and driver
  • 10/12/13 wrench
  • Allen hex key set
  • Drain bucket for coolant
  • Needle nose pliers and side cutters
  • Trim removal tools or a thin plastic 

I advise getting a series of plastic sealable bags and grouping all fasteners from each component removed.  By this i mean keep all the screws related to the console in a bag, then seal it, tape it to the console and write 'console' on the outside.  This helps match the fasteners with the piece.  Otherwise it will become impossible to match the correct screws to the correct piece, and inevitably a pile of left-over fasteners will be the result, along with annoying rattles and squeaks.  You'll also need some rags to protect interior parts and a box or two to keep all the pieces as you remove them.

These instructions are for a right hand drive car but the same instructions apply for a left hand drive car, just with the sides reversed.

Step 1 : Remove the Centre Console

The Centre Console has to come out to provide access to the rear air vent duct, which has to be removed as it is attached to the HVAC unit.

Remove the gearstick knob by pulling up firmly

Remove the leather boot around the gearshift by clipping it out.

Remove the plastic surround by pulling upwards and unclip the wiring for the gearshift indicator.

bmw e39 evaporator gearshift removal

 

Remove the rear cupholders by opening them and gently pulling the entire unit out - be careful as these are fragile.

Remove the rear air vent by pushing down and then pulling out. 

bmw e39 evaporator vent removal

 

Remove the access panel around the parking brake by undoing the screws.  You may need to move the seat backwards and forwards to get access to the screw heads.  Unclip the park brake leather boot and pull the boot back over the handle.

bmw e39 evaporator park brake ebrake removal

 

Remove the rear screws from behind the air vents and on the lower sides of the console.

Remove the ashtray.

Remove the hazard switch and door lock switch by pushing up from underneath, and unfasten the wiring from the switches.

bmw e39 console removal

 

Remove the climate control panel by sliding in a trim removal tool on either side and pulling outwards.  Unclip the wiring harness from the back.

bmw e39 remove climate control heater ac unit

Remove the lower switch panel by pushing outside.  Undo the wiring harness from the switch panel.

Remove the carpet trim panels by pushing towards the back of the car. 

bmw e39 remove center console carpet trim panels

Remove the retaining screws from the front of the console.

At this point it's easy to remove the audio unit.   Again, push in a trim removal tool and release the unit, and unfasten the wiring connectors.  Different cars have different units.

bmw e39 remove radio audio mid unit

bmw e39 remove radio audio mid unit

The console should now be free to be removed.  Recline the front seats as far back as they will go. Shift the gear knob out of Park (into approximately Drive).  Remove the console by pulling up at the back and towards the roof.   If it does not lift freely check for screws still in place or wiring harness connections still connected. Don't force it - it will come out easily if everything is unfastened.   Feed the console over the parking brake and gearstick and remove through the passenger door.

bmw e39 remove center console

 

At this point the console will be out of the car, and you're ready to move onto the next stage - removing the dash pad.

Step 2 : Remove the Dash Pad and Instrument Panel

Removing the dash pad is necessary to remove the HVAC unit.  Removing the dash pad also involves removing the lower pad (knee pad) as well as the instrument cluster, and the passenger airbag.

As you will be removing the air bag, it's necessary to disconnect the battery.  If you have electric seats, ensure they are in the full back and reclined position, then disconnect the battery in the luggage compartment of the car.

Lift the passenger airbag cover by sliding a trim removal tool down each side, and lift the panel upwards.  It is attached to two fabric straps, so place it on the dash.

Remove the airbag by undoing the 4 fasteners holding the airbag down to the underlying support frame.  Disconnect the wiring harness plug and lift out the airbag carefully.  Store somewhere safe, base side down (do not store bag side down in case of accidental discharge).  The fabric straps retaining the cover will be released once the airbag is removed. There is also a small torx screw in the lower left which needs to be removed eventually, so it can be taken out now.

bmw e39 passenger airbag removal

Carefully lever off the trim strips (wooden or other).  These are easy to break, so be sure to lever off a little at a time along the length.  Don’t start at one end and try and pull off, as this will likely break the trim in half.

