HVAC Control Head in dash--multiple issues (Mostly fixed)

[Schurkey]

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.

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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.

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Verify the two rows of circled solder joints on these circuit boards!

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"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.

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Newer "MAX" control head has wires attaching the two circuit boards, and only one electrolytic capacitor.

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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.)

 

[Schurkey]

Having blown fifty bucks on control heads that aren't helping me any, I decide to investigate the vehicle wiring. NOW I'm on the right track. I trace Circuit 2 which starts at the electrical power center under the plastic cover by the right hood hinge/firewall area. Circuit 2 has a fusible link at the power stud. Fusible link is good. There's power all the way through that wire to Position 12, but it's not “good” power, it's marginal power. If there's a load on the circuit, the voltage plummets like a paralyzed falcon.

There's a 3-wire connector (C323) behind the glovebox. [Edit: Service manual says it's "below" the HVAC fan motor. No. That's where the wire bundle comes through the firewall. This connector lives near the ECM, behind the glovebox.] I pull it apart. BINGO! Two of the three wires seem OK, but the red one—circuit 2, my problem child—has melted the connector, and burnt the wire ends. (Green goes to the A/C clutch relay, and black/white stripe is a ground.)

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The other half of the connector:

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I clipped the wire ends off of Circuit 2 at that connector, then plugged that connector back together as the other two wire connections are OK. Circuit 2 gets re-routed through a brand-new Weatherpack connector. There's no excess of wire, so I can't pull this connector out farther to show more detail.

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I now have proper power at the control head, and two of my four control heads work except for the tiny light bulbs for night operation. The too-new unit with MAX button works, and has night-lights, and one is genuinely defective, I guess.

After examining the original control head, I did touch-up the solder at several points where the one circuit board plugs into the other. This will be the third time I've re-flowed the solder there. I expect it to last the usual two years before the display goes intermittent again.

The other working control head got the solder re-flowed at two connections, again, where the one board plugs into the other.

At some point real soon, I'm going to replace all the electrolytic capacitors on the pull-out circuit boards. Four each on the older boards, only one on the newer board. Electrolytic capacitors have a typical service life of about 20—25 years. They may leak, and corrode the circuit board traces. They may “vent” (industry term—my word for it is “explode”) where the housing is scored exactly for that purpose. And they may die silently—drifting capacitance value, or holding the correct capacitance but dramatically increasing the internal resistance. Point is, they have AT LEAST four failure modes, a recognized service life...and these caps are WAY beyond their service life expectancy. The largest capacitor on my original control board is leaking onto the circuit board, it's gotta go--and as long as I'm in there, the others are gonna vanish along with the failing one. I'm saving one more reply panel for that work.

[Later Edit]
Got brave, plugged-in the later ('92, I think) control head with the "MAX" button. Works perfectly, I like being able to control the recirculation actuator manually, and even has front-panel lights that actually light up. Until I get the original control head re-capped and the three unbelievably-small light bulbs replaced, this is what's in the truck now. (...maybe forever.) A potential down-side is that the recirculation door activated by the A/C compressor pressure is kind-of a safety feature, I don't know if this control head will defeat that, or if the pressure switch on the compressor will still move the recirculation actuator.

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[/Later Edit]

 

[Schurkey]

(Future Use)

 

[scott2093]

I have one of those blue esr meters and it's been pretty great taking the guesswork out of finding tired caps and surprising how many are still kicking along.

Of course some caps esr ratings may have very well been within the acceptable range back then and is now considered bad according to the standards on the meter but I'm too lazy to look up specs for those old ones..

 

[scott2093]

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.

I'm trying to follow along but it's unclear to me which manual you are referencing regarding this statement.

 

[GrimsterGMC]

Having blown fifty bucks on control heads that aren't helping me any, I decide to investigate the vehicle wiring. NOW I'm on the right track. I trace Circuit 2 which starts at the electrical power center under the plastic cover by the right hood hinge/firewall area. Circuit 2 has a fusible link at the power stud. Fusible link is good. There's power all the way through that wire to Position 12, but it's not “good” power, it's marginal power. If there's a load on the circuit, the voltage plummets like a paralyzed falcon.

There's a 3-wire connector (C323) behind the glovebox. I pull it apart. BINGO! Two of the three wires seem OK, but the red one—circuit 2, my problem child—has melted the connector, and burnt the wire ends. (Green goes to the A/C clutch relay, and black/white stripe is a ground.)

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The other half of the connector:

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I clipped the wire ends off of Circuit 2 at that connector, then plugged that connector back together as the other two wire connections are OK. Circuit 2 gets re-routed through a brand-new Weatherpack connector. There's no excess of wire, so I can't pull this connector out farther to show more detail.

