Things just keep getting more and more strange. I just checked slot #4 again, and now it shows no voltage. But the compressor is still not functioning.

I then checked for voltage between slots #1 and #2. At idle I had about 9v. But the voltage dropped as engine speed increased. The checklist (unless I read it wrong) says the voltage should increase with engine speed. Does this indicate a bad speed sensor? If so, where is it?

Thanks.
-Brett

The coolant temp sensor (really a switch), in my car has 3 pins. Only one pin connects to the Klima relay, one is ground, and the third one connects to the aux fan relay. If you disconnect all three, the aux fan relay will not work and so your fan would not work on high speed. When the temp is high, then the switches close, so disconnection works to fool the Klima relay BUT disconnection for the aux fan relay actually will break the current path of the relay coil.

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Saumil S. Patel

Oops !! That thread was for diesel and your is a gas engine, the gas engine does not have this throttle cut-out switch. So dont worry about pin 4. The rest should still be o.k.

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Saumil S. Patel

I just ran through the checks again, and this time the results stayed the same. No voltage on #4. Between slots 1 and 2 there was 9.5V at idle, and went DOWN with increase in RPM.

What does this indicate?

-Brett

Oops again, that number is different for diesel and gasoline again. It should be about 9V for gasoline, so your engine speed signal is fine.

Was your compressor speed signal o.k. ? The symptoms you have indicate one of these signals should be bad.

When you are all done, I will have to modify this to include gasoline as well. Sorry !!

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Saumil S. Patel

Hi Saumil
What I am calling the coolant temp. sensor/switch (I may have it wrong) is a blue bodied one with two prongs/connections that resides on top of the engine. The first time I pulled it (with the car's electric power "on") the aux. fan started running. The next time I did that, nothing happened. I'm assuming the fuse for the fan is fine because when I jumped the A/C fan switch (down by the refrigerant pressure switch) the fan came right on.

I was beginning to wonder if the aux. fan switch trouble and the A/C clutch trouble are related somehow.

-Brett

I think I missed that step (compressor speed signal check). I'll go out and check right now.
-Brett

Is it possible that you have a different sensor/switch arrangement in your car ? Someone familiar with this car can pitch in here. Or could it have been a coincidence that the fan came on when you pulled the cables out. Note that atleast in my 87, 260E, the fan can come on from different sources. One is the engine getting too hot (this is the coolant temp sensor), and this always turns the aux fan on at high speed (the current for the fan comes from aux fan relay in the relay box) AND another by the climate control unit, if the settings are for highest a/c and highest blower speed, this also turns the aux fan on at high speed (the same relay supplies the fan current) AND the other is if the refrigerant presure becomes too high, and this is the pressure switch (most likely on your drier), this turns the aux fan on at low speed (a different relay in the box handles this circuit).

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Saumil S. Patel

Between slots #9 and #11 I'm getting infinite resistance. So that indicates a bad sensor on the compressor, or a break in the wires. Is that correct?
-Brett

You should get a low resistance between pins 9 and 11. So yes either the sensor is open or you have a broken wire somewhere.

Now since we are finding car based differences, you have to make sure that your compressor has a speed sensor and that the wires of the sensor arrive at the pins 9 and 11. To double check, verify connectivity between sensor connections at the compressor and pins 9 and 11.

This finding fits very well with your symptoms. And if you do find that the compressor speed sensor is broken, I am not sure what you can do about it. You have to call the dealer and see if they can just replace the sensor for you, it is a serviceable part but may be so old that it is not available anymore. Otherwise, will need a new compressor.

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Saumil S. Patel

I've found that the speed sensor is located on the back of the compressor. My usual supplier has them (approx. US$70). It has white and red wires leading from it to a harness on the top-front of the compressor. On top of those is another set of wires - blue (was dirty, might have been green), red and white. The white and red went to slots 9 and 11 (can't recall which was which). But (this seemed odd to me) each (red and white) showed essentially no resistance to its own slot (which is how I determined slot ownership), and only limited resistance to the slot of the other - perhaps that is how it is supposed to work?). I don't know where the blue/green wire goes. There is also a black wire in the mix, which appears to be a ground.

But here is an odd thing. As I was trying to check these for continuity, my results kept changing. Then I plugged the Klima back in, and the A/C worked for a minute or two. Then I checked continuity again (showed fine). I plugged the Klima back in, and the A/C started working and didn't stop this time (several minutes of A/C - what luxury!)

So now I'm thinking that I have a bad/dirty connection or a weak wire down at, or around, that harness on the top-front of the compressor. Testing there (nudging wires and such) seems to change my results and the run/not run condition of the compressor. Either that, or perhaps an intermittent problem in the sensor (belt tension seems fine, so I'm ruling out that the sensor is actually doing its job properly).

I'm going to go out now and clean the contacts, make sure the connections between the wires and the harness look good, etc., and see if that fixes the problem.

-Brett

I forgot to mention, that now I am getting low resistance between 9 and 11 (was infinite when the compressor did not engage). It would seem to me that these new (low resistance) results appear to be consistent with the red/white resistance measurements I mentioned above.

-Brett

I cleaned the contacts and all around the harness. I found there was a blue wire on the bottom too. The connections looked sound, so I limited my actions to cleaning.

It seems to have worked. I gave it a test drive a couple miles away and back, and I was actually getting cold with the A/C on full.

-Brett

Great !! Otherwise, if all sensors are o.k., then the problem would move to the Klima relay itself.

The way the speed sensing works is that when you start the car, the compressor is turned on briefly either right when the car starts or after a few seconds (depends on the type of car). Then a check is made for similarity in the speed of compressor and the speed of engine. If the diff is larger than about 30% then the compressor is shut off. And all this takes roughly between 3 to 4 seconds. In some cars, this test is then reperformed few minutes later, I think atleast 3 more times, before giving up until the next restart. This process occurs after every restart.

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Saumil S. Patel