Where is this module located on a 300SD?
Is it a relay?

I am still having trouble with my heat.

I only get heat if I unplug the AC compressor under the car.
With it plugged in I get hot air through the defrost and cold air through the vents.

Monovalve & CCU appear OK.
With ACC OFF and key ON I get 11.65V & good ground at monovalve plug.
Using Jeremy's trick of applying 12V to monovalve directly
I can hear the coil activating inside.

Thanks,
Jeff

__________________
1984 300SD 300,000 miles--two tank WVO setup
2.88 diff & 500SEL anti-squat rear end

The electronic temperature control module is located under the lower dash carpet panel in the passenger foot well.

Remove the four metal and one plastic screws and the carpet panel will pull out. This is the same procedure as if you were getting to the blower motor.

The module is a silver box near the passenger fender and is bolted onto a metal frame with a clear marking that says temperature regulator in German.

I don't think this is a part which frequently malfunctions.

Are you saying your heat is directed to the proper vents when you disconnect the compressor? If so I don't know how to further direct you. The temperature regulator might be an okay place to start if you know your CCU is functional.

I had a problem similar to what you describe. Heat only came out of the defrost vents and cold air was coming through all front vents. As you may know you will never get hot air through the center vents. But you should get hot air through the side and foot well vents.

After exhaustively testing vacuum pods, the CCU, the temperature regulator, vacuum lines, etc. I determined the problem. There is a plastic connector on the control arm for the main air flap in the HVAC box. It connects to one of the six vacuum pods under your dash.

The plastic connector had broken with age causing the main air flap to remain in the cooling position (down) even with the heat on. The vacuum pod was performing but wasn't connected to the thing it was supposed to move.

If you want to see if this is your problem, remove the drivers side lower carpeting panel below the dash. Look up from below and you will see a pink vacuum pod on the left side of the HVAC box. This is right next to the A/C expansion valve. I can e-mail you pictures if you want. They are too big to upload here. Check to see if the pod is connected to the plastic control arm.

If it's broken let me know and I can tell you more about how to fix it. I think this will become a big problem for high-mileage W126s in the coming years, especially the diesels where increased levels of vibration will cause the plastic pop clip to break.

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1984 300 SD 384,000 Miles

Mbzjag,
Yes, the main air flap pod was not attached to the arm.
The plastic pin is half gone. I still don't get why I am getting
good heat when I unplug the AC compressor. Perhaps part of the
function of that pod/flap is to shunt away cooled AC air.

My understanding is that the AC is always on as a means of
taking moisture out of the air. Last year when I went the whole
winter with the AC unplugged, I had a problem with the windows
fogging up.

I sent you a private email and can resize your images and post them here.

So, what is the function of the silver module/regulator?


Thanks a million!
Jeff

__________________
1984 300SD 300,000 miles--two tank WVO setup
2.88 diff & 500SEL anti-squat rear end

That is incorrect. The ACC has five operating modes. The a/c operates in only three of those modes.

It interprets the position of the temp selector (except at the detent positions) and operates the monovalve to maintain the set point temperature, referencing the in-car temp sensor. It also regulates blower speed when in Auto selection and controls the center vent flaps and the recirc flap.

Whoops, I had an old e-mail address in the system. Please re-send and I'll e-mail you the pictures.

Tangofox is right. If you choose EC or OFF the compressor will not turn on. Choose the EC button and I bet you will not get cold air and the compressor will stay off.

When my plastic pop clip was broken the system did seem to do funny things. Sometimes the heat would be really good out of the defrost and other times not so good.

Yes, I think your entire problem is due to airflow. If that flap is shut and your compressor is engaged (excluding buttons EC and OFF), you will feel cold air. You are right in thinking that airflow direction is critical here.

I wouldn't bother about the temperature regulator. My mechanic said he's never seen one go bad. I'd bet your problem is only with the pop clip.

Getting it fixed is not an easy proposition. I took the entire HVAC box out and rebuilt it. That involves a lot of work which includes dash removal and lots of wiring, etc. You will be down to the firewall. You'll see in my pictures which hopefully you can post here for people at a later date.

I couldn't find any reference to this problem on the internet.

I don't think there is any way to remove the fastener. It slides onto the metal rod which goes all the way to the other side of the HVAC box and through the main air flaps. The pop clip is part of a plastic "sleeve" which fits on the end of the metal rod. There is no way to wiggle it off because of the way the plastic fits together. You'll see what I'm saying when you take it apart. Therefore, the entire thing has to be disassembled. It's a huge job.

Then you've got to think about finding a replacement. A junk yard might work but then you're dealing with more 20+ year old plastic which might break. Plus 20 hours labor getting it out of a parts car.

The dealer sells a $170 lever kit which includes all the plastic knobs and fasteners on the vent controls in and around the HVAC case. I would buy it if you are going to disassemble.

And watch out for the legroom flaps during dis-assembly. I sheared both of mine on the little knobs which fit up through the holes. The right one is $50 and the driver's side is $250 from the dealer!!!! Don't break it. But then again, putting the old plastic back when you've done so much work isn't all that satisfying either.

There may be a way to drill a small hole where the old plastic pop clip was. Then put a small screw to the vacuum pod control arm fastened with a nut. That would be a great economical and time saving solution. But I don't think it would be easy to get under there and try to drill a hole in the plastic lever.

The last option would be to tape or somehow fasten the control arm up during the winter months. Then let it fall during the summer months. Kind of like the legroom door coat hanger trick. But then you will be in trouble during tail months like Sept/Oct when some days you need heat and others you need A/C. The legroom door flaps aren't as critical. Unfortunately what we are discussing drives the whole system as you can see.

It was surprising to see this inferior part on the car. Unless someone finds a way to do a cheap fix I think a two cent piece of plastic is going to drive a lot of these cars to the crusher in the near future.

__________________
1984 300 SD 384,000 Miles

The plastic tab that goes through the arm is only half broken on my unit.
I will definitely try for an easy fix before tearing everything apart.

The center AUTO button on the far right always ratchets up to high and
never changes. So maybe a module problem? In any event I'll pull a spare
this weekend at the 50% off PNP sale.

I know your mechanic says he's never seen one fail, but I know one thing:
eventually, EVERYTHING fails. It'd be good to have one on the shelf just in case.

Will report back later with photos & results.

Thanks all for good info,
Jeff

__________________
1984 300SD 300,000 miles--two tank WVO setup
2.88 diff & 500SEL anti-squat rear end

For every temp control module that fails, there are probably a thousand failures in push button units.