Exploding A/C temp regulator
 

Trying to diagnose the A/C in my '85 300D.

Installed a known working temp regulator (from my other car!) in the bad car. As soon as I applied power.... I heard a loud POP from the temp regulator box!:eek: (Now this box rattles just like the one that I pulled out did!)

Anybody have any idea what could cause the temp regulator box to blow like that?

I'm afraid I've let the magic smoke out of too many of these boxes!:silly:

I've read that non-functioning aux. water pumps can make them go kaplooie.

Actually, I had heard the same thing.... therefore I had already had the aux. water pump unplugged from previous troubleshooting before I tried the other temp. regulator.

Could a bad blower motor do the same thing? I can make the blower motor run though by jumpering it.

By "temp regulator" you mean the thing in the dash with the pushbuttons, the ACC? I ask because I'm not as familiar as I'd like to be with the ACC system in our cars. My unit had a bunch of fried traces and did not work completely. It came to me in that condition. I wired around the problems and resoldered everything (like the cruise control cure) and now everything works.

My guess would be a short somewhere, as Larry has already suggested. The ACC doesn't have many electronic things, mostly a bunch of small relays. Possibly one of them went pop. I would imagine that they control the monovalve, aux water pump, compressor, and blower motor. There is another controller behind the passenger side dash, or so I have read, plus a box of dropping resistors for the fan, on the passenger side inner fender. If you can open the thing up and figure out what went "pop," I may be able to help more with the diagnosis.

Jeremy

No, I've actually already put a different ACC pushbutton box in there. The temp regulator box I'm talking about is the silver Bosch box back behind the glovebox. The other black box behind the glove box is the blower regulator box (already replaced it as well!) The resistor box under the hood also appears in good shape and is not shorted to ground.

After further investigation... the temp regulator box that went POP still appears to work in my other car. I did pull the box open though to see what happened inside. There are a series of large metal plates standing up across the back of the circuit board. Each plate comes from a small plastic covered block on the board. The black plastic covering that block popped off at one of the plates... but nothing else appears to be fried.

Anybody know what these plates are for?

I guess I got the power shut off to it quick enough!

Go here http://mb.braingears.com/ and you can down load the factory manual for the climate control.
Also go to your public library they may have a good manual selection. Mitchell Manuals are in lots of libraries and are sometimes easier to use than the factory manuals because they are re written, even the wiring diagrams.

I'm not absolutely positive if this is what you are describing. But in nearly all electronics assemblies with large metal "plates", these are used as heat sinks for heavier current draw. It sounds as if you have a short that can fry things. Do you have a multimeter and know how to use it? (Not an insult, just trying to determine your experience with electronics.) I would check the pins at the connector for a short. Let me know if you need help.;)

From everything you mention, I am wondering if you got ahold of the wrong ACC controller for your car.

Sam,

I do have a good multimeter and some electrical knowledge (I'm a mechanical engineer, not an electrical;) )...but by no means an expert. So any advise would be greatly appreciated.

The original CC unit I pulled out had burnt circuits. The replacement CC unit I have is the identical same part #. The original temp. regulator box was rattling when I removed it (which is why I replaced it with the one from the good car). Turns out it is rattling because the same piece popped off in it. The blower regulator box was replaced also with a used one because the original one had been tampered with so I figured I'd be safe. I have also checked the resistors and they don't appear to be shorting out and are clean.

I had ruled out the CC unit because the original temp. reg. box was already blown before I ever plugged the new CC unit in.

Now you can probably understand why this is driving me CRAZY!!!:silly:

From studying the wiring diagrams, there could be several things causing this: The ACC controller, blower motor controller(provides "auto" fan control)(same area as temp controller), blower motor, temp. sensor, a/c compressor clutch solenoid and low pressure switch, and via the ACC controller: the aux water pump and flap solenoids.

From reading your post, you mentioned something about the blower motor not working. Is this one of your reasons for troubleshooting this? If your blower was drawing too much current, it may have taken out the blower motor controller.

Probably the best approach would be, with power off, to set your meter on ohms and check each of the pins, at the temp controller connector, to ground and see what kind of resistance readings you get. Some of these will vary depending on ACC pushbutton settings. Inside the temp. controller, sometimes you can trace the circuit tied to that heat sink and also the outgoing pin it is tied to. With heavy current draw, these will be the larger, more direct traces. Also, you might pop open the blower motor controller and see if something is fried inside.

It Works!!!
 

Thanks for all those who helped.

FYI to complete the thread.

Finally got it working today. Turns out that some previous owner had been screwing with the wiring. Two of the pins in the right plug on the ACC were switched. Opened the plug up, switched the pins... all works fine!

Glad to hear......
 

that you were able to sort things out! ;)

Don't you just love the band-aid "fixes" of previous owners??:rolleyes: Hopefully, you don't come across any more. (I ran into a few on mine.)

Have been having an overheating problem on the main circuit board in my current ACC box (pushbutton). The heating seems to be concentrated near the relay located in the middle of the three toward the rear of the board. At this point I have not been able to decide which high current device this particular relay supplies.

Have disconnected the monovalve, the aux water pump, and the A/C compressor. This seriously limits the answer to the heater fan, but it runs quite well when hooked directly to +/- 12v. Sooo, has anyone figured out what each of the relays is hooked to, and is there any high current device in the the other (temperature) control box behind the glove box? Have not had it opened or even out of its normal location.

Thanks in advance for any ideas.

I'm not sure what relay goes to what. But, here are the possibilities I see in the wiring schematic: Fresh/Recirc Flap Control, Speed 2-4 Enable Input (Auto Fan operation & Defrost)

Can you see a number of any kind? Relays are usually marked K1, K2, etc.....

I don't readily see any high current draw situations involving a relay in the ACC controller.