Remove the glovebox by undoing the bolts that hold the glovebox to the hinge bar.  Remove the circlips holding the restraint strap and damper, and disconnect the harness clip for the rechargeable light.  Lift out the glovebox.

bmw e39 glovebox removal

Remove the outer Instrument Panel trim by removing the screws at the top of the trim.  Disconnect the wiring harness connectors for the light and other switches.  Again, each connector is unique and only does in one way.

bmw e39 instrument cluster trim removal

Remove the Instrument Panel by undoing the screws now revealed by the removal of the trim.  Disconnect the wiring harness connectors.  This is done by releasing them carefully – some have rotating levers which release the connection.

bmw e39 instrument panel removal

The panel will come forwards and can be lifted out once the wiring harness is disconnected.

Unfasten the row of screws holding on the lower dash trim panel (sometimes called a kneepad).   Remove all these screws along the length.

bmw e39 lower dashpad removal

Once all the screws are undone, pull the lower dash pad forwards, taking care with the audio system supports.  Lift the lower dash pad out through the passenger door.

Repeat process for the smaller trim on the drivers side

bmw e39 lower dash pad drivers side removal

Remove the A-pillar trims by prying off the airbag badge, which reveals a screw underneath.  Remove this screw and carefully lever off the A-pillar trim.  There is no need to disturb the door seals.  Lever from the windscreen side as the door frame side has a breakable seam to release the airbags.   Take care as the airbags are located underneath the full length of the A-pillar, and the sunroof drain tubes if the car is fitted with a sunroof..  The clip holding the center of the trim can be seen in this photo.

bmw e39 curtain airbags

There is a ‘bowden’ cable connecting the center air vent with the HVAC controls that must be released before the upper dash pad can be removed.  This must be disconnected.  Extreme care must be taken as the cable end and clips can become brittle with time.  This is up underneath the dashboard and must be located – this is easy because the cable is green.

bmw e39 bowden cable centre air vent

Release the cable by slipping the end over the connection and twisting the clip gently to release.

bmw e39 bowden cable air vent removal

Remove the remaining screws holding in the upper dashpad and release it from the support frame.   Lift out through the passenger door.

bmw e39 dash pad removal

Remove the foam insulation out from the top of the dashboard carefully – don’t tear it around the vent openings.

Step 3 : Remove the Dashboard Support Frame

Take careful photos of the locations of all wiring, cable ties and placement of routing of the harness in general.  Then start cutting all cable ties to release the harness from the support frame.

bmw e39 hvac wiring harness clips

bmw e39 hvac wiring harness detail

Release the fuse box from the support frame by undoing the plastic fasteners.  These can be troublesome – if they do not release you can cut them, but be sure to put them on your list for replacement.  There is also screws on each side holding the fuse box to the frame.

bmw e39 fuse panel removal

There is no need to undo any of the harness for the fuse box, it should just drop free of the frame.

Start undoing all of the frame fasteners, starting with the steering column support.

bmw e39 steering wheel harness detail

There are additional bolts underneath holding the steering column – search and find all the bolts holding the steering column.  Once free, the wheel will float around – don’t put pressure on the wheel once it is unsupported – such as using it as leverage to get in and out of the car.

bmw e39 dash support frame removal

Undo all the nuts holding the frame to the car on each side and on the transmission tunnel.

bmw e39 centre dashboard support removal

Lift out the frame once it is loose from all fasteners and remove via the passenger door.  Use care to ensure that there are no cable ties holding the harness to it.

Step 4 : Removing the HVAC Unit

Start by removing the self-closing plastic rivet that connects the center air-duct to the HVAC unit.  This may need to but cut, but it should be possible to pry the rivet upwards to release.   The part is NLA but an ordinary rivet can be used to refix it during reassembly.

bmw e39 hvac unit removal

It is not necessary to undo all wiring harness connections to the HVAC unit, as it has it’s own wiring harness.  Locate the connections between the HVAC unit harness and the main car harness and disconnect those. 

It is time to move into the engine bay and disconnect the coolant and AC lines that go into the HVAC unit.

Drain the radiator by releasing the radiator drain plug.  Be sure to have a large container at the ready to catch all the coolant released.

The AC lines should be evacuated.  If you have a set of AC gauges you can test the static pressure beforehand.  In some localities it is illegal to vent refrigerant gas to the atmosphere.  You may need to call a mobile AC technician to do this for you (or have it done before you start the job).

Begin by releasing the ducts from the cabin air filters and cabin air filter boxes.  This is a simple clip, but different model E39s have different designs.  The duct between the firewall and the cabin air filter housing can be release by twisting upwards and releasing.

