'92 C2500 HVAC Controls Gone

[DiGiTaL D]

I searched, but couldn't find an answer.

On Saturday, I started my truck and the HVAC controls came on like normal. I turned on the defrost and a second or so later they quit as if the fuse blew. I checked the fuse for the heater/AC (25 amp) and it was fine.

I took a short drive without heat or defrost and got a bite to eat. After breakfast, we got back in the truck and the HVAC controls came back on and stayed on, worked like normal. It was fine for the rest of the day.

Didn't drive the truck at all on Sunday.

Yesterday morning I drove it to work. HVAC controls worked fine the whole time. When I came out after work and started it up, the controls were gone again. Drove the whole way home without any heat and it sucks.

About a year ago, maybe more, I swapped the control box with a junkyard unit to replace a broken button on the original. The original one worked when I took it out and the junkyard unit has been working fine until Saturday. So I figured I got a spare, I'll just put that in and get my heat back.

No joy. I've tried both the junkyard unit and the original main board and both the junkyard control head and the original control head (they come apart into two pieces). Nothing works.

All fuses are still good. I have literally done nothing to the dash or wiring harness any time recently that would cause any of this. I did a complete dash swap over a year ago and also replaced the blower motor and the resistor pack since, but since doing those jobs everything worked perfectly. Now it just up and quit.

The one thing I noticed since this started happening is this: when the HVAC controls are non-functional and the headlight switch is OFF, the button backlights on the HVAC control head will be ON. Turning the headlight switch ON shuts the button backlights on the HVAC controls OFF. Its reversed which made me think it was the electronics on the control board that were bad. Apparently that is not the case.

Is there another control board somewhere? I have a Chilton manual but it only covers the manual controls for the C/K series, not the electronic controls so I don't even have a wiring diagram to work from.

Its cold and I need heat. Any ideas?

Thanks.

 

[someotherguy]

There's a power connection that I can't tell you exactly where to find it anymore (too many years past) but you'll find it in the harness heading towards the blower motor/ECM area, it'll have a connector all on its own, want to say 2 wires, red and black. This one goes bad every now and then on the older interior trucks (88-94) and you'll lose power to the HVAC controls.

Tried to find some kind of decent reference pic but this is the best I could come up with and it's kinda crummy, sorry.

You must be registered for see images attach

Richard

 

[DiGiTaL D]

Thanks for the post. I'll check it out tomorrow and advise. I'll probably have to take the glove box out to get some better access.

I don't have a factory service manual but I did pick up a Haynes manual today which has a wiring diagram that includes the electronic HVAC controls. I see it gets power from the "battery junction box". What is considered the battery junction box? The one up under the driver side footwell on the firewall, or the one on the firewall inside the engine compartment on the passenger side?

 

[nogalus]

Behind the ash tray is the connector for the control head.

If you remove the ash tray and wiggle, twist, and push on that connector
I would wager that you will see your
control blinking on and off.

So it was for me. I tightened up the
connectors with a couple of wire ties.
No problems for about six months.

Hard wiring would be best, I suppose!


Good luck, amigo!

 

[Dropped88]

the main power feed where there is always a problem is directly behind the glove box i believe its a 3 wire connection red,green,grey with the red wire being a bigger gauge and the terminal usually melts into the plastic making a bad connection and a pain to unplug

 

[someotherguy]

the main power feed where there is always a problem is directly behind the glove box i believe its a 3 wire connection red,green,grey with the red wire being a bigger gauge and the terminal usually melts into the plastic making a bad connection and a pain to unplug

That's the connector I'm talking about...sounds like your recollection of it is more recent, and accurate, than mine.

Richard

 

[Dropped88]

That's the connector I'm talking about...sounds like your recollection of it is more recent, and accurate, than mine.

Richard

Yes I got very familiar with all the dash wires when I pulled all the tbi stuff out of the wiring

 

[DiGiTaL D]

Behind the ash tray is the connector for the control head.

If you remove the ash tray and wiggle, twist, and push on that connector
I would wager that you will see your
control blinking on and off.

So it was for me. I tightened up the
connectors with a couple of wire ties.
No problems for about six months.

Hard wiring would be best, I suppose!


Good luck, amigo!

Thanks man, you were spot-on. I just pulled the ashtray out, played with the connector, and bingo, got my controls back. Would have had this fixed sooner but I've had the flu (that awesome 10% effective flu shot did not work for me) and have been out of commission the last couple days.

Thanks also for the information concerning the connector behind the glove box. I'll pull the glove box and check that while I have the truck in the garage.

Everybody have a happy new years!

 

[DiGiTaL D]

Yeah this might be a problem...

You must be registered for see images attach

You must be registered for see images attach

Since I don't really see the point to this connector even being there, I just deleted it. Spliced the wires together and soldered them up. Don't think this one will be a problem again (this is the one behind the ashtray). I'm probably gonna do the same on the other one behind the glove box when I get time.