1988 K1500, AC + heat, (Option C60) the usual push-button control head with the "blue" vacuum-fluorescent electronic display panel. Apparently the '88 “black” control head is a one-year deal, the '89 and '90 have grey plastic faces but function the same. This control head design has a long history of problems. The control heads new enough to have a “MAX” button (91—94?) had a substantial re-design; they may be more reliable as a result.
I have had repeat problems with the display section of the original control head. I've re-flowed solder joints twice, problem fixed each time...for about two years. Then the display goes intermittent again. I've had the truck for 20+ years, so the reliability of the display isn't that great. I stuff my thumb in a certain place on the control head, and the display functions again...for awhile. This display problem does NOT affect the function of the control head--all the buttons work, hot or cold, low or high fan speed...everything works like it's supposed to, I just can't see what's what on the display. The display has been intermittent again and was on my "list of things to do" for far too long.
Circuit board with four electrolytic capacitors circled.
Verify the two rows of circled solder joints on these circuit boards!
"Horizontal" board pulls out of "vertical" board. Vibration or other relative movement seems to break the solder joints. Note that two of the three HVAC lights (round, black 1/4-turn sockets on smaller board) are visible in this photo. Commonly burnt-out.
Newer "MAX" control head has wires attaching the two circuit boards, and only one electrolytic capacitor.
However, the most-recent problem was MORE than just an intermittent or "dead" display. None of the buttons worked. No fan, no air movement, all three actuators for the air doors didn't move, no display. NOTHING on the HVAC worked This is an entirely-new situation for this vehicle.
I have the downloaded '88 service manual, and the downloaded '88 service manual supplement. My paper '88 service manual (complete set) was printed in '86, it's FULL of errors and omissions...which the service manual supplement was supposed to have corrected.
THE 1988 SERVICE MANUAL SET IS NOT TO BE TRUSTED! Verify using the '89 Service manual set for confirmation!
I'm looking through the downloaded-from-the-link-on-this-site '88 service manual (GMC, not Chevy) and it doesn't discuss the HVAC control head diagnostics. The downloaded Service Manual Supplement does have some diagnostics. I also have the '88 “Electrical Diagnosis” book as well. The '89 service manual is better in all regards for HVAC service.
I'm looking at the illustration of the HVAC control head electrical connector, and it doesn't match mine. Circuit 2 powers the control head, is a large red wire, and in position "12" of the 12-pin-wide (5 wires, a space, 3 wires, a space, and then 4 wires) connector body. All the way to the left on the illustration, in the group of five wires. I don't have a red wire there. My heavy red wire is on the opposite end, in the group of four wires, but labeled position 12 molded-into the connector like it should be.. The service manual screwed-up the wire positions.
My hunch is that the control head isn't getting full-time “12 volt” power like it should. That's the purpose of that red “Circuit 2” wire in Position 12. When I test voltage with the control head disconnected from the harness, I'm surprised that it's getting battery voltage. But when I test voltage with the control head plugged-in...there's only 3 volts.
And that's when I jumped to the wrong conclusion, and wasted about a week's worth of diagnostic time and more than fifty dollars.
I figured that something the control head controlled—perhaps the control head itself, or an actuator, or the A/C system—was drawing so much current that it was dropping the voltage at the control head below what the control head needs to operate. And I know that I've had previous problems with my control head.
So I go to my favorite Treasure Yard and grab the only control head he has that even remotely fits. Unfortunately, it's a newer one with the “MAX” button, so not directly interchangeable. [Edit: Yes, it is interchangeable, and gives manual control over the recirculation door.] Then I go to my second-favorite (much larger, much more expensive) Treasure Yard and buy two broken control heads (the only ones they have) hoping I can make one good one out of three potentially-damaged ones.
I plug each one in...and get the exact-same “nothing” results. I swap the circuit boards around...nothing. Deader than a smelt.
(Continued.)
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I have had repeat problems with the display section of the original control head. I've re-flowed solder joints twice, problem fixed each time...for about two years. Then the display goes intermittent again. I've had the truck for 20+ years, so the reliability of the display isn't that great. I stuff my thumb in a certain place on the control head, and the display functions again...for awhile. This display problem does NOT affect the function of the control head--all the buttons work, hot or cold, low or high fan speed...everything works like it's supposed to, I just can't see what's what on the display. The display has been intermittent again and was on my "list of things to do" for far too long.
Circuit board with four electrolytic capacitors circled.
You must be registered for see images attach
Verify the two rows of circled solder joints on these circuit boards!
You must be registered for see images attach
"Horizontal" board pulls out of "vertical" board. Vibration or other relative movement seems to break the solder joints. Note that two of the three HVAC lights (round, black 1/4-turn sockets on smaller board) are visible in this photo. Commonly burnt-out.
You must be registered for see images attach
Newer "MAX" control head has wires attaching the two circuit boards, and only one electrolytic capacitor.
You must be registered for see images attach
However, the most-recent problem was MORE than just an intermittent or "dead" display. None of the buttons worked. No fan, no air movement, all three actuators for the air doors didn't move, no display. NOTHING on the HVAC worked This is an entirely-new situation for this vehicle.
I have the downloaded '88 service manual, and the downloaded '88 service manual supplement. My paper '88 service manual (complete set) was printed in '86, it's FULL of errors and omissions...which the service manual supplement was supposed to have corrected.
THE 1988 SERVICE MANUAL SET IS NOT TO BE TRUSTED! Verify using the '89 Service manual set for confirmation!
I'm looking through the downloaded-from-the-link-on-this-site '88 service manual (GMC, not Chevy) and it doesn't discuss the HVAC control head diagnostics. The downloaded Service Manual Supplement does have some diagnostics. I also have the '88 “Electrical Diagnosis” book as well. The '89 service manual is better in all regards for HVAC service.
I'm looking at the illustration of the HVAC control head electrical connector, and it doesn't match mine. Circuit 2 powers the control head, is a large red wire, and in position "12" of the 12-pin-wide (5 wires, a space, 3 wires, a space, and then 4 wires) connector body. All the way to the left on the illustration, in the group of five wires. I don't have a red wire there. My heavy red wire is on the opposite end, in the group of four wires, but labeled position 12 molded-into the connector like it should be.. The service manual screwed-up the wire positions.
My hunch is that the control head isn't getting full-time “12 volt” power like it should. That's the purpose of that red “Circuit 2” wire in Position 12. When I test voltage with the control head disconnected from the harness, I'm surprised that it's getting battery voltage. But when I test voltage with the control head plugged-in...there's only 3 volts.
And that's when I jumped to the wrong conclusion, and wasted about a week's worth of diagnostic time and more than fifty dollars.
I figured that something the control head controlled—perhaps the control head itself, or an actuator, or the A/C system—was drawing so much current that it was dropping the voltage at the control head below what the control head needs to operate. And I know that I've had previous problems with my control head.
So I go to my favorite Treasure Yard and grab the only control head he has that even remotely fits. Unfortunately, it's a newer one with the “MAX” button, so not directly interchangeable. [Edit: Yes, it is interchangeable, and gives manual control over the recirculation door.] Then I go to my second-favorite (much larger, much more expensive) Treasure Yard and buy two broken control heads (the only ones they have) hoping I can make one good one out of three potentially-damaged ones.
I plug each one in...and get the exact-same “nothing” results. I swap the circuit boards around...nothing. Deader than a smelt.
(Continued.)
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