On the left hand side, pull the rubber grommet covering the three coolant lines backwards over the coolant lines.  This will likely be stiff and take perseverance.  Don’t use prying metal tools as the underlying coolant pipes are soft allow and can be damaged easily.   Work the rubber backwards until it is released from the firewall and the clamping screws can be seen.  Remove these.   The coolant lines can then be pulled from the pipes – it is a good idea to have a container underneath as more coolant is likely to leak out.

On the right hand side, release the AC lines by undoing the bolts that hold the hard AC lines to the connector in the firewall.  Work the lines gently backwards to release them from the firewall once the clamp is removed.

There are three threaded bolts protruding through the firewall which hold the HVAC unit to the firewall from the side.  One is adjacent to the AC lines, one is adjacent to the coolant lines, and the other is in the center behind the engine.  This has a plastic cap on it which can be twisted off before removing the nut.

At the completion of this process you will have disconnected the AC and coolant pipes, and undone the nuts.  You may find that the cowl trim panel disintegrates into crumbling pieces – this is a common E39 fault, and you may need to order a new one.  As this is a common repair parts are available and not too expensive.

bmw e39 fresh air intake filters removal

In this image you can see the ducts removed but the coolant lines and rubber grommet still in place.  Note the crumbling cowl centre piece, which supports the cruise control cable and protects the wiper mechanism from engine heat.  This panel was replaced during the repair.

Once the lines and nuts holding the HVAC unit to the firewall are released, undo the two nuts holding the feet of the HVAC unit to the transmission tunnel, and the entire unit should lift out.  If it does not come freely, stop and check for remaining nuts (especially in the firewall area) or a wiring harness connection still attached.

The unit will lift out in one piece with it’s own wiring harness attached.  In these images excess cable ties were removed which was not necessary.  The wiring harness should still be mostly in place at this point.

bmw e39 hvac unit top front

Top view showing the outlets for the center air vent and defrost vents

bmw e39 hvac unit left side

Left hand side view showing the coolant pipes going into the heater core.

bmw e39 hvac unit right side

Right hand side view, showing the AC pipe work.  The Evaporator is behind the black panel.

bmw e39 hvac unit underside

Underneath showing the two pipework connection panels and center locating screw that holds the unit to the firewall.  Also showing are the two evaporator drain plugs which connect in with the drain pipes in the transmission tunnel.  The large ducts on the side connect to the rear passenger footwell ducts.

bmw e39 hvac unit front

Front view showing the two fresh air intact ducts (which feed from the cabin air filters), and the three coolant pipes, center fastener and AC pipe connection plate.  The two round housings cover the blower motor and fans.

At this point you should have virtually nothing left in the interior of the car, except for the steering wheel.

bmw e39 dashboard console removed

Step 5 : HVAC Unit Disassembly

Once the HVAC unit is out of the car, it is necessary to partially disassemble it to remove the AC Evaporator.

Start by releasing the air recirculation Bowden Cables, carefully.  I broke the ones in the image here and had to replace them, which was time consuming and costly.  Release by gently twisting the clips and sliding off, after sliding the cable connection up over the plastic lever arm.  There are two cables each with two ends, so 4 ‘unclips’ to complete.

bmw e39 hvac unit bowden cable routing

bmw e39 hvac unit bowden cable routing

It is a good idea to photograph the positions of these for reassembly reference, using the color coding and cable routing to ensure it goes back together correctly.

Remove the front alloy plates and air intake ducts (white) and unfasten screws holding the blower motor covers on. 

bmw e39 hvac blower motor

Release all the screws holding the top cover of the HVAC unit on (see 4 released screws, but there are multiple locations around the perimeter of the top cover).  This will also release the entire blower motor assembly.  It’s not necessary to separate this completely if you have enough workspace to spread it all out, but still attached by the harness.   The brown staining on one side of the blower motor indicates dirt ingress from the cabin air filter, either because the filter was not seated properly, or because the ductwork was not attached properly.  The cabin air filters not only ensure the air entering the cabin is odor and contaminant free, it also prevents damage to the blower motor and evaporator from ingress of debris.  It is vitally important to keep the cabin air filters changed and inspected with each service.

Remove the side cover of the Evaporator revealing the TX valve and pipework.

bmw e39 hvac tx valve

Evaporators have a temperature probe which fits between the fins – this should be removed by pulling backwards on it to release the probe. 

bmw e39 evaporator casing

Once the top cover is off and the side panel removed, it should be possible to lift out the Evaporator and inspect it.  This photos shows the top cover still in place.  Notice the blockage of the fins on the right hand side, which was where the dirt on the blower motor was.  This increased foreign matter on the evaporator increase the chance of corrosion when condensate clings to the dirt/fluff.  This evaporator shows clear signs of corrosion and bright green oil seeping through.

