A/C compressor clutch, or the whole compressor?

[D_Ryan]

All;

Now that I have the air flow going where it is supposed to when it is supposed to (bad vacuum elements for footwell ducts and center vents), it's time to restore operational A/C.

Car is a California-spec 1985 300TD, passed along from my father. Just under 230K on the clock, with a relatively new motor (<40K). I do not know if the A/C compressor on the car is original or not. Car is not converted over to R-134.

Early in the summer I took the car to a M-B dealer for A/C service, as it was not blowing cold when the season started. They purged, filled (with issues), and attempted to leak test. Allegedly, only leak detected was very minor. Bigger issue was that according to the tech, he could not keep the compressor running to completely charge the system. Allegedly the compressor shaft was bent, and that was what prevented the compressor operating continuously. Quoted cost to repair - $1.6K

I just came in from under the hood. Removed the spade connectors from the pressure switch on the Receiver/Drier. 10.6 volts on one lead (12.0 with the motor running), and 0 on the other. When the lead with voltage on it is connected to the pressure switch, no voltage is detected on the other terminal.

Followed Larry Bible's advice, and jumped the two wires with the motor off, key in the On position. No evidence of the compressor clutch engaging. Blower works correctly, and fan speed increases/decreases as it should.

Can I safely assume a new compressor is the order of the day, or can only the clutch assembly be obtained/replaced (is this not worth it given the age of the compressor itself?) Are there additional checks I should make to confirm my diagnosis?

Thanks as always

[Brian Carlton]

Doesn't this vehicle have a Klima relay?

If so, then you need to perfrom some tests at the Klima relay, with the relay removed to determine if the compressor will engage. Jumping the pressure switch won't engage the compressor directly.

[D_Ryan]

Brian;

Yes, it does have a Klima relay. I did a remove/replace this morning, just to check for corrosion on the contacts.

What is the nature of the test to be performed. I have a volt/ohm meter handy...

Thank you
r/
Dave

[Brian Carlton]

Originally Posted by D_Ryan

Brian;

Yes, it does have a Klima relay. I did a remove/replace this morning, just to check for corrosion on the contacts.

What is the nature of the test to be performed. I have a volt/ohm meter handy...

Thank you
r/
Dave

On that vehicle, the Klima takes a few signals from certain components (kickdown switch, PBU, temperature) and determines whether to engage the clutch. I don't have the schematic for that one, but, one of the sockets is 12V. Another socket goes to the clutch. You need to jump the 12V supply socket with the socket that goes to the clutch and see if the clutch will engage. You can do this with the key switch on and the engine shut down, if you like. Or, you can do it with the engine running, just to confirm if the compressor is turning when the clutch engages.

If it engages with 12V directly applied, then it's not a clutch problem, but a control problem.

[D_Ryan]

Brian;

In addition to jumping the contacts in the relay socket, won't I also have to have the pressure switch on the receiver/drier jumped at the same time?

I believe that the system is low on pressure because the tech couldn't get a full charge in due to the compressor problem.

Once again, Thank You

[Brian Carlton]

Originally Posted by D_Ryan

Brian;

In addition to jumping the contacts in the relay socket, won't I also have to have the pressure switch on the receiver/drier jumped at the same time?

I believe that the system is low on pressure because the tech couldn't get a full charge in due to the compressor problem.


Once again, Thank You

Once you jump those contacts, the compressor will engage, no matter whether the system is charged or not. If the system is low on pressure, don't let it sit there and run, obviously.

But, this test should eliminate every electrical control in the system and apply 12V directly to the clutch. The only possibility of this scenario not working is if the pressure switch is inline with the 12V line to the clutch. Again, I don't have the schematic, but I doubt it. Just to be sure, you can perform the test, and, if the clutch won't engage, you can jump the contacts and see if it engages.

[D_Ryan]

Brian;

Wanted to express a heartfelt Thanks!

Pulled the Klima relay to begin looking for 12V on one of the sockets. Never could find it, and the highest reading I was seeing was 4.6V on socket #2. That seemed completely wrong from some of the other postings I searched regarding the Klima relay in later model diesels.

Went into the fuse box, and while Fuse #8 (Blower/Compressor) was good, the ends were corroded. Cleaned them, and also cleaned/adjusted the fuse terminal itself. Now I was seeing 12V on socket 5, and reinstalled the Klima relay.

With the pressure switch bypassed, and the motor running (yes, I know, bad for compressor longevity, but it seemed there are so many inputs to the Klima relay it was the only option, I didn't think I couldn't test it with the Klima out), I had the Mrs. briefly activate the A/C button on the ACC, and I could here the compressor clutch engage. Had her immediately switch the ACC to off (compressor couldn't have run even three seconds).

System is low on R-12, so no cooling is taking place, but at least the compressor clutch is now operating. Seems the root cause was insufficient voltage from Fuse #8. It was your recommendation to check the Klima that enabled me to find it.

Thank you once again.

[Brian Carlton]

At least you know the clutch is OK and you don't need to R&R that.

And, it appears that the Klima and the PBU are functioning properly if it engages the clutch via the PBU.

So, now what was the reason the tech couldn't fill it up properly?

Bent compressor shaft? I'd bet against it.

[D_Ryan]

Brian;

I suspect that when the compressor kept cutting out due to voltage to the clutch being insufficient to hold it energized, that he aborted the process.

Not intimately familiar with the process, does the compressor need to be running as the systems is being charged to get accurate pressure readings?

r/
Dave

[BIGRED]

So in the end...they would have charged you $1600 and said there is still more wrong and continued to replace part after part.

Had a friend with a 600 with an A/C issue. Took it to the dealer for service and asked to have it looked at. Quote was over $3,000 to fix it. He didn't have the time to leave it, went to the office, maintenance person said give me the keys....turned out to be a fuse.

When the manager was contacted with the information, he was honest enough and said we would have charged you the quoted price and then gone hunting.

(shaking head) its a scary world out there.

[Brian Carlton]

Originally Posted by D_Ryan

Brian;

I suspect that when the compressor kept cutting out due to voltage to the clutch being insufficient to hold it energized, that he aborted the process.

Not intimately familiar with the process, does the compressor need to be running as the systems is being charged to get accurate pressure readings?

r/
Dave

I've never done it myself, but, I do believe that the compressor needs to be running while you fill the system. Of course, it won't start until the pressure comes up above the low pressure cutout point.

[Brian Carlton]

Originally Posted by BIGRED

(shaking head) its a scary world out there.

That's certainly the truth. The dealer is noted for this behavior. Just slap a very big number on the problem. The customer will pay for it 90% of the time.

What other choice does he have (in 90% of the cases). He's not going to do it himself, and he has no idea where else to go.

So, he pays.

I don't.

[locry]

i almost fell for that ploy too,

i had a leaky compressor that was diagnosed as a lost cause, they quoted me a new one to the tune of around 18,000Php=360USD. When all i really needed was a shaft seal replacement, which i did with the help of this board ofcourse. it only cost me 800Php=16USD for the shaft seal (the seal is much cheaper in the US i think) and a few bucks for soap and some shop towels to clean up after the mess.