93 190e AC is Cold, then blows hot, Heat blows hot, then cold

Hmmm, I think there might be some relays you could try replacing that might help. This car has a pretty (very) complex heating/ac system, so you really need an EXPERT (I mean it) to trouble shoot it. Your average mechanic (even a good one), is going to be left throwing parts at it in hopes of a good result...

All that said, a common issue is the RPM sensor on the back of the AC. This device preserves the serpentine belt in case of a compressor seizure. The symptom you describe with your ac cutting out but then working on restart are identical in case of the RPM sensor. Actually if you leave the car idling (ie don't rev it up) the AC should continue to work for longer then 10 minutes if the RPM sensor is the issue.

Let us know what you learn. Marty

Agree. The compressor has nothing to do with the heater problem. However, the "10-minute-OK" after ignition reset sounds like the typical RPM sensor problem.

On W201 (and other engines with single serpentine belt), the AC relay shuts off the compressor clutch when it senses a big difference (20%-30% IIRC) between engine and compressor speed. This is to protect the belt in case of seized compressor. This shut-off logic is only reset after turning off ignition.

Other than this shut-off reset, there is nothing in this climate control system that is reset by ignition. Therefore, I would assume this is a symptom by accident, not by design.

I would suspect a few other places:

1. The temperature dial on the push button unit. If you move it far away from your usual setting, does the cold/hot, hot/cold still exist?
2. Switchover valves dying. This assembly converts electrical signals (from push button unit) to vacuum (then drive elements to change temperature and air distribution). It is behind grove box.
3. Blender door potention (sorry about my spelling) meter dying. This is the worst nightmare in W201. The console/dashboard needs to come out to change this inexpensive part. When this fails, it often comes with never ending clicking sound behind the grove box. The system is trying to compensate by quickly switching between cooling and heating.

You need the climate control diagnosis manual to debug. There are many chapters. I can't retype all and I don't remember all either.

snipped-for-privacy@gmail.com wrote:

your usual setting, does the cold/hot, hot/cold still exist?

Yes - But the same thing, hot or cold for a few minutes then switches to the opposite.

The shop took the dash off and "found a short" a few months ago when I had it in for the same problem, I'm guessing they didn't check the parts behind the glove box. They did adjust the center vents and made them blow harder. But, of course, that didn't fix it either. Sigh They have been pretty intent on selling me a new compressor, even though that won't affect the heat -- and when the AC *works* it's very very cold.

I'll send this reply to them and see if they can figure it out :)

Update: The shop says it's the potentiometer. Is that valid?

Thanks!!

Yes. Potentiometer is possible, which is item #3 in my previous post. Unfortunately, this is a very time consuming job, requiring tearing up the dash/center console to access this cheap part. How many hours of labor does the shop estimate? My estimation is 8-10 hours. Part probably cost 10% of that.

Mine is causing the rapid clicking behind grove box, but I can live with it.

Do not replace the AC compressor.

snipped-for-privacy@gmail.com wrote:

You will end up replacing that too I will bet. How would the Potentiometer work for 10 minutes and fail? How would it magically start working by turning the car off?

It wouldn't.

Marty

If they have to tear out the dash to replace his potentiometer does it make sense to replace the evaporator while they're in there? I seem to recall that some mid-90s evaporators eventually developed a leak around the gasket.

Josh

Yeah, I think eventually I have to replace that too. I am waiting...

His symptom does not match any typical single component failure, so I have to guess there is some unpredictable behavior somewhere. The ignition-off-on reset is the typical symptom for RPM sensor but that does not explain the heat. The mechanical variable resistor (like potentiometer, the temp dial on push button) may develop unpredictable response. It may cause big swing even with minor movement.

The problem involves both cooling and heating, so my guess is in the blender control. Potentiometer is the famous culprit around that area. I am not sure if blender door resets at ignition off but I assume it does because it is vacuum controlled. If the door resets, potentiometer (which tells push button control where the door is) goes with it.

I agree I make a lot of assumpti> >

Good point.

Since a potentiometer is really a simple device (a variable resistor), I wonder if you can test the resistance from the wire? I don't know if that is any more accessible?

I agree with Wan-ning Tan, that this is most likely multiple problems.

Another thing to check is the serpentine belt and it's tensioner. If this is loose, then the compressor RPM sensor might trip.

Aside from that, you really need to take the car to someone who KNOWS the systems in this car and can test/troubleshoot it.

Good Luck, Marty

I had a similar problem on my 1987 300E. It was a bad KLIMA valve in my case.

The air conditioning would blow cold for about 10 minutes then blow ambient air. The compressor had shut off. If I shut off the ignition and immediately restarted the car, it would blow cold again for another

4-5 miles then blow warm air again. Could be recycled that way over and over again.

A fairly experienced and reputable independent shop said I needed a new compressor after numerous fixes, including replacing leaking expansion valve and service valve minor leak repairs. Since the car had a new compressor less than 10,000 miles before, I declined.

I made an electrical jumper per the factory manual, removed the KLIMA relay, and tested each system with the jumper wires. Everything checked out, so I bit the bullet for a new KLIMA relay ($160.19 at MB dealer). The air conditioning has worked fine for two summer seasons (100+ degrees here near Fresno) now.

Swapping easy to replace parts is usually not the way to fix something, but in my case the easiest fix was what ended up being the problem.