AC & Heater Switch Overheats/burns up/??RELAY SETUP

[stringpickin]

Hey and Good day!

I dont't know about you, but it seems like I have to replace my AC/Heater Switch about ever 2 years. It dies the same way every time. It gets hot, I smell that favorite electrical stuff is burning smell, and zap the fan quits working. I learned many years ago, don't reach under the dash and grab the Plastic Wire Plug that plugs in to the AC switch right as this happens. It burns like 60.. I once could see smoke coming from there and new what was happening and thought might start fire, so I tried to yank it off. Don't Do that unless you absolutely have too..

What is strange is it never burns the fuse out. The switch goes first.. And also it does actually destroy the actual contacts of the switch because it gets sloppy when you flip it up and down afterwards..

So....Anyway.... Anyone else have that problem? I have heard a couple of people tell me they have that I have talked to here and there on the street.

That leads me to the question. Has anyone here come up with a slick and easy way to wire relays in to the system so that the juice of the Fan Motor and the Compressor Clutch so that the AC switch is only used to throw the relay and the juice runs through the relay for the two???

I am sure I can figure it out eventually, but I was hoping maybe someone here could help me out, save me a bit of time and give me some good advice on what they found as they accomplished it.

Or maybe there is another fix that is out there because something else is causing this problem that I for the life of me cannot seem to figure out?

In either case, I would appreciatte your help, advice, and time..

Thank you very much. If you want to post here, that is fine, but if you want to email me at stringpickin@sbcglobal.net that is fine too..

Moderators. Please bare with me as I post this same topic to the Blazer section, and any other section that includes this same AC system so that I can possibly hit someone that can help. Thank you.. String..

[cdowns]

most people with electrical problems can solve alot of them by simpely cleaning thier grounds, dirty grouns cause high resistance==heat+fire=melted wires
and you could also lubricate the bearins in the fan motor for a better spinning blower that will give you less resistance and less electrical load==less heat

[Longhorn Man]

The trucks with A/C already have a relay in the wiring.

[stringpickin]

Thank you guys for the info.. Yes, agreed bad grounds can cause tons of trouble.. That is already been checked and rechecked.

In fact I put a complete new wiring harness in it last summer hoping to solve this problem and a few others..

But, I did not see a relay anywhere in the entire system for the Blower or the AC compressor.

The only thing I am aware of is the switch that is located on the back of the ducting that is a big push button switch that activates the Compressor, but it is merely a switch that ties in to the system that allows the AC switch to be used soley for the blower when using the heater.

If you know of some relay, do you have a part number or do you know where the location of it is located. Thank you much for your help..

String

[john]

Heres a diagram of the four season A/C John

[stringpickin]

THANKYOU JOHN.. You know I have never owned the wiring diagram for my truck. I always thought I knew it pretty well. So now I see in the diagram a relay for the blower motor.. Makes sense. I am wondering why I didn't notice it when I put the new harness in.. hmmm.. John, can you tell me where the relay is generally mounted? approx. location under the dash I would definatley assume.. Thanks again.. String..

[GMC Jim]

On my '71 GMC; the relay is mounted on the firewall; just at the lower right corner of the glove box. Am not sure the design amp rating for the blower motor switch is ( have never had to replace one!); but the amp motor does draw 16.5 amp per the GM Chassis Service manual. The air conditioning is protected by a 25 amp fuse.

[stringpickin]

Thank you Jim for your response. I can only assume this baby has been missing one for the last 18 years. I build this truck up from 84 to 88, and used a cab off of another truck so as to have the stock AC.. I bet you the body guys removed that thing when the stripped the cab out, complete color change on the paint.. Complete ground up restoration. Well as complete as I could afford.. lol.. Well, pretty simple now, just get the right parts and wire them in. Thanks again for the help..

[mrein3]

I converted my non-AC truck to an AC truck.

The heater controls I bought used had a burned up switch like you have seen.

I ordered a new switch and the connector for the back from LMC. When I went to wire it up my fan went low in the low position, medium in the medium position, then low in the high speed position.

After tracing wires and looking at the wiring diagram I noticed I was missing a high speed fan relay.

I went to auto zone and purchased the same relay I use for my headlights. I wired that in and mounted it out of sight on the inside part of the heater/ac box.

I wonder if lots of amps are running through the switch when the fan is on high speed. Similar to the headlight switch without the relay upgrade.

I'm hoping that by running the energized side of the relay straight off the battery (through a 20 amp fuse) I won't burn up the switch. And if I ever lose both headlight relays I have one spare on-board. I'd only be missing high speed fan until I got to the auto parts store.