300e compressor not engaging - PeachParts Mercedes-Benz Forum

J.HIDALGO · #1 03-09-2003, 11:04 PM

I already did a search and come up with some info. Since I do not have AC pressure gauges to check for low r134 (it was converted about two years ago) I jumped the low pressure switch (s31/1) and the compressor still does not engage. That leads me to believe it is NOT low refrigerant pressure preventing the compressor to engage. I also already checked for belt slipping or loose (it is NOT).
The problem, according to my son (he drives this car now), started after I replaced a bad fuel pump relay. So I suspect, I somehow, messed up the klima relay since it is in the inmediate area.
I am troubleshoting the compressor cutoff procedure in the automatic climate control manual, specifically job 83-605, and my question is this: Does the vehicle has to be on (running) through the whole procedure or just the key in the on position (car NOT running)?
Any other suggestions? or tips?

Mike Murrell · #2 03-09-2003, 11:36 PM

J. H.

Just in case you haven't seen this. It might help.

http://www.peachparts.com/Wikka/AcDiags

J.HIDALGO · #3 03-10-2003, 03:53 PM

Already saw that article.
Anybody else?...please!

TIA

stevebfl · #4 03-10-2003, 06:01 PM

The pushbutton controller grounds the Klima relay through the low pressure switch. Verify that this is happening at the low pressure switch.

If the signal is proper, then the various signals should be checked at the Klima relay. I don't know how the book says to do it, but I bypass the Klima relay (with it out) and that makes the compressor turn. With it turning one can then verify the compressor speed signal. The engine speed signal can be verified without the comp jumped.

J.HIDALGO · #5 03-10-2003, 07:56 PM

I jumped the low pressure switch. Compressor did not engage.
I followed the instructions on the manual and I checked both pressure switch (S31/1) connections for battery voltage. I got 12.4 v on one and xx mv on the other one. According to the book, if voltage is found in only one connection, I should check the high pressure of the air conditioner. What I do not know is, should it be approx. battery voltage on both or ANY voltage.? My understanding is that if I jumped the low pressure switch, the compressor should engage. Is that correct?
All the steps described in the book come back OK...except for one. The speed sensor (L4) did not give me the nominal value of 0.3 V AC. It gave me NO Volts. The resistance was .907k. The nominal value is 530 to 650 resistance. May be I messed up that test? Is that what you meant when you said "verify the compressor speed signal"?
The only way the compressor engages is by jumping terminal 5 and 7 on the 12-pole coupling...

J.HIDALGO · #6 03-10-2003, 10:08 PM

I just went back and tested the speed sensor (L4) again. It passed! I was measuring in V DC instead of V AC. It gave me .3 V AC. For those of you that have the manual, this is procedure 83-605 step 4. All the steps passed the tests. I think I need a new klima relay... However, I am still not sure if jumping the low pressure switch should engage the compressor. Anybody knows for sure?.
Once again, the only way to engage the compressor at this time is by jumping terminal 5 and 7 in the 12-pole coupling (klima relay removed). I still need to check the high pressure of the air conditioner.

stevebfl · #7 03-11-2003, 08:51 AM

You do not quite understand the significance of the readings at the low pressure switch.

According to what you have said it would appear that your problem is the pushbutton controller.

The key here is what is taking place at the low pressure switch. The switch is the conduit for the "ON" signal from the PBC (pushbutton controller). That "ON" signal is a ground signal.

One needs to measure the voltage with the system hooked up. The Klima sends out a 12v signal and the PBC grounds it. If one read 12v on one side and nothing on the other the switch would be open. If one jumped it the resulting point will either look like 12v (if the PBC is NOT grounding) or 0v (if it is grounding - actually it won't pull it all the way to 0v). Anyway to remove the PBC from the decision, ground the Klima signa at the low pressure switch.

One must watch out using these instructions as the similar systems of some diesels are different and they send a 12v signal from the PBC to the Klima instead. Grounding that circuit will burn a circuit board in the PBC.

300EE320 · #8 03-11-2003, 12:20 PM

J.H.

While it sounds like you are on the right track by looking at the electrical system, it would be really easy to push on one of the schrader valves just to confirm that there is some freon in there.

I just got the A/C going again on the 90 300E and I had been chasing electrical because I didn't want to buy the gauges. Turns out the system was 100% empty...

djankov · #9 03-11-2003, 02:14 PM

I replaced my Klima/Kickdown Relay and it did not help. I then replaced my PBC and still no luck.
I bought the R134a kit at Walmart for about $25 which included a gage and bottle of R134a. Turns out my system had zero psi. I recharged the system and I now have A/C.
If you still need the PBC, try:
http://www.benz-store.com/push_button_operating_units.html

They have the most reasonable prices with a 3 year warranty.

J.HIDALGO · #10 03-11-2003, 02:25 PM

DH,

Did you have a leak? I will try it today but I doubt that is my problem since when the compressor was engaged, the air was cold.

Steve,

You probably knew this was coming, and I have an idea, but to avoid "Grounding that circuit will burn a circuit board in the PBC." how would I ground the the Klima signal at the low pressure switch? Would the grounding then engaged the compressor?

Would the temperature sensor inside the cabin prevent the compressor from engaging? Actually, I think the "sucking" fan behind the glove compartment does not work, or it is malfunctioning.

stevebfl · #11 03-11-2003, 02:41 PM

Read the above threads again. I always suggest identifying the "On" signal before proceeding. If you cross the two low pressure terminals and there is no compressor then there are two possibilities at that connection. One is the 12v from the Klima is still open (PBC not doing job) or the voltage will be real low .2v or so and will go back to 12v if the A/C is turned off. If the voltage is pulled low when the A/C is turned on at the PBC then the problem is outside.

You can verify where the voltage comes from by pulling both wires from the low press switch. One will be 12v. Pull the Klima relay, it should then turn off the 12v. If it doesn't then the 12v is coming from the PBC and you have the other "On" signal

300EE320 · #12 03-11-2003, 03:13 PM

I suspect that my leak was at the compressor. I converted to 134a and recharged with a leak sealer additive. Still holding after 6 weeks.

I do think you are on the right track. When I jumped my low pressure switch the compresser engaged with zero freon in the system. Steve's the guy to follow here!

Good luck.

J.HIDALGO · #13 03-11-2003, 08:00 PM

I got the following readings:

Individual terminals disconnected:

#1 lead 12.5 V A/C OFF
12.3 V A/C ON
#2 lead .940 V A/C ON
207 mV A/C OFF

Both leads connected:

#1 Lead 12.5 V A/C OFF
.994 V A/C ON
#2 Lead 12.5 V A/C OFF
.988 V A/C ON

Klima Test:

A/C OFF KLIMA ON 11.51 V
A/C OFF KLIMA OFF 20.0 mV

A/C ON KLIMA ON 11.18 V
A/C ON KLIMA OFF 26.3 mV

What gives?

stevebfl · #14 03-12-2003, 10:24 AM

It looks like from your first reading the switch is open. This means you are out of refrigerant.

The next readings say that the PBC is pulling the Klima to ground. I can't say from these why the comp doesn't come on with the low pressure switch jumped.

I would verify the switch/pressure thing first, although from what you are saying it won't work even when it does have pressure.

J.HIDALGO · #15 03-12-2003, 08:37 PM

I hope I am just low on refrigerant.! I will report results....
Thanks to everybody. Wish me luck!