ACC II - bad temperature sender or bad thumbwheel?

[thorsen]

It's a pretty hot one today and I can't get the 1980 280SE cooled down. The AC is working fine (ac clutch cycling, return hose to compressor is cool) but I think the problem is in the temperature sensor chain. This car has the ACC II that blends in heated air to keep the cabin at the temperature set on the thumbwheel, so it's not an issue of the AC not working.

It's about 90-95* in my shop and the total resistance of the chain is as follows:
Thumbwheel set @ 85* = .821 K ohms
Thumbwheel set @ 65* = .452 K ohms

The resistance of the individual sensors is as follows:
Ambient sensor on firewall = 99.6 ohms
In-car sensor = 39.4 K ohms
Thumbwheel set @ 85 = .570 K ohms
Thumbwheel set @ 65 = .005 ohms

I can't find the values that each sensor is supposed to test at. I am thinking the ambient sensor on the firewall might be bad based on the low reading - does anyone have any ideas?

[thorsen]

Duh - I hate it when I do that. It's covered in 83-609.

[thorsen]

On a lark I went to Radio Shack and bought a 2200 ohm resistor - and my DVM measured it at 800 ohm. I didn't know DVM's can go bad but I guess they do. I bought a new DVM and retested and found my thumbwheel would go open circuit around 65 degrees. I bought a thumbwheel repair kit from unwired tools and i suspect that will correct my problem.

[meltedpanda]

good luck, if you do fix it let us know , this would be a major accomplishment!

[thorsen]

I got the thumbwheel repair kit from UnWired Tools today. It was easy to fit the new potentiometer and I had it all back together in less than 30 minutes.

I was getting ready to throw the old potentiometer away when I decided to take it apart. I spent a few minutes cleaning the insides of it and put it back together without any extra parts. Surprise - it was within spec again. Now I guess I have a spare.

[thorsen]

The new thumbwheel fixed it. I had 55 degree air blowing out of the vents yesterday.

[thorsen]

The problem returned. Strange how it only seems to happen on 90+ degree days. Tomorrow morning I will be following 83-611 to balance the sensor chain, but I have a feeling the problem is internal to the servo. I will be using 83-610 to test the feedback potentiometer.

To add insult to injury, the driver's window wouldn't go down. I got that fixed easily enough - I pulled the mechanism out of the door and cleaned all the gunk off, followed by a re-greasing.

[thorsen]

The sensor chain balanced out perfectly and the feedback potentiometer tested correctly too. At this point I am thinking either one of these:
1. Sensor chain failure only during extreme heat
2. The servo is not fully closing off the supply of hot water to the heater core. during mild weather the AC can overcome the additional heating but when temps go past 90* it just can't compete.

We have a cool spell going on right now but the next time it heats up I am going to retest the sensor chain. Sometime this week I am also going to put an additional valve in the supply line for the heater to positively stop the flow of coolant to the heater core.

Somewhere between those two I should find an answer.

[thorsen]

The weather in Chicago has been down right pleasant and I am still waiting on the temps to climb past 90 again. I have the thermometer from my digital volt meter taped to the metal pipe that supplies the heater core so I will be able to tell if the heater is getting hot water.

I've been keeping myself busy with other stuff. Today I made new sunroof sliders for the ones that are NLA.