A/C Compressor Engagement Issues
 

Ok. Why does the ac compressor clutch not engage on initial start or after CCU was turned off to on, until rpm's are raised above 2k??? Someone said it has to see 750 to engage. This is well above. Once engaged it stays cold inside and seems to cycle normally. It's just the initial on command that's an issue. The KLIMA has been replaced with no change. This thing is driving me nuts.

On my Mercedes there must be a pressure transducer that only allows the AC pump to be engaged if the pressure is within the allowable range. If there is not enough refrigerant, it will not engage. If there is too much refrigerant, it will not engage.

I am not an expert, but my AC does work well and I service it myself.

Does engine rpm affect the refrigerant level?

LOL

If the CCU in the SD models (W126) is like those in the 123 and 124 models of the mid-80s (which I am familiar with), upon startup, Klima looks at the tach signal and requires a minimum of 600 RPM for 10 seconds before it will energize the clutch. Is the car's tach working normally? If Klima doesn't get the right signal (in '84 or '85 Mercedes started running the tach signal through the EGR electronics) it won't turn on the compressor.

That's the only thing I can think of that is RPM-dependent.

Jeremy

Early 124 Klimas have a 10 second compressor delay. Heaven knows why. IIRC later Klimas are a direct replacement and don't have this delay. Maybe your 123 Klima has a similar delay. Does the compressor not engage even 10 seconds after starting the engine?

Sixto
87 300D

Another example of why I am glad I have a 'type one' MB AC...

You know what? I have never thought about it, but I am so impatient that 10 seconds seems ridiculously long when the inside of the car is 200 degrees inside. I do know that the a/c vent temp is the coldest of all but one of the cars I have ever owned. My 1995 Taurus had the best of all. It may be that the rpm sensor signal to the Klima is false, but the tach works perfectly. Tach signal on a W126 comes from a trans bell housing-mounted sensor, right? So, the sensor on the front of the engine is for IP adjustment and A/C signal ???

Hardly blame you for that lack of patience under those conditions.. the same time warp happens when people are sitting and waiting for their glowplugs to go the longer FSM time in really cold weather.. .. the 45 seconds.... that is why I always tell them to " time it with your watch '..

My SDL waits 5 sec or so after starting. Probably a good idea to not load the engine down right after starting. The logic is slightly flawed though because the HVAC head cranks the blower up to high.

What sensor on the front of the engine? Does an '85 300D have an rpm sensor at the crank pulley and flywheel?

Sixto
87 300D

85 is on the flywheel only IIRC.

-J

The "starter union" had it put in their contract.

I also have one on the crank pulley IIRC. It looks to be a hall effect sensor. It also has one on the bell housing. And its the 85 W126, not a W123

300SD. Starting to need reading glasses :(. I'm not aware of a front sensor on an 85 SD either. Are you sure it isn't the holder for the dealer rpm tool? If there are wires, where do they lead?

Sixto
87 300D