87 SDL AC compressor clutch problem
 

It's warm enough in San Jose to run the AC but the compressor won't engage. It gets 12V across the dryer pressure switch but that doesn't do it. Battery voltage into the wire at the pressure switch that goes to the compressor doesn't do it either. I measure 18.4 kilo-ohms between the wire at the pressure switch (skipping the pressure switch, of course) and ground. Does this mean that the clutch coils are shot or could it be something else? Can anyone with a 603 with working AC provide a comparable resistance value?

If it turns out to be a bad clutch (sucks because the compressor and clutch were replaced by the PO within the last 5 years) someone please say that replacing the clutch can be done without removing the compressor.

Thanks,
Sixto
95 S420
87 300SDL

I will try and check the resistance for you as soon as I get home from work... ETA: 8 hours.

The resistance sounds way high. I don't have the exact specification handy, but something in the vicinity of 5 ohms is about where you want to be. The coil draws a couple of amps current - you can use ohms law to determine where the resistance needs to be. I suggest you try measuring just the clutch coil directly at the compressor terminals.

Are you sure about the wiring and operation of the compressor circuitry? I'm familiar with the 124 of the same vintage, but have the (probably mistaken) impression the 126 is similar. On that car, the signal from the PBU passes through the pressure switch on its way to the Klima relay. To measure clutch coil resistance you pull the Klima relay and measure from that point. Fastlane lists a Klima relay for your car, so I think I'm on the right track here.

Can the clutch be replaced in situ? Probably. On a 124 with the OM603 it can be done, but requires either a veerrrry low profile clutch removal tool, or pulling the radiator. I'd imagine there's more room to work on the 126. Worst case, plan to pull the radiator. Yeah, it's a pain, but cheaper and easier than dealing with refrigerant issues.

- JimY

My bad. I thought the compressor wiring was the same as the 83 SD. As Jim points out, the pressure switch feeds the Klima relay so there isn't a direct line between the pressure switch wires and the compressor. Back to investigating...

Thanks,
Sixto
95 S420
87 300SDL

Sixto -

It's pretty easy to diagnose this stuff with the wiring diagram and just a few simple tools.

The Klima socket has +12v and also the compressor clutch line. By jumpering the two together (sorry, you'll have to lookup the wire numbers yourself...) the compressor should switch on. You can then determine whether cold air is produced, bubbles are in the sight glass, etc.

Also, the Push Button Unit signal is present at the Klima socket (after passing through the pressure switch, of course). From it you can determine whether the PBU is ordering the compressor to turn on - this determines whether your problem is inside the car or under the hood. One trick - on the 124 some cars send a +12v signal to the Klima to activate the compressor, others ground this lead. I don't know which your 126 will do. On the 124 line its +12 for gassers, ground for diesels. You should be able to figure it out just playing around with a DVM and switching the compressor on/off.

Other signals at the Klima include overheat (shuts off the compressor, should also activate high speed auxilliary fan), transmission kickdown, engine RPM signal, and the dreaded compressor RPM sensor.

Good luck,

- JimY

coil resistance
 

Folks,

I pulled my old compressor out and the coil measured 2.7 ohms.

Chuck

If the coolant level is low the compressor won't kick in. Maybe it is as simple as adding coolant?

Joe

Took a solid look at the wiring diagram and redid some tests. The compressor's fine. It cools the cabin when I hot-wire it. The Klima relay isn't getting the appropriate ground signal from the PBU. So the next step is to yank the PBU and check for a signal to the compressor.

Sixto
95 S420
87 300SDL

Pull the KLIMA relay and carefully open it. Check for burnt connections. Mine was kaput. also, check the A/C hoses from the back of the Compressor. I had a leak from both sides of the manifold hose.

Did more directed investigating today. No ground signal from pin 7 of the ACC controller and no connectivity from the controller to the Klima relay socket. D@mn!

I haven't used the Klima relay for almost a year. I fit a simpler relay that bypasses the compressor cut-outs because something was intermittently tripping the Klima relay. In retrospect the replacement relay might have drawn more current than the PBU and wiring could supply.

Now to scratch my head.

Sixto
95 S420
87 300SDL