 

bmw e39 evaporator casing leaks oil

This photo shows the Evaporator lifted free from the housing, showing the heater core behind it.  The heater core is in perfect condition and does not need replacement.  Each car should have the heater core inspected carefully – a preventative fix should be considered if any signs of core failure are shown (such as corrosion, presence of coolant, cracking along seams.

This photo also shows the Evaporator probe location and the tell-tale green AC oil present in the bottom of the evaporator housing.  This oil was leaking through the drain channels and what allowed identification of the Evaporator failure as the source of the oil.

Also present in this photo is evidence of leaves and other debris – these should not be present in the Evaporator housing if the cabin air filtration system is in place and working correctly.  Matter such as leaves trap moisture and allow corrosion to start, and corrosion is what killed this evaporator.  Take the time to clean out all of the air ducts so that they are factory fresh.  If you have a cabin smell problem, now is a good time to rub some cleaner of the scent you like in these places.

This section marks the turning point of the project – everything from here on is putting the car back together again.

Step 6 : New Evaporator and Re-installation

When purchasing a new Evaporator, make certain it is the exact size, shape and has the sealing foam in place.  You should also purchase a new TX valve as it is cheap, and a failure of the valve would lead to a repeat of this entire process.  TX valves can get clogged if any part of the AC system fails and should be preventatively replaced.  Use new O-Rings in all the AC assembly, and use a little AC oil to lubricate the O-rings when putting them back in.  They are all different sizes so be careful to match them up correctly.

This is the TX valve prior to refitting the side cover of the unit.

bmw e39 tx valve cover removed

It’s a good idea to wrap that wiring harness which has unraveled with new tape.  This was was done later in the project, when clipping all the wiring harness back together.

When refitting the Evaporator, do several test fits to ensure correct fit and routing of the pipework.  When you are confident it is fitting correctly, carefully push in the temperature probe, checking to make sure that it goes between fins and doesn’t damage the evaporator itself (a few fins will be slightly bent).

This picture shows the flow of air within HVAC unit, and shows the new Evaporator in place, with the blower motor and recirculation connected but not properly back in place.

bmw e39 airflow hvac detail

These pictures show the reassembly process from each side

bmw e39 hvac unit partial assembly

bmw e39 hvac unit partial assembly heater core

bmw e39 partial assembly heater core evaporator installed

These are before the top cover and outer vents go back on.  Once the top cover is back in place, carefully reconnect any wiring harness connectors to valve actuators and take care to find homes for all the various screws removed in the process.  Some of these are not easy to find homes for, but you shouldn’t have any left over.  A torque limited electric screwdriver with a torx fitting will help speed reassembly.

At the completion of the reassembly process,  you should have a HVAC unit looking exactly like the one you previously pulled out.   I staged the parts just to sanity check where everything fitted and make sure I understood everything, as quite a bit of time had passed between first disassembly and commencement of reassembly.

bmw e39 interior dashboard center console

This car was previously a smokers car, and I also took the time to clean out the upper dashboard pad air ducts with compressed air and disinfectant.  The car now has a pleasant smell after sitting in the sun instead of very stale tobacco.  After trying many things over the years to rid the cigarette smoke from the interior, taking the time to blast out the air vents with the air and disinfectant cleaner squirted through has been the most effective yet.  I should have chosen a slightly different fragrance in hindsight but it is much better than old smoke.

Also note the corrosion on the dashboard support – this increased markedly when removed from the car.  BMW mustn’t be worried as they didn’t paint the metal.  If I had more time I would probably have sanded it back and applied a simple coat of spray-can silver or black to prevent further corrosion.  Once back in the car I don’t expect it to corrode much more, but it is curious why BMW fitted a steel part without coating it with any type of rust protection at all.

I also took the time to give the dashboard parts and centre console a very decent scrub with interior cleaner.  Normally you can’t reach all the nooks and crannies and having the parts out of the car made it simply to get a very good clean going.  Years of muck around the center console were removed where the seat normally blocks good access – these light coloured interiors are hard to keep looking pristine.  Take the extra time to give it all a good scrub while it is apart.

Step 7 : HVAC, Dashboard and Console Reinstallation

Replacing all the parts is just the reverse order of installation.  Here’s where all those bagged fasteners will help out immensely as you fit each part with the correct fasteners.

