heater fan - always on?

SirRobyn0 · Jan 12, 2022

Turbo4whl said:
@SirRobyn0

To repeat what has already been stated, early squares without AC, the fan always runs. Not many differences between the 73's and 74's. My 1974 Jimmy I bought new in 1974, the blower was on with the key. No off for the fan.

I'm sorry somehow or another I missed that or it didn't sink in properly. I have not seen a 73 or 74 recently so that would likely be why I don't remember that. Thanks for correcting me.

Ellie Niner · Jan 12, 2022

Yep. My non A/C 1975 GMC operates like this. Sliding the HEAT/DEF lever to OFF just redirects airflow to a port in the heater box that blows it upward under the dashboard; blower keeps running at whatever speed you have it set at.

I always thought this was a carryover from the "Astro Ventilation" days, which was what GM dubbed their flow-thru ventilation system after they took the wing windows away on cars circa-1968.

Dejure · Mar 4, 2025

byproxy said:
heater on my 73 works great. have both high and low speeds and heat and defrost.

problem is that the fan always runs. the "off" position doesn't actually turn the fan all the way off.

i'm assuming this isn't normal? presumably i'd need a new switch? just checking before i dig into the control unit. the guy i bought it from said it's always been that way and was assuming the fan just stayed on all the time in these trucks.

Sadly, photos of it are long gone. That aside, I solved the problem of a dead switch by enlarging the rectangular hole and installing a three position rocker (12 volt 3 position rocker - center was off, up high and down low.

When done, I had a rocker that lighted when power was applied to the fan (low or high). In the up position, it went to high. In the down position, it went to low. The middle position, the fan and light was off. Genuinely off.

Because the frame is pot metal, filing the shape to accommodate the swap to a non-stock, easily swapped switch, went, relatively, fast.

I got rid of ribs sticking up on both sides of where the original switch came through, then enlarged the hole to center the switch, after I knew it would mount flush.

Getting rid of the constant on mode added decades to the life of the switch.

It looks like I'll be doing the very same thing with my current rig. The fan won't come on, and a switch is one of the first places an old electronics hack would look (it did all the initial heater work for the last 47 years).

Novapwr · Thursday at 6:44 AM

My 78's blower motor always runs. When levers are both to the left it still runs on low. In warm weather I just unplug the resistor on the firewall. It will only work on high then. I did see someone mention the heater shutoff valve they installed. I'd like to see that. Was there one on the heater hose from the factory? In summer it gets too hot due to water still circulating through the heater core.

75gmck25 · Thursday at 7:18 AM

There was a heater shutoff valve on some vehicles, but I’ve never figured out which ones.

I found a two port vacuum-operated heater bypass valve several years ago. With vacuum applied, the heater water flow went from the engine to the valve, made a u-turn and went right back to the radiator, without going to the heater core.

It worked fine to change the water flow, but the air flow still seemed to pick up some heat, so it was never really cool air unless the A/C compressor was on. I ended up connecting it to activate the bypass valve using the vacuum line for recirculated air, so it bypassed the water only if the system was in Max A/C mode. It worked, but I took it off later during one of my many engine modifications.YMMV

Ricko1966 · Thursday at 7:48 AM

Novapwr said:
My 78's blower motor always runs. When levers are both to the left it still runs on low. In warm weather I just unplug the resistor on the firewall. It will only work on high then. I did see someone mention the heater shutoff valve they installed. I'd like to see that. Was there one on the heater hose from the factory? In summer it gets too hot due to water still circulating through the heater core.

I wouldn't feel comfortable shutting off water flow through the heater core,it acts as a bypass and does not bypass the same on all vehicles or even all SBC some vehicles blocking it creates hot spots,and other issues. As @75gmck25 said he used, many later model SBC vehicles used a bypass valve,not a shut off valve so the water still circulated through its normal path,creating a bypass,but didn't flow through the heater core. A little research will find a valve pretty easy.

ali_c20 · Thursday at 9:01 AM

I run this in my K5 and will put one in my C20.
https://www.oldairproducts.com/product/25-1018-bypass-heater-valve?srsltid=AfmBOoqHIF7WgXedrJNAlOwguFikFcyPcjRaRaWSSX-Y3mUbSZdQBMvb
It's available in push or pull cable to open. Cable is not included, but they are cheap . Engine warms up faster with valve closed. If you're running in a hot coolant situation you can add a shot off low temp coolant with opening the valve.
Works perfect since 4 years.

Ricko1966 · Thursday at 10:39 AM

Looks like 1999 escalade would do the same,with nothing but a vacuum hose for switching

bucket · Thursday at 11:21 AM

Ricko1966 said:
Looks like 1999 escalade would do the same,with nothing but a vacuum hose for switching

I've got one of those that I'll install on my Suburban. One day, lol.

Novapwr · Friday at 6:34 AM

I'll get one of these valves and put it in. I have the 250 motor. Did they come with one of these valves from the factory? Just curious.

AuroraGirl · Friday at 7:32 AM

Ricko1966 said:
Looks like 1999 escalade would do the same,with nothing but a vacuum hose for switching

On a manual heat (so no AC) youd have to either rig up a vacuum switch or go underhood and have a vacuum line off your engine either capped or hooked up

for a non AC vehicle, using a ac vacuum ball might be wise so then you can keep the heat out of the heatercore even when its accelerating with large throttle

Im not sure how much temp this would lower the cabin, but id imagine it would be considerable when moving slowly , less radiant heat from the dash

AuroraGirl · Friday at 7:40 AM

Dejure said:
Sadly, photos of it are long gone. That aside, I solved the problem of a dead switch by enlarging the rectangular hole and installing a three position rocker (12 volt 3 position rocker - center was off, up high and down low.

When done, I had a rocker that lighted when power was applied to the fan (low or high). In the up position, it went to high. In the down position, it went to low. The middle position, the fan and light was off. Genuinely off.

Because the frame is pot metal, filing the shape to accommodate the swap to a non-stock, easily swapped switch, went, relatively, fast.

I got rid of ribs sticking up on both sides of where the original switch came through, then enlarged the hole to center the switch, after I knew it would mount flush.

Getting rid of the constant on mode added decades to the life of the switch.

It looks like I'll be doing the very same thing with my current rig. The fan won't come on, and a switch is one of the first places an old electronics hack would look (it did all the initial heater work for the last 47 years).

My control panel is shot because the first time i attempted to move one of the levers it broke
and i didnt realize the fan was supposed to run, but I dont want it to either way

You must be registered for see images attach

1980 no AC switch, I just went inline that brown wire and it turns it on and off with that red switch
I just used the unhooked connector and a multimeter, found whatever went hot with key on but not key off

Dejure · Friday at 1:41 PM

Groan.

Fortunately, if you own a drill, some bits and a saber saw/jig saw, you could make your own plate to replace the damaged one.

A good sabersaw (bayonet blade types) will cut nice, straight lines, with practice, for