W126 A/C no longer cooling

There is an old saying that deaths come in threes.   I’ve not had any deaths in the family, but I had the A/C dead in three cars at the same time.   The easiest was the A/C in my 450SLC that just needed a re-gas.   The W126 A/C has been more problematic.   Just as I got the system in my 560SEC mostly finished, the A/C stopped working on my 300SE.

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.

W126 A/C Testing

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.

W126 A/C Troubleshooting Guide

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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