Heater, Vents, A/C Control Panel

OceanSpray · March 29, 2019

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

txab · March 29, 2019

Disconnect battery again, then re-connect. Turn key to on, but don't start. After a minute or two see what happens. Just leave the controls alone during this period

OceanSpray · March 30, 2019

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).

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.