Heater, Vents, A/C Control Panel

[JohnB]

1990 GMC C1500 Sierra, TBI 5.7 L. Passenger side header (Pacesetter Shorty) gasket has been leaking for months. Crappy fiber gasket just failing apart. Decided to replace gasket with AC Delco multi layered metal #12550033. OK. First thing I do is disconnect ground cable on battery. Then start to work. Remove the K&N air intake. Removed bolts and spark plug wire brackets. Disconnect the #4 and #2 spark plug wires to get better access. Header bolts OK and easily removed since headers were installed in frame-off restoration in 2016. Old gasket just shattered in about 30 pieces. Header can be eased away from block but to make it easier, I decide to remove the 3 old and rusted bolts holding header to Walker Y pipe, and replace with new grade 8 bolts and brass nuts and lock washers (that was 2-days of fun).

Get everything back together OK. Go out for road test and I notice the heater, vent, air display is crazy light up and nothing works. See attached photo. No fan. Vents not opening. No air. No heat control. Wondering if I inadvertently jiggled or disconnected some wires in the engine compartment. Checked for anything obvious. Jiggled wires at connectors. Popped a few connectors. Checked the under panel heater fuse (OK). NADA, NOTHING. Pulling the fuse shuts down the display.

When I reconnected the battery, did I fry something ? Anybody see anything like this? Any ideas on a fix ? Or am I heading to an auto electronics repair shop. I'll be searching forums for anything similar, but most of these responses have to do with a loose connection at the panel... either light up or not light up.

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[ima93chevyguy]

1990 GMC C1500 Sierra, TBI 5.7 L. Passenger side header (Pacesetter Shorty) gasket has been leaking for months. Crappy fiber gasket just failing apart. Decided to replace gasket with AC Delco multi layered metal #12550033. OK. First thing I do is disconnect ground cable on battery. Then start to work. Remove the K&N air intake. Removed bolts and spark plug wire brackets. Disconnect the #4 and #2 spark plug wires to get better access. Header bolts OK and easily removed since headers were installed in frame-off restoration in 2016. Old gasket just shattered in about 30 pieces. Header can be eased away from block but to make it easier, I decide to remove the 3 old and rusted bolts holding header to Walker Y pipe, and replace with new grade 8 bolts and brass nuts and lock washers (that was 2-days of fun).

Get everything back together OK. Go out for road test and I notice the heater, vent, air display is crazy light up and nothing works. See attached photo. No fan. Vents not opening. No air. No heat control. Wondering if I inadvertently jiggled or disconnected some wires in the engine compartment. Checked for anything obvious. Jiggled wires at connectors. Popped a few connectors. Checked the under panel heater fuse (OK). NADA, NOTHING. Pulling the fuse shuts down the display.

When I reconnected the battery, did I fry something ? Anybody see anything like this? Any ideas on a fix ? Or am I heading to an auto electronics repair shop. I'll be searching forums for anything similar, but most of these responses have to do with a loose connection at the panel... either light up or not light up.

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If you have access to another HVAC unit then I would swap them and see if that works, then you would know if the problem is in the unit or lies elsewhere.

It would be really bad luck if the unit fried coincidentally at that time, but is possible.

 

[DerekTheGreat]

Sounds like it died. I was going to suggest taking the fuse out & putting in back in after a few minutes to see if a reset might it help it but it looks like you already tried that. Your buttons are minty though, if you get another one, would you mind sending your old one to me?

I've recently acquired a radio out of a 90 to 92 truck and being that mine is an '89, all my buttons are black, so my HVAC buttons will stand out. However, if I had your old unit, I could make them all match by putting the face of yours onto mine or at least the buttons.

 

[David89gmc]

Unlikely you fried the controller unless you shorted something. Even then the display probably wouldn't even light up.
Carefully check the wiring loom to the controller and make sure nothing is damaged or disconnected. Also check the AC compressor wiring.
Try removing the controller and see if there's a reset button on it. Removing power doesn't reset all electronics, sometimes one needs to reset the board with power applied.
The manuals on this site should get you far enough to know whether the controller is damaged.

Good Luck!

 

[JohnB]

Thanks for the response guys. Before seeing your posts this morning, I tried a couple of things yesterday, and apparently the problem has been resolved.

I decided to pull the heat control panel unit out of the dash to check it out. Disconnected the battery. Pulled heater fuse out of its slot. Removed the dash trim bezel (unplugged wires), then unbolted the heat control. Pulled it out as far as I could, and realized I needed to remove radio to access the wire clips. OK, but didn't have time to continue. Put everything back together, installed heater fuse, reconnected battery, and guess what? Everything started working again! Even the radio controls, which I've had intermittent power on/off issues, was working just fine.

Doing some YouTube research last night, several videos mentioned how the main dash wiring harness rests on top of the heater, A/C control connection causing intermittent problems and a poor connection. Other videos speak to the poor design of connectors, harnesses, and poor grounding wires, and are a source of a variety of dash panel issues with this series of GM trucks. My pulling the control unit out probably repositioned the wiring harnesses.

One response to my post described a control unit reset by disconnecting battery, re-connect, turn key on but don't start, wait a minute or 2, then start up (thanks TXAB and DerekTheGreat).

Hopefully the harness jiggling fixed the problem. I'll monitor the issue. I intend to pull the radio at some point. Another problem I have is the defroster vent is not opening completely. I need to remove the radio to access the vent actuators which have a tendency to stick (another YouTube fix). Plus, on this truck, I have a working factory tape cassette player in the center of the dash. I'm thinking of removing radio and cassette and replace it with a single or double DIN radio/CD, but defroster vent is the next issue to address.

Oh, and I also need to replace the header gasket on the other side as well. Don't we just love working on these old trucks.

 

[AK49BWL]

The wiring connector at the back of the HVAC controller is just an extension of the circuit card, much like expansion cards in a computer (or to be exact, identical almost to the 5-1/4" floppy drive data cable connector)... The "pins" will tend to erode or corrode over time, and chances are that's what happened - when you pulled on the module you probably shifted the connector just enough to make contact properly again. If you pull the radio/amp/cassette, would you be willing to sell it? Looks like that one would have the 5-band EQ on the tape deck if I'm not mistaken.