New AC compressor not coming on - PeachParts Mercedes-Benz Forum

DieselPaul · #1 02-06-2012, 02:47 PM

I replaced the AC compressor on my 87 300D with an eBay remanufactured unit. We replaced the compressor and as many O rings as we could get to. The dryer and the pressure switch were replaced with napa altrom parts that actually had 124 part numbers. The system was vacuumed down for 5 minutes, let sit for 5 minutes, no loss of vacuum. So we then let it vacuum down for 45 minutes. It took just under 3 cans of R134 so it's got around 32-34 ounces in it, which as far as I know is about Right.

However the compressor never comes on.

If you unplug and Plug the klima back in the compressor will come on for a second or two and then shut off. So the clutch is good.

I tried 3 different klimas, they all do the same thing. The one came off my SDl and is known to be fully functional.

I put a multimeter on the new pressure switch it read 0 ohms. On the SDL it's reads .01-.02.

I've tried the climate control on all3 settings that run the compressor and nothing changes.

The only strange thing it does is if you put the fan on auto it will run really fast the go to a low speed.

My friend and I are both stumped, any help would be appreciated.

DieselPaul · #2 02-06-2012, 02:53 PM

I just jumpered pins 5 and 7 and the compressor comes on.

I am thinking just doing sixtos conversion to an ice cube relay but my AC worked before. So I'm not sure if something else is up.

compress ignite · #3 02-06-2012, 03:00 PM

Did the replacement have one?

[AND is the air gap correct?]

EDIT:

O.K. you just "DID" test step #3 (Jumpering 5+7)

AND test step#4 is the Speed Sensor on the Compressor.

DieselPaul · #4 02-06-2012, 03:06 PM

It had a speed sensor, but I didn't know how to test it. I'll go try that now. however my car idles at more like 600 rpms, not 750.

compress ignite · #5 02-06-2012, 03:27 PM

[Are you using the (do you have?) FSM ?]

The Klima compares the speed of the Compressor to the speed of the Flywheel
and if they're out of It's "specs" ,No clutch engagement.

If your Idle is @ 600 RPMs ,Does not the '87 still have the "Turn and Click"
Idle speed adjustment Knob?(So, that you can turn the Idle up to 750 RPMs)

DieselPaul · #6 02-06-2012, 03:47 PM

With the motor off i get infinite resistance out of the new compressor at the klima connector. At pin 1 and 2 on the compressor itself I also get infinite resistance. On the old compressor at pins 1 and 2 I get 570 ohms.

So does that just mean the speed sensor is bad, or do they come out of adjustment?

Also can the sensor be replaced without evacuating the system again or does the sensor form a plug? It looks like I could undo the 4 compressor bolts, shift the compressor to the side and replace the sensor. I'd have to take the connector apart to leave the clutch in place but nothing impossible.

compress ignite · #7 02-06-2012, 06:28 PM

(Remember the FSM,Most of the time,requires you to remove your Cranium and
install the Deutschlander Operating Chip)

[Add to that we're talking about Two(2) different speed sensors:
One on the Comp and One on the Flywheel.(There's also one on the Speedo)
AND you're testing the Comp. Speed sensor at a location other than that
recommended by the FSM.And I'm not so assured of the results.]

PROCEDURE:

Set DMM to A/C Voltage and then check Sensor @12 point connector.

[I HATE it ! When I suspect the "rebuilders" just R+R the OLD Sensor on the Reman Unit.]

It's a "Hall Effect" rotational sensor.
So there's an "Air Gap" between the end of the Sensor and the Compressor's Shaft.
[And given that scenario,I'd imagine there's No Seal Between the Sensor and the Refrigerant.]
[My luck so far has held, on Mercedes, So I've never had to replace a Comp. Sensor.]

DieselPaul · #8 02-07-2012, 12:18 PM

I tested the one on the compressor itself and according to the FSM thing you sent me, that is bad. It reads infinite resistance on the multimeter.

What is the point of comparing the flywheel speed to the compressor speed? I always turn the AC off when I thrash on it. I think sixtos relay mod eliminates the use of the speed sensors.

The conpressor has a warranty, I asked the seller if he would supply just a new sensor, however I'd almost rather reuse the OEM one off the old conpressor.

sixto · #9 02-07-2012, 02:05 PM

Since a single belt drives everything, an internally seized compressor will take out the belt and the water pump leaving you stranded. Comparing compressor and engine rpm is the car's way of knowing when to hold and when to fold. In my case, I trust my judgment to determine that there's a problem with the AC compressor and switch to 0 or EC to continue with the trip. In 250K miles the AC clutch failed and the compressor leaked. I suppose all that's left is for the compressor to seize

Hopefully this rebuilt compressor reset the clock.

Sixto
87 300D

DieselPaul · #10 02-07-2012, 06:18 PM

Hello
I asked the head mechanic in my warehouse he said
If you by pass the electric direct to the coil and the compressor working good and cool that mean the compressor and ac system working Ok but if you hook the wire on the plug of compressor and the clutch not engage you have to check the electric system it is not the cause from the speed sensor switch because the speed sensor on the rear of compressor will working only the compressor has too much pressure or too much high speed and it will working when the magnetic inside the swash plate in the compressor turn too much speed and it will make the speed sensor close the circuit and send the signal to the electric system and cut off the electric to the coil
In your case my head mechanic said the compressor Ok and coil Ok and if you check the ohms on the speed sensor is nothing ohms that mean it's Ok ,
You have to check your ac electric in the car system.
Please let me know
Thanks,Tommy

This was their reply to me saying their speed sensor tests bad.

ah-kay · #11 02-07-2012, 06:55 PM

Quote:

Originally Posted by DieselPaul

Hello
I asked the head mechanic in my warehouse he said
If you by pass the electric direct to the coil and the compressor working good and cool that mean the compressor and ac system working Ok but if you hook the wire on the plug of compressor and the clutch not engage you have to check the electric system it is not the cause from the speed sensor switch because the speed sensor on the rear of compressor will working only the compressor has too much pressure or too much high speed and it will working when the magnetic inside the swash plate in the compressor turn too much speed and it will make the speed sensor close the circuit and send the signal to the electric system and cut off the electric to the coil
In your case my head mechanic said the compressor Ok and coil Ok and if you check the ohms on the speed sensor is nothing ohms that mean it's Ok ,
You have to check your ac electric in the car system.
Please let me know
Thanks,Tommy

This was their reply to me saying their speed sensor tests bad.

This is a looong sentence which I have difficulties to comprehend. Let me ask you a few questions.

1) Why you need to change out the old/bad compressor?
2) Was the Kilma relay working before, i.e. it was controlling the old/bad compressor OK?
3) How you managed to charge 3 cans of R134 without the compressor ever running? I always jumped the freon ( low ) pressure switch at the recv/dryer to charge. If it never ran, how you charged it?
4) Have you checked the low pressure switch? Is it open circuit or closed?

I have modified the Kilma relay on a SDL to by-passed the speed sensor. The modify kilma only monitors the low pressure sensor and ignored the rest. It has been running for 2 years+ will no effect.