Start with the HVAC unit and relocate the three locating threads into the holes on the firewall.   Make sure the feet match up with the threads sticking up from the transmission tunnel.  Double check to see that the condensate drain holes fit into the drain tubes, and the floor ducts marry to the HVAC unit ducts properly without gaps.   BMW used chunks of grey foam for support when originally installing the HVAC unit – its not certain what these are for – and mine were badly disintegrating.  They were re-used as much as possible, but the debris from the foam deteriorating had to be vacuumed up.  Re-fasten the nuts in the firewall (replace the plastic cover on the centre nut) and the transmission tunnel and reconnect the wiring harness, tracing each wire to ensure it has found a matching connector.

Once the HVAC unit is secured, place the dash support back in, taking care to locate it correctly on the mounting points.   Check to make sure you don’t have any left over bolts or nuts – this support is there to save your life in the case of a crash.   The steering column fastening is tricky with access so take it slowly and get it correct.   When the structure is correct, reposition the wiring harness using the reference photos and replace all the cable ties that had been cut previously.  Check and double check, as finding out a connector won’t reach when in final finishing assembly will be very frustrating.  Refix the fuse box to the support structure ensuring it is located properly and no wires are crimped, twisted or otherwise located incorrectly.

When everything is ready, re-install the foam insulation pad over the top of the HVAC box vents, and reinstall the dash pad by lifting it into the car, pushing the rear of the pad down towards the windscreen and lowering it onto the dash support.   Be sure to re-connect the green Bowden cable from the center air vent to the HVAC box.  Refasten all the screws, re-install the instrument panel and instrument panel trim, replace the air bag and refit the airbag cover.  Make sure the fabric straps are in place before lowering the airbag onto the locating bolts – it must be fitted first.  Take your time with fitting the wiring connectors – although they are robust you don’t want to break any of the plastic locating or locking tabs.

When the upper dash pad is back in place, the A-pillar trims can be refitted, and the lower dash pad replaced on both sides of the car.   

Before refitting the centre console, ensure that you have replaced the rear air duct and reconnected the various wiring harness connectors that originate from the HVAC unit.  Trace each wire to make sure there is no empty connectors.  There is one long wire which goes down to the parking brake.   When everything is ready, replace the center console by using the reverse of the action used to take it out in the first place.  Shift the gear lever down, feed it in and push it up under the dash to make it fit snugly with the lower dash pad.

Refit the glovebox and reconnect the strap and damper to ensure the action is correct.  Remember to reconnect the glovebox light and rechargeable light.

Fit the lower carpet panels, the rear air vent, the hazard and door lock switches and remember to reconnect the cigarette lighter cord.  Replace the gearshift trim surround, gearshift boot and knob.

Refit the temperature control unit, ASC and other switches and refit the audio system.  In each case the connector for the unit should be in the right place with sufficient slack to connect it up easily before returning the component to the correct slot.   Once all switches are in place, reconnect the battery and start giving it a systems test to make sure everything is hooked up correctly.  Any non-functioning switch should be investigated for an unfastened connector.  Don’t start the car at this point!

Replace the dashboard trims (wooden or otherwise) by placing in the tabs, then giving a firm push keeping the force even across the length of the trim.  At this point it should look like a car again:

bmw e39 interior complete

Again, it’s a good idea if you can invest the time to give each piece a thorough clean before re-fitting.  This will give the interior a fresh feel.  I also shampooed the carpets (after this photo was taken) as the grey foam used to support the HVAC unit had left staining in the carpets.  However the instrument panel was dust and lint free, the wood trim shone with a light furniture wax, the leather boots where clean and dirt free and the windscreen was given a thorough clean on the inside.

Do NOT, however, test the AC as the system is empty.  You may want to put some tape over the button if you’re not the only one driving.   All that is left is to refill and get read for a test drive.

 

Step 8 : Coolant refill and AC regas

Refilling AC gas is a topic all of it’s own and I won’t cover the finer details of that here.  However, if you have opened the AC system the Receiver/Dryer will need to be replaced.  The R/D is a can of dessicant- of the type you see in the box of a new pair of shoes – and the job of it is to act as a reservoir of refrigerant gas and also to soak up moisture in the gas.   Leaving the system pipes open to the atmosphere will allow the dessicant to absorb moisture from the atmosphere, so a new R/D will be needed.

