Heater speed issue - The 1947 - Present Chevrolet & GMC Truck Message Board Network

Dan-K · 01-08-2026, 02:42 PM

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

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

Quote:

Originally Posted by Dan-K

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

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

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

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

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

01-08-2026, 02:42 PM	#1
Dan-K Registered User Join Date: Aug 2009 Location: Ft.Collins, Co. Posts: 110	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 ?

01-08-2026, 05:59 PM	#2
MARKDTN Registered User Join Date: Jan 2011 Location: Chattanooga, TN Posts: 2,260	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. __________________ '83 K20-TPI '73 C10 '79 C10-ex-diesel(SOLD) '07 Tahoe(Son driving) '14 Suburban-DD '71 C10-current project

01-09-2026, 01:47 AM	#4
Sheepdip Senior Member Join Date: Nov 2018 Location: Waterford California Posts: 2,893	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.

01-09-2026, 03:03 AM	#5
Richard Senior Member Join Date: Dec 1999 Location: Southern California Posts: 3,346	Re: Heater speed issue 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. Attached Images __________________ Richard 1972 K10 Custom Deluxe SWB Fleetside My build https://67-72chevytrucks.com/vboard/...d.php?t=800746 Last edited by Richard; 01-09-2026 at 03:28 AM.

01-09-2026, 02:14 PM	#6
mrein3 Registered User Join Date: Feb 2000 Location: Center City, MN, USA Posts: 3,266	Re: Heater speed issue 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. Attached Images __________________ '70 cab, '71 chassis, 383, TH350, NP205. '71 Malibu convertible '72 Malibu hard top Center City, MN

01-10-2026, 01:00 PM	#7
Dan-K Registered User Join Date: Aug 2009 Location: Ft.Collins, Co. Posts: 110	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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