Well this weekend my dad pulls out this blue service manual dedicated to the 123 climate control. I did some reading, some testing, and was able to improve my heat-stuck-on problem, but it's still there. I'm wondering if anyone can shed some light. Here's what I did.

I tested the interior thermistor at 75 and 85, couldn't do 65, the people thermometer considers that "out of range" aka "you're dead if your 65 degrees". My mom's baking thermo whent from 75 and it checked out within range at those temps. I just dunked it in water with the thermomter in there as well and took ohm readings.

The ambient sensor: It was about 97 degrees after my car idling with the hood up for a while and it checked out, like 207 ohms or close. I removed it and checked it in my hand, not froma point in the car.

The dial potentiometer: This is interesting. I tested it at the plug under the blower. At first, it was reading from 700 to 15-1600 ohms and I'm thinking I got my problem. Well I adjusted it so it's not about 200-1000 ohms. I won't go the full range the manual says, but in this range, I would think I should have little heat as the resistance chain should be low. I'm getting a warm car instead of a hot car. It's tolarable but if the AC is on, if flip flops from heat to AC.

Other things I did. I plugged the thermistor back into that air hose. I used to work awesome just sitting above the plastic piece above the passenger foot well. Dont' know what has changed. This helped but gave my wild flip floping between the air and AC.

I zip tied the floor vents open so it would react more quickly to AC temp changes. Then I relocated the thermistor so it is sticking out below into the passenger well a few inches. That worked better, still flip floping but the delta is much less. I'd put it back on the dash, but with the defrost vents stuck open, it renders the climate control useless as it just blows hot air right on it and shuts the heat down or cold air and shuts the AC down.

Anyway, the best I can tell, somewhere I have more resistance than I should in the chain. I'm wondering if maybe the thermistor is flaky in it's lower range? Just sitting there with the doors open, it was bout 65 out and the thing was reading where it should for that temp but I couldn't actually use the thermomter to test it.

What about jumping the ambient sensor in the interim? That should knock a few hundred ohms off of the chain and make it less likely to heat.

Are there any other things I should ohm out? I'm not awesome at reading the electrical diagrams.

Thanks,

Frank.

__________________
'82 300SD

I have some climate control diagnostic documentation that I received from Larry Bible. It proved helpful to me and based on your post, it might help you narrow it down to the real issue.

Send me you fax number in a private message and I'll shoot it to you.

JCD

I have a 82 300SD and experienced climate control problems shortly after I purchased the car in 88.
I found that all the problems I had were caused by the poor design of the Climate Control Module. The problem was the soldered connections between the three printed circuit boards within the unit were broken. The boards are held in place by the layout of the module.
I corrected the problem by resoldering the connections, extra heavily. The connections between the boards should either be made using contacts or wires. Solder connection between boards is hobby shop quality.
I have not had any other problems with the Automatic Climate Control since. If I do, I will redo the connections by drilling holes through the copper lands and use wire.

Another electrical problem I have had and easily repaired is the aux fan in front of the radiator. The problem on this was simply frozen brushes. The fan was dead, even with 12 VDC placed across the fans power leads. I disassembled the unit and found the brushes frozen in place. That looked like they had only worn down around 25%. I broke them loose, cleaned the brush guides and have had no problems since.