Loss of all AC/Heating controls on 91 K2500

[farmer joe]

The AC was useless when I bought the truck (broken refrigerant line) but the heater has worked fine for years until last winter, when the AC/Heater control section in my dash started dying. None of the buttons have any noticeable affect on the heating system or blower. Screen and most of the lights behind the buttons are all dead. The only sign of power in the entire module is the light behind the Hot<>Cold rocker switch, which still comes on with headlights.

This happened a few of times last winter and on each occasion the module returned to life at startup, though not necessarily the NEXT startup. On one occasion, I noticed the panel die as I tried to switch the fan from low (where it always tended to be at startup) to medium, shortly after starting the truck. This seemed like an anwer at the time, so when the controls returned to life I made a point to never touch the fan switch after that. They did work for a while after that, but eventually they went way when I wasn't looking, despite not using the fan switch. Eventually they just didn't come back. Now it is winter again.....

The A/C troubleshooting section of my 1991 C/K Pickup Service Manual has so far offered very little help in diagnosing this problem. In one optimistically named "Control Head Diagnosis" section, it had me check for power to the module. I removed the module from the dash, and then checked for 12v. between terminals 12 & 8 of the module connector with ignition off, and between terminals A & 8 with the ignition on. The requisite voltage was present as expected. The manual gave me directions for how to proceed if I did NOT get the voltage as expected, but gave no more concern to diagnosing a control head that was not working despite having the proper voltage and went merrily on to diagnose air-flow doors and other parts of the A/C control system. grumble, grumble, grumble........

So I see numerous threads regarding heater blower problems on this forum, which leads me to my first question:
Could some problem in the blower or the relays that control it be the source of the dead module?

I feel like I'm in a Catch-22. I don't want to buy a brand new module if something in my blower is going to fry it soon after installation. However most of the blower/relay troubleshooting stuff I find in my manual seems predicated on having working controls in order to carry out the tests. Which is why I haven't even begun to disassemble my way into the blower.

Also, I still have NOT discerned from my manual exactly where the two relays that control it are physically located, so my second question is:
where are they?

Any thoughts or suggestions for a path forward would be gratefully received.

Joe

[speedygonzales]

The pins in connectors can relax for lack of a better term. The tension the female pin places on the (male) pin over time can be less and less. Combine this with oxidation on the pin and you have an intermittent connection that comes and goes. This can explain why you measure voltage at the controller but it still does not work. When you measured voltage, you probably removed the plug and probed in the front of the connector for voltage. Yeah it's there but it doesn't make contact with the controller.

I did not see where you set your controls to "ON" and wiggled the connections from the control head all the way to the motor. If you do this and find that one connector will cause it to come on when wiggled, consider replacing the pins. They are easily found on the internet. Once you know the style you are looking for.

One other thing you can do is power directly to the device you are testing from the battery. Make up 2 jumpers and put them directly from the batter to the fan motor for example. Or other devices.

[farmer joe]

Thanks for the reply. The issue of pin contact did occur to me, though I didn't mention it. The test I was able to do - once I had the dash area apart - was to squeeze my hand around behind the module while it was plugged in, grasp the connector plug firmly, and wiggle it up and down as far as it would go. During the test, none of the lights or control screen on the module came on or even flickered briefly.

For what it's worth, the connector fits fairly tightly onto the prongs in the back of the module. It takes an effort to push them together or pull them apart. While it still could be the problem, I have at least some reason to believe it is not. I also can't see any other way to test for this, short of getting a known good module and plugging it in as a test. I may wind up buying a new one, but I'm not at that point yet.

I haven't opened up the dash enough to trace the wiring to the fan yet, because I started out knowing the module itself wasn't working properly and the fan had always run reliably and sounded quiet. From browsing this forum, I'm now starting to look at the fan and particularly the relays as a potential source of the issue. Once I get access to them I may well use a jumper direct from the battery for testing.

