The W126 Klima relay is more of a control unit than a simple relay.   It has multiple inputs that determine if the A/C compressor should not engage.   These include:

• During full throttle
• When the engine is very hot
• Invalid compressor speed (e.g slipping belt)
• Not enough coolant in the system

This additional functionality makes these relays more sensitive than the little silver ones used in the previous car.   I also had to change the same relay in my 560SEC.    It is important to note that the V8 relay is different to the one used in the Inline six.

The relay is located next to the fuel pump relay near the fuse box.  It is a simple swap out.

I bought a KAE relay.   I had previously replaced the relay with a rebuilt unit about two and a half years ago.   As it has already failed, I went with a new one.   The KAE unit is made in Germany so I hope it lasts longer than the rebuilt unit I went with.

With the new relay, the A/C is now cool again in the 300SE.    I may gut the innards of the broken relay so I have an easier way of jumping the ports for troubleshooting if/when I need to.

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I took the car in for the gas pressures to be checked and the gas was not below minimum spec.   A tiny bit was added but my mechanic told me he thought it was probably an electrical problem.

Mercedes-Benz USA published a very good Service Manual for the Automatic Climate Control in the W126.    There are often copies on USA eBay and some bad scans are available too.     The relevant section is Testing compressor protective cutout, in the addendum for the 1986-1991 models.    This manual is pretty much a must have if you want to work on your W126 A/C.

There is a step by step test procedure I used to isolate the problem.

1. Test low pressure switch on receiver/drier.   Test OK
2. Testing supply voltage to the control unit (Klima relay).     Test OK.
3. Testing control voltage for A/C compressor from low pressure switch.   Test OK.
4. Bypass relay by bridging pins 5 and 7 and check that compressor cuts in.   Test OK.
5. Test RPM sensor for A/C compressor.    The required value is above 0.3V~  My reading jumped around a bit but was nearly always above that reading.
6. Test RPM signal (TD).   Test OK.

There is a separate troubleshooting guide for the diesel engines that is different.  It is easy to use the wrong one as the labels are on prior pages.

Based on these tests, it looks like I require a new Klima relay for my W126 A/C.   This is rather annoying as I replaced the relay back in December of 2017.   At the time I purchased a rebuilt unit as I figured I would get a rebuilt factory one instead of an aftermarket unit.  I ordered a rebuilt aftermarket unit, so this time I have gone with a new aftermarket unit.

I had hoped to swap the relay with the 560SEC, but the relays are different from the straight six to the V8.

Right now I am driving around with the relay bypassed.   It is rainy and humid and I need the defrost capability, it not the cooling.

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My first step was to see if the system could work.   Overseas readers may not know that in Australia it is illegal to sell freon to the general public.   R12 has not been available for many years.   Therefore, I booked the car into a mechanic who does A/C work to have the system converted and gassed up with R134A.   First steps were quite positive, when the Klima relay was bypassed, the system would blow quite cold if you jiggled the ignition key a little.    That left some obvious repair items though.   I had a new ignition switch (the electrical part) fitted, and ordered a new relay.   I also ordered a rebuild climate control unit.

While the ignition switch was out of the car, I planned to put in a new tumbler.   It used to be possible to purchase these from the Mercedes-Benz classic center.   They would then set them up to use the right key for your car.   The part has been NLA (No Longer Available) for years on the W107, but I purchased it for my 300SE about 18 months ago.   In that time, it is now NLA for the W126 except for the 1990/1991 models.

The new freon had dye inserted, so I was able to check if the system was holding pressure.   In general, it was, but there was a small amount of dye around the compressor.   In particular, between the two halves of the compressor.   The mechanic who gassed up the system had seen success with a product that can be added to the A/C system to seal up minor internal leaks.  We added that component and I waited for the relay and climate control unit to arrive.

The Climate control unit is quite easy to fit.   I have done this job on my 300SE when I was troubleshooting its climate control system.   First, the ashtray is removed, then the radio.   The dash panel can then be pulled down and out to expose the climate control unit.

it is easier to remove the connections to the other switches first.   Care must be taken to keep the fiber optic cable with the switch, so the back lighting will work when it is re-attached.

In the series 2 cars, the climate control unit is screwed into the back of the wood panel, and there are two big connectors that connect the unit into the cars wiring harness.

I am getting quite fast in removing this part of a W126 dash after doing it quite a few times on the 300SE.  Taking the dash apart also allowed me to fix a few other annoyances.   The first was the illumination of the gear selector when the lights are on.   It is a tiny 1.2w bulb that slots into a hole in the underside of the unit at the front.   Not only was my bulb blown, but it had come out and was floating around in the dash.   The second was plugging in the cigarette lighter properly so I could charge my phone.

I had couple of these bulbs in stock, so it was an easy change.   The Ashtray illumination was flickering as well, so I changed that bulb while I was there.   After putting the dash back together, I had a MBCNSW twilight drive to attend.   I had originally planned to take the 250SE, but the weather was rather poor.  Based on this, the 560SEC was the better choice, especially as I could test the Climate Control.

My results were generally positive, but with some more work to be done.  My 560SEC climate control sort of works.   The climate control unit regulated the temperature well, although once or twice it went from cooling to heating.    The new relay worked fine, and I had no problems with the ignition switch.   The three main problems I had were that the A/C just wasn’t all that cold, the centre vents are not working properly, and the electric fan never kicks in for the condenser.  I’m pretty confident that the centre vents will be the vacuum pods under the dash.   That is a job for another time.   As the weather heats up I need to get the electric fan working as the car gets quite hot during prolonged idle.   After coming so far, I also wold like to get the A/C cooler than it is now.

Next step is to take it back to test the freon level, and see if that sealer has worked at all.   I also need to see if the Aux fan will work if plugged into 12v directly.

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Coincidently, at the same time I needed to change the Klima relay, I had another relay problem.   Symptoms were very hard starting and poor running when cold, as well as the ABS light on.   This normally points to the Overload Protection Relay.   The Overload Protection Relay protects the sensitive electrics on the car, in this case the engine ECU and the ABS controller.

They are known to go bad after a while and my old one  is date stamped 1996.   This relay is fairly cheap (about $50-60) and protects very expensive components, so if it has not been changed in a while its always worth having a spare on hand.  There are a few different part numbers that vary on year so it is important to get the right relay for the car.    It has a 10W fuse in the top which is worth checking.  Sometimes just this fuse is bad, although most often the whole relay needs replacement.

I now have no more ‘urgent’ repairs on this car.   There are a few smaller items that need attending to that I can do slowly.   For example, the left hand bulb in the instrument cluster is not working.   I need to pull the cluster anyway to fit a gear with one less tooth to make the odometer more accurate.

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