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- - Heater speed issue (https://67-72chevytrucks.com/vboard/showthread.php?t=862317)

Dan-K

01-08-2026 02:42 PM

Heater speed issue

I replaced the fan speed switch on my heater control (non a/c) and the resistor on top of the heater box but the fan only works on Full-blast , what else could it be ?

MARKDTN

01-08-2026 05:59 PM

Re: Heater speed issue

Can't speak to non A/C, but on an A/C truck it's the blower relay is bad. I can't find a good diagram to see if they have one or not and where it is.

LockDoc

01-08-2026 10:03 PM

Re: Heater speed issue

Quote:

Originally Posted by Dan-K (Post 9417755)

I replaced the fan speed switch on my heater control (non a/c) and the resistor on top of the heater box but the fan only works on Full-blast , what else could it be ?

High speed bypasses the resistor. I would check the terminals on the switch with a multimeter and see if you have power in all of the different switch positions. If you do then it would have to be the wiring to or from the resistor or the resistor itself.

Sheepdip

01-09-2026 01:47 AM

Re: Heater speed issue

https://www.cjponyparts.com/old-air-...ng%20(Tier%201)

They were all pretty much the same in that era but of course you have to buy the one that fits. I believe 68-72 just about any GM product used this one. And of course it's on the back side of your heater duct box.

Richard

01-09-2026 03:03 AM

Re: Heater speed issue

1 Attachment(s)

If I recall correctly a heat only system does not use a relay for high speed. Both of my 72's were originally heat only and did not have the relay. The relay and an additional harness are used on the A/C trucks. Note the sale of replacement A/C harness and A/C blower relay. Possibly different blower motors or fans were used on the two configurations? The original A/C pieces I obtained have the relay.
Back at OP question. Possibly a wiring issue? Maybe a faulty new part. Unfortunately that is something that has become to common.

Edit- I took a screenshot of the wiring for the blower and added it. This is in the electrical section of this forum. This is heat only, no relay indicated in circuit. I did not include the motor side, orange wire just travels to blower motor.

mrein3

01-09-2026 02:14 PM

Re: Heater speed issue

1 Attachment(s)

When I converted my heater only truck to factory AC, the used AC wiring behind the fan switch and in the connector to that resistor was all melted. I bought new plastic connectors and made new wires.

I copied the image in the link Sheepdip posted. All that part does is turn 12 volts into heat and 3 and 6 volts or some other lower voltage. That is how it reduces the speed of the fan.

That heat is tough on undersized wiring after 50 years. Take a good look at all those wires and plan on replacing them.

Dan-K

01-10-2026 01:00 PM

Re: Heater speed issue

I think I will check the wire connection , it looked a bit suspect when I had it all apart . Thanks for the Ideas / replies .

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