A/C not working on high speed fan setting

[engineer_gregh]

Worked fine for the past several years. Blows fine on low and medium settings, just quits blowing on high. Suggestions are appreciated.

[Steeveedee]

Seems like the switch spends most of its life in the high position. I vote the switch is bad. Easy enough to check.

[leddzepp]

Check your ground on the relay, located behind the glovebox as well.

[Sweeet67]

Mine just did the same thing. I replaced the relay, problem solved.

[engineer_gregh]

Thanks for the responses. I ordered a new NOS switch. I'll update the thread once I solve the issue.

[GMCPaul]

Pull the high speed fan relay from off the top of duct work behind glove box. The coil on it's probably burnt into rather than it being a fan switch problem. 2 screws to remove and then you'll know
Paulo Jr @ GMCPauls

[Steeveedee]

Does high fan speed use the relay? I thought that high speed was direct battery voltage, and didn't use a resistor.

[GMCPaul]

Originally Posted by Steeveedee

Does high fan speed use the relay? I thought that high speed was direct battery voltage, and didn't use a resistor.

The A/C trucks use both a resistor and a high speed fan relay. The non-A/C trucks use a resistor only. I did mistakenly describe checking the resistor in above post rather than the relay. The high speed fan relay is a silver relay mounted on passenger side of air duct behind glove box on side of it near the top rear of duct. Because it's attached to the plastic duct it has it's own ground wire under 1 screw check it to make sure it's unbroken, if it's connected to ground then use a meter to check the relay for correct functioning I've seen test procedures on google before that describe this in detail. We've found it's rare for the switches to fail and far more common for the high speed fan on the A/C equipped models to fail.
Paul Jr @ GMCPauls

[Steeveedee]

OK, I see. My switch was stuck in the "off" position, so the choice of what to replace was obvious. Good to know about that relay. Thanks for the explanation!

[Hotcorner]

I have same problem. Best way to access the relay?

[Sweeet67]

If you can contort your arm, and reach screws on the top, you don’t have to remove the glove box. Otherwise, just save yourself the frustrations and pull the box liner and then you will have easy access to the relay. It’s right on the far right side from the seated position.
Remember to clean the ground wire that will be under one of the screws.

[engineer_gregh]

Replaced the switch and solved the problem. The switch was burnt at the M connector but ohm's checked bad across all terminals. It burned the plug but it was still useable. Looked to be the original switch. Replaced with a new GM part ordered off eBay. Here is the link to the new switch.

https://www.ebay.com/itm/GM-1965-66-...53.m2749.l2649

[davischevy]

As Paul said, In the last two trucks I had high speed issues with, it was the relay. In one truck, somebody took the relay out and never put it back and the other was poor ground.

I just now worked on one that the switch would not go on high speed. Somebody had put a screw with a head in the bottom hole, and the switch could no too to the high position.

[Andy4639]

I have fixed I don't know how many fan switches that either quit working are they fall a part. Not a lot to them once you take it a part and know how the parts go back together. Clean the copper contacts inside good and put some electrical grease in it and it works like new again are at least the ones I've done have.

[Jim Boy]

Ok. before I start drinking again I hope someone can figure my problem out. My fan never worked on high but i'm not a real fan of AC and our cabs are small so never really needed it. With Covid 19 and self isolation a man has to do something so I thought that I would get into solving the problem. Fan works on low and medium, nothing on high. I did the 12 volt test at the relay and have power there. Here is the problem, when I remove the ground wire from the relay, the fan motor works on high. Now low speed is low and medium seems to be medium but when I switch to high the motor comes on but the rpms drop a bit from the medium position. I've read many articles about making sure the ground is in place properly but none ever say take the ground off. Hope someone has the smarts to figure this out for me. Dam Covid19 anyway. Solve my problem so we can both have a good day. Stay safe eh. Ron

[mrein3]

Originally Posted by engineer_gregh

Replaced the switch and solved the problem. The switch was burnt at the M connector but ohm's checked bad across all terminals. It burned the plug but it was still useable. Looked to be the original switch. Replaced with a new GM part ordered off eBay. Here is the link to the new switch.

https://www.ebay.com/itm/GM-1965-66-...53.m2749.l2649

FWIW, I now stock that plastic connector. When I say stock I mean I keep one handy because that connector melts way too easily. I suspect that the heat in the resistor along with the perhaps one size too small wire builds up heat and melts at the switch. I also keep a new switch handy.

[tdangle]

I just fixed the same problem with mine. Worked on the high speed, but I smelled something getting real warm. took the switch apart clean and greased it. Cleaned the connectors and pulled the plug apart and cleaned everything. Works great now, just the plastic at one of the connections is a little warped from being overheated.