Fitting a new R/D is best achieved by releasing the Right hand side headlight partially.  Use a towel underneath to protect the bumper paint.  This gives access to the three bolts holding the R/D in place.  Use new O-Rings when refitting and take care to ensure that there is no bending/twisting pressure on the hard AC lines when refitting.  You will need to carefully align the rotation of the R/D in the clamp that holds it, so the hard AC lines go back in.  The best way to do this is to mark the position of the old one in relation to the clamp before removing.  It should all go back together as it came apart.  If you have some AC oil use a little to lubricate the O-Rings before pushing into place.

Refill your coolant by mixing the appropriate ratio and pouring the coolant mix into the filler.  I always use the Lemforder blue coolant as it is to OEM specifications.  You can’t just pour it in = it will overflow from the expansion bottle without filling the entire system.  This is circumvented by setting the internal fan speed to low, the cabin temperatures to max, and putting the car in the ‘ON’ position (don’t start it – there’s not enough coolant yet!).   This activates the supplementary cooling system pump, which is an electric motor.  You should see a little jet of coolant emptying into the top of the expansion tank fill neck.   This will drop the level in the expansion bottle as coolant is pumped around the system.  Now is also a good time to check for coolant leaks onto your floor from the newly reinstalled HVAC box – in case you bent/broke/cracked a coolant pipe in the heater core.  Take the time to check the floor of the car and underneath the dash on the left hand side to be sure there is no coolant leaking out, and all the joints are tight.   Once the coolant is full, put the cap on properly and start the car and bring it up to temp.  (Again, check the AC is off).

Switch the car off and let it cool down, and recheck and refill the water level as needed with a water/coolant mix.  At this point the car is driveable with the AC switched off.

bmw e39 AC gas vacuum and fill

This photo shows the car being prepared to be filled with AC gas, with the headlight not yet refixed after installing a new Receiver Dryer (R/D).

Before refilling the car with AC gas it is necessary to perform a leak test.  If you have no AC tools or experience, at this point you should contact an AC technician and get them to take over. 

You can perform an easy leak test if you have access to a Vacuum Pump and a set of AC gauges.   Connect the high side and low side gauges to the Schrader valves near the R/D (car switched off).  The valve sizes are different for the high/low pressure sides so you can’t make a mistake.   Then connect the Vacuum Pump to the center (yellow) hose on your AC gauges, and switch the pump on.  Vacuum Pumps rely on oil to work effectively so be sure to keep an eye on the oil level of the pump.  You should be able to pump the Vacuum level down to about –25 inHg.  I like to leave the pump running for about 30 minutes.

Vacuuming the system down achieves three things.  The first is that it removes air from the AC lines – an AC system will not work with air in the lines.  The second is that it removes moisture from the system.  This is because the lower the air pressure, the lower the boiling point of water.  If you’ve ever seen a documentary on climbing Mt Everest, you will have seen that climbers cannot make a hot cup of coffee above a certain altitude because the lower air pressure at altitude means that the water is only slightly warm before it starts boiling.   In a Vacuum the water will boil off at room temperature and the pump removes the humidity from the system.

The final thing that a Vacuum does is check for leaks.  When the pump has been running for 30 minutes close the yellow valve on the AC gauges and switch the pump off.  Leave the gauges connected for as long as possible – overnight is great.  Take a photo of the Gauge reading for reference, and check back at the end of the time to see if the needles have moved (or the numbers have changed if you have a digital gauge).  If the amount of Vacuum has decreased you have a leak in the system, and you’ll have to go into leak detection mode.  That’s another article in itself, but the first places to check are the R/D connections and the connection to the firewall.  With a big leak, you may be able to hear it.  With a small leak, you’ll need to use other methods.

A professional will often test the AC system with pressurized Nitrogen – if you know how to do this and have the equipment, it will work well.  But if you can hold a good vacuum overnight then you’re able to proceed to the filling of the system with r134a.  I won’t detail that procedure as there are plenty of articles detailing how to do it.  If you have access to shop-bought r134a cans, you can fill it yourself.  If you live somewhere where the sale of AC gas is regulated, you’ll have to take the car or get a mobile technician to do the fill.

Step 9 : You’re Done

If you made it this far, give yourself a hearty congratulations and open the celebratory beverage of your choice.  While this is a long procedure and contains many different aspects, it is quite satisfying to complete and complete well.  The E39 is a well engineered car which can be pulled apart and reassembled without breaking lots of parts, if you take your time and do things properly.   Nothing is more satisfying on a hot day than the efficient icy-cool blast of cold air from your BMW vents, so if you find yourself needing to do this job collect the parts, do your research, get the spare time and go for it!