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I now have proper power at the control head, and two of my four control heads work. One is the too-new unit which I haven't tried, and one is just plain cooked, I guess.

After examining the original control head, I did touch-up the solder at several points where the one circuit board plugs into the other. This will be the third time I've re-flowed the solder there. I expect it to last the usual two years before the display goes intermittent again.

The other working control head got the solder re-flowed at two connections, again, where the one board plugs into the other.

At some point real soon, I'm going to replace all the electrolytic capacitors on the pull-out circuit boards. Four each on the older boards, only one on the newer board. Electrolytic capacitors have a typical service life of about 20—25 years. They may leak, and corrode the circuit board traces. They may “vent” (industry term—my word for it is “explode”) where the housing is scored exactly for that purpose. And they may die silently—drifting capacitance value, or holding the correct capacitance but dramatically increasing the internal resistance. Point is, they have AT LEAST four failure modes, a recognized service life...and these caps are WAY beyond their service life expectancy. The largest capacitor on my original control board is leaking onto the circuit board, it's gotta go--and as long as I'm in there, the others are gonna vanish along with the failing one. I'm saving one more reply panel for that work.

The one in my '88 burnt out exactly the same as this and it was only due to having a paper copy of the electrical service manual that I even thought to look there. Quite often my display will go crazy and it just takes a good slap up side the dash board to put it right so I am guessing a cracked soldered joint as well.

 

[Schurkey]

Note: I added text to Posts 1 and 2, and also a photo to Post 2.

I'm trying to follow along but it's unclear to me which manual you are referencing regarding this statement.

Photo 1. '88 Service manual supplement.

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Photo 2. Page 1B-9 of the SMS.

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Photo 3. Actual harness connector from my '88 K1500.

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"1" and "12" are oriented the same in the service manual illustration and in the photo of the connector, but the connector is photographed with the tab on the bottom, not on top ("upside down".) "12" with the red wire is part of the group of four, not the group of five. The group of five is at the other side of the connector--"5" to "1", not "12 to "8".

I said the service manual screwed-up the wire positions. More accurately, they screwed-up the "spaces" between the wire groupings in the illustration.
12--9 is a group of four. Then the space, 8--6 is a group of three, a space, then 5--1 is the group of five.

Note also that the connector illustration has four "open" holes with no circuit number or wire color. ("1" and "5" easily visible in the photo. "B" and "C" on the lower row harder to see.) Those same holes in the actual connector have heavy white wires. I have no idea what those white wires "do". Perhaps they go into the harness and dead-end there.

 

[someotherguy]

The factory manuals are by far the best source of info for working on these trucks, but they are definitely not perfect. I'm reminded of looking in my 1994 set of manuals and seeing references to checking the vacuum lines to the controls while troubleshooting A/C issues. Vacuum lines to the controls. 1994. Yep...

Richard

 

[scott2093]

Note also that the connector illustration has four "open" holes with no circuit number or wire color. ("1" and "5" easily visible in the photo. "B" and "C" on the lower row harder to see.) Those same holes in the actual connector have heavy white wires. I have no idea what those white wires "do". Perhaps they go into the harness and dead-end there.

My head hurts...
I can't even find a year yet where those are. My 93 is all out of sorts too.. I'm having the best luck tracing a 90s circuit with some changes oddly enough.

re pressure switch....In the 93 manual there is obviously no blk/white going to a single wire pressure switch in that 157 circuit. It's actually empty in the drawing. Same with the 92 afaik...
But my real life 93 has a tan wire there (old circuit 157) that feeds the temp door motor .
And I'm not talking about the other tan wire I have 2 spaces down that feeds the mode door motor. So I have 2 tan wires at different terminals that do what 1 tan wire used to do since it was supposedly spliced in the 93 manual......
No service manual has that 2 tan wire set up that I can find. ..
Unfortunately my ocd has me wanting to draw my circuit out for no apparent necessity.
Why all the changes over those years? It's weird that some of these head units work with some of the differences...

 

[Schurkey]

But my real life 93 has a tan wire there (old circuit 157) that feeds the temp door motor .

Temp door? Might want to confirm it's not controlling the recirculation door actuator ("air inlet valve").

If it really is the temp door, I'm kinda shocked. I went through all the buttons on my "too-new" MAX-button controller, and it seemed like everything worked like it was supposed to, and controlled the proper actuator. The one thing it did not do that I thought it should, was that I have to hold the temp-select button to make the temp change from hot to cold, or cold to hot. The original could be held for a second or two, and released--the actuator would continue to move in the selected direction until it went all the way, or the button was pushed again.

I don't know if the newer controller has a fault, or if that's the way they're intended to work.