Unfortunately I have a Dr.'s appointment in a couple of days, so I need to reassemble what I currently have apart, at least far enough to make the drive. It will be Wed or so before I will be free to tear into the fan and relays. I'll report back what I find when I get there.

I joined this forum almost 10 years ago, and it has helped me solve several puzzling issues over the years. Mostly I haven't had to even post, just browsing a few pages of posts usually gave me enough ideas to solve my problems. I believe this is the first time I've actually reached the point of posting my problem.

Thanks for the help.

[68 TT]

The fan speed is controlled through a series of relays and the resistor mounted on top of the heater blower motor housing right behind the glove box liner so if you remove the glove box liner you have good access to the wiring.

If you need another heater control panel let me know. I have the spare one from my 1990 dash that you can have since I converted my truck to a 1994 dash, wiring and HVAC system during the drivetrain swap.

[farmer joe]

Thanks so much for your offer. I haven't found a replacement yet, but I'm sure I need one. Since my last post, I took my module apart to inspect the circuit board. While the main board looks OK, the display screen has a burned area right next to where the fan speed is shown.

Given that damage and the number of posts about fan problems, I figure that I need to check out the fan relays and resistor before installing a replacement. I haven't gotten to this part yet as I have had to be out driving frequently this past week. I'll be diving into it this weekend.

I really appreciate the offer for your spare. I live a long way from the nearest big cities (Spokane & Wenatchee) and the couple of salvage yards in my area have none available. If you pm me an idea of a fair price I'm sure I can pay that and shipping.

Here is a pic of mine for comparison:



Thanks again!

[Second Series]

I had a similar problem with the Radio in my '88 k2500. I opened up the radio and cleaned the button contacts. A pencil eraser works. You might give that a try for an easy fix. The '88 has the manual heater controls, but my '90 k1500 has the one in your picture and the same radio as the '88. The radio and heater controls look like they have the same type of buttons.

[farmer joe]

Thanks for the suggestion. I think my switches must be different. I had the module apart as far as I could get it, and the switches were all part of a board section that had no obvious way to disassemble it. And the switches were each covered by a nice little rubber cap. I really didn't want to try pulling on them.

I still appreciate any and all suggestions

[68 TT]

Originally Posted by farmer joe

Here is a pic of mine for comparison:

Attachment 2064820

Mine is slightly different and does not have the recirculation button so they are not compatible for a swap out. I know from looking into new ones that the two styles have different wiring connections and do not interchange so you will need to find one with your button configuration.

[Second Series]

The switches with the rubber caps are membrane switches. There is a carbon disk in the rubber cap that contacts the copper trace. You can clean the trace with an eraser. I don't remember how I cleaned the disk, maybe a small amount of alcohol on a cotton swab, but try not to get alcohol on the rubber.

[farmer joe]

Originally Posted by 68 TT

Mine is slightly different and does not have the recirculation button so they are not compatible for a swap out.

I'm not really surprised. The guy at the salvage yard looked up compatibility and there were only a couple of years that were interchangeable, even though many years of trucks appear to have ALMOST the same component. Thanks just the same for the offer.

Originally Posted by Second Series

The switches with the rubber caps are membrane switches. There is a carbon disk in the rubber cap that contacts the copper trace. You can clean the trace with an eraser. I don't remember how I cleaned the disk, maybe a small amount of alcohol on a cotton swab, but try not to get alcohol on the rubber.

I'd certainly try to clean the switches if I was considering putting the original module back in. However yesterday I found a source for used parts that has more than one replacement available, much cheaper than a new one. So I think I'll just order one that has been tested to work.


I pulled the relays and resistor out and gave them a close look. Nothing seems to be wrong with any of them, so I'm now ready to risk putting a good one in and seeing what happens. And I will absolutely come back and report the results.

Thanks again ya'll for the helpful advice. This forum has never failed to be a help when I've come here looking for info.