Summer temps are here - Commence with the AC questions.
 

Hi all,

My 1995 E300D is having some AC issues and was wondering if you all have any suggestions. Here are the symptoms. Sometimes the air kicks on and somtimes it doesn't. When the air is on, I can see the compressor clutch engaged and spinning - when the air is off, no clutch engaging or spinning. So, I'm wondering if this is parhaps a switch or sensor issues? I think I am going to pull the clutch to see what it look like.

I have a set of gauges, but the readings aren't so clear. When the car is running, but the AC is off, I am getting normal to high readings on both the low and high pressure sides. I can't get the AC to come on so I can get those readings. Today I managed to get the AC to come on while I was out driving, so when I got home I left the car running and tried to get the gauges on. I only managed to get the low side on (I think I would have mangled my hand in the belt trying to get the high side on) and appear to have gotten a normal reading (40 psi).

Any suggestions?

My guess is it's electrical.
don't pull the clutch, connect 12V to it and verify it works fine when there is power engaged to it.
since you get no air at the same time as you get no compressor, I would say there is a problem in your control unit.

Don't attempt to connect the high-side connection with the compressor running. The pressure can be very high and you won't be able to connect the fitting without a whole lot of force. I connect both fittings before starting the car.

You should see the same moderate pressure on both ports when the system stabilizes after shutting down the compressor. As long as there is any liquid refrigerant in your system, this pressure is entirely dependent upon refrigerant temperature and not at all affected by the refrigerant quantity.

Certainly check for power at the clutch, and if you have pressure in the system you certainly can power it with a 12V line for long enough to check the pressures. If there is low refrigerant in the system, you will starve the compressor of oil so don't run it very long if the high-side is too far below specs while running.

Do you know if there is a compressor speed sensor on your car? These can be problematic from what I hear. I don't have one on my '96.

Good idea. I will give that a try and let you know what I find. Thanks!

Matt,

Good point - it's hard enough to get hook up the high side when the system is off. I will put some juice to the clutch and see what I get and also check out the pressures if that works. I don't know about the speed sensor, but i will find out. Thanks!

Mark

Update
 

Well, I got to looking around yesterday and I found that several of the wires leading to the compressor have lost their insulation - so I suspect my problem is in faulty wiring. I still need to put 12 volts to the clutch to see what I get, but seeing those wires looking so bad was disappointing. I know my model is known for the bad wiring harness, so now I am wondering how many other wires look like these - insulation brittle and falling off. Getting a wiring harness quote from Phil - I dread the thought of replacing it, but I suppose it would be worth it in the long run.

Update
 

Well, I finally got the opportunity to test out the compressor clutch today - I put 12 volts to it and it seems to be working just fine. So it would appear that my problem is electrical. While I had the juice to it, I hooked up the gauges to see what I'd get. With the compressor not running, my high and low pressures we not exactly the same (86 psi low/100 psi high). However, once I engaged the clutch, the cool air came on and the system was allowed to stabilize, I had the following pressures (32 psi low/185 psi high) - which seemed pretty normal to me. The outside temp was about 78 degrees F and the system was producing cool air at about 52 degrees F.

Any thoughts on a next step? Should I assume that the problem is further up the electrical line somewhere because (as I indicated before) the crappy insulation is coming off. If so, I guess I am in for a long rewiring job. Or, should I rule out any other options first?

Yes, I believe there is a compressor speed sensor here. My wiring diagram shows a RPM sensor on the compressor - is that it? Regardless, I believe that this area of the wiring harness is prone to these problems - not to mention that the whole harness has been a problem in this model. Trying to decide if I want to replace the entire harness or chase down this problem.

My diagram shows the wires from the compressor going to the "AC Compressor Control Module" - does anyone know where that is located?