Kevin in Atlanta

Kevin (jetson8859) and I spent some time today chasing down a cooling only with the temp slider all the way to the left. Otherwise full heat. This was also observed watching the setting motor arm move to full hot once the temp slider moved off the full cold microswitch.

We tested the outside sensor - it was fine.

I found related thread replied to be Alan for testing the temp at the head: Head unit only - on head unit module pins 13 & 14.

The head unit on those circuit board connectors started and stayed at 320 ohms. Not good. I pulled another known good HVAC head unit and compared readings and they were inline with Wally's write up as well as others. 1000 ohms swing from cold to hot.

So, we are left to think the rheostat (?) has failed. Assuming we are right, can this be repaired?

StratfordShark

Are you certain different head units from different years have same pin-outs?

Reason I ask is that full heat at any setting off cold would be symptom of either break in circuit, or one of the sensors drifting high out of spec. A constant 320 ohm instead of Max 1000 ohm would force the setting motor to cold position wouldn't it?

Mrmerlin

for testing and final assembly I would suggest to cut the push connector out of the loop under the left fender.
cut the sensor wires and solder them then add some heat shrink.'

NOTE the connector has a habit of internally corroding and short of removing the 4 pushed in connector pins and resoldering them to the wires,
its just as easy to cut the wires and solder and will result in a positive connection

Kevin in Atlanta

Yes - 88 vs 87 heads. The reading at the connector at the head read 740 (it's cold here in GA). That to me means the entire sensor loop is ok. The reading at 12 and 13 on the setting motor connector was 320 as well.

I agree with your logic, but I can only tell you what I found.

StratfordShark

Regarding my earlier comment about the way the system responds to resistance, I've just checked WSM and resistance at controller pins 13, 14 should go from approx 750 ohm (cold end) to around 1750 ohm (30C hot end).

That's why I'm puzzled by your 320 ohm. If that measurement's correct then it would be driving the setting motor to full cold, not the full hot you are experiencing.

I would test total resistance at setting motor plug 2 (closest to arm) pins 4 and 12. What resistance do you see there at cold and hot ends of slider travel?

Kevin in Atlanta

I took a look at the connector you reference and they are in splendid condition.

Mrmerlin

Kevin the part that unviewable is the back side of each side of the connector,
the wires corrode in the rear sides of the socket pins or pull free of the pin.
The slide together part of the pins can/will also corrode.

Read this for better understanding

https://rennlist.com/forums/928-foru...highlight=HVAC

Kevin in Atlanta

I took a good look at the backside of each and they were pristine. No sign of any separation where the wires enter the connectors. I tugged and the wires did not move. Since the temp sensor circuit tested good I think there is no need to do for elective surgery. :-)

Mrmerlin

ok good, lets find out where the failure is

StratfordShark

It's a good system when fully functioning, but relatively simple so should be straightforward to troubleshoot.

The setting motor is controlled by the total resistance of three elements in series, so you should first look at resistance of total chain being presented to the setting motor (this also includes connectors in the test). If this is out of spec (should be too high if system going to full hot) then the problem is likely to lie in one or more of:

external temp thermistor/connector
internal cabin temp thermistor/connector
temp slider of control head

These are in order of likely fail point, with the most common issue usually the external sensor connector.

If the total resistance os within spec, then it's very likely there is fault in comparator electronics within setting motor itself. If you are familiar with electronics this is easy fix as the motor is easy to open, and the circuit board is old school with big discrete semiconductors that are easy to remove/replace. I think there are 4 identical NPN transistors. You can test each one to find which has failed. I don't know how common this failure is relative to sensors, but quite a few people on here have experienced it so it's not a crazy improbability!

I'm still puzzled by the 320 ohm you are finding at controller, irrespective of temp selected by slider. I'm sure you're being diligent in measuring this correctly, but this is not a failure I've heard of before. Even if the controller has failed, your full hot would suggest another failure with the external or internal temp sensor in addition to the rheostat failure! As you have access to 'good' control heads, how does your system respond when you replace your suspect original with a known good one?

I think that about exhausts the possibilities, so let us know what you find as you go through the measurements.

Kevin in Atlanta

I could not repeat the 320 readings last night. Not sure we how got those., What I do have on the 87's head unit at 13 and 14 is Full Cold 2.7k and Full Hot 24 (no K) I am a bit bewildered. The readings were not consistent. Weird and frustrating at the same time.

So I put the 88 head in the 87 and the behavior persisted. The 88 head results at the circuit board were 684 and 1.727 respectively.

I am going to circle back to the temp sensor ring today.

StratfordShark

The 88 resistance readings are fine, so maybe best to use this while you troubleshoot the rest of the system.

Check that total resistance at setting motor, then you can drill down to the separate sensors by removing driver side of console, and locating plugs for the internal and external sensors by following loom back from setting motor.

Kevin in Atlanta

Checked the inside temp sensor at T31 3.7 checks out ok.

At the setting motor connector (pin 4 and 12) with the car on or off full cold 4.8k and full hot is 5.8k. That is 1k higher than I expected.


Oh and my setting motor died Luckily I have another one .

StratfordShark

Ok sounds as if connectors all intact anyway.

also the motor (before it failed - how did that happen?) was correctly moving to full hot even at the lower resistance presented to it at full cold. That resistance is too high, so I suspect you have problem I had - one of sensors has drifted high out of spec over time. In my case a new internal sensor was the fix. You need to measure those sensors separately and see which is too high. Please search here for where people have posted correct resistances of each sensor at given temperatures.

Kevin in Atlanta

Setting motor failed full hot even though temp was just off the full cold position.

The internal temp sensor was 3.7k at the T31. Ambient temp today is 50ish. Yesterday I think the outside temp sensor was 740. Am I right to think that the inside sensor should have been closer to the outside temp sensor reading?

I just checked another inside temp sensor I have and it ohm'd 3.4k.

Thanks for all your help.