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Old 11-21-2020, 11:18 PM   #1
farmer joe
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Join Date: Jan 2013
Location: Oroville, WA
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Loss of all AC/Heating controls on 91 K2500

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
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Silverado Trim Pkg.
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Old 11-22-2020, 12:26 PM   #2
speedygonzales
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Re: Loss of all AC/Heating controls on 91 K2500

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.
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Old 11-22-2020, 06:20 PM   #3
farmer joe
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Re: Loss of all AC/Heating controls on 91 K2500

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.
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