A/C and Auxiliary Fan on 560sl - PeachParts Mercedes-Benz Forum

sms · #1 05-25-2003, 11:03 AM

I just finished replacing the a/c compressor and receiver dryer. The a/c hadn't been working for many years and it was clear that from lack of use that the compressors seals were leaking.

Anyway, I evacuated the system and tried to re-charge with r-12, but couldn't get the refrigerant pressures up to more that 65 on suction side and 50 on high side. The clutch engages if I short it, but then shuts off soon after.

I was wondering if this could have something to do with the aux. fan since it doesn't come on unless I short it out as well. Is the fan always supposed to come on if the a/c is on?

Any other suggestions as to why I can't charge the system would be appreciated.

Thanks,

Steve

stevebfl · #2 05-25-2003, 11:26 AM

The pressure you give indicate the compressor isn't turning.

You talk of jumping the compressor and then it dies. Why does it die? Where are you jumping it? Do you quit jumping it? Do you need to jump so,ething else?

sms · #3 05-25-2003, 11:48 AM

Steve,

I agree. The compressor only turns momentarily when I jump the low pressure switch, but then stops. That's why I'm think there must be some sort of electrical problem like a relay or sensor. Just don't know where to start with that sort of thing?

sms · #4 05-26-2003, 04:52 PM

I believe now that I can rule out several things.

1) Aux fan- The aux. fan only comes on when temp goes above limit. Since I can't get it to charge up, high temp is not an issue yet.

2) Pressure switch- Compressor does come on momentarily when switch is shorted, but the stops soon after.

3) Expansion valve- Don't believe expansion valve is obstructed since there would be big differential between high and low side pressures.

What I can't rule out.

1) Relay problem.

2) Fuses or fusible links. I checked one fuse that I believe is for a/c, but don't know if there are others.

3) Other sensors.

4) Compressor oil. Accidentally added some oil to the suction side and not just the discharge. Don't know what impact that might have?

Any suggestions would be welcome. Thanks!

stevebfl · #5 05-26-2003, 05:16 PM

Don't worry about the refrigeration untill you can get it to stay on.

The compressor is controlled by the Klima relay and the V8 doesn't use a speed sensor on the compressor. As such about the only thing that should make the Klima ignore your "on" command would be a wrong temp reading.

I usually pull the Klima and jump the output to make the compressor run. With the Klima out the other inputs to it are available for testing. Get your wiring diagram.

sms · #6 05-26-2003, 07:49 PM

Can you tell me where the Klima relay is located and what it looks like on the 560sl? Also, does a wrong temp reading mean that the internal temp sensor may be bad?

Steve

stevebfl · #7 05-27-2003, 08:39 AM

The Klima relay is behind the glove box on the 560SL. It is a little smaller and a little thicker than a pack of cigarettes. it is black plastic.

Through your actions, as described, you have by-passed all the A/C related controls (those inside the car that work for climate control). The Klima takes that signal and passes it on as long as all the engine factors are OK. These factors vary for different models but the single cam V8s do not have speed sensors so the main factor Klima is watching is engine temp. The dual temp sensor is at the front of the motor in the center.

sms · #8 05-27-2003, 09:13 AM

Steve,

Is there anyway to directly test the Klima or the engine temp sensor? Is the engine temp sensor the sensor that is located in the top of the water pump housing or is that the coolant sensor? I replaced the coolant sensor last year.

My CD ROM manual shows an air conditioning control unit labeled N6. Is this the Klima?

Thanks again,

Steve

stevebfl · #9 05-27-2003, 09:43 AM

There is no direct test other than performance. One might state that if the control signal goes in but the "comp on" signal doesn't come out then it is disfunctional. The problem with that assessment is that its job is to cut the compressor for precise reasons. These reasons are only semi-spelled out. Temperature is the obvious (from the wiring diagram) criteria. I am pretty sure that engine speed both slow and high are monitored and battery voltage (or charging rate).

The temp switch involved is S25/5.

sms · #10 08-08-2003, 01:53 PM

Hi,

Here's an update on my a/c problem. I found that nothing was wrong with my r12 compressor installation. I simply didn't charge the system properly and it wasn't warm enough. The weekend in May that I tried to charge it it was only ~ 55F. Also, this time I statically charged the high side 1st with liquid from my r12 tank and then ran the compressor and charged the rest of the way from the low side with gas. Works great now!

Steve

1991300SEL · #11 08-08-2003, 02:35 PM

Steve(SMS):

Just curious. Back when the ambient temps. were cooler, what fan setting were you using?

Were you by chance attempting to charge using the lower fan setting?

On my 126, the compressor will cycle more often when the low speed fan is on(bottom button on my interior control panel).

If I use the middle or upper fan speed(much higher air volume), the compressor does not cycle near as often.

I seem to recall a comment awhile back that cycling more frequently @ low fan was a positive sign. I'll see if I can dig up this post and add it in here afterwards.

My 126's cycling characteristics are different than the domestic/asian models I've owned and all are/were R-12 systems.

1991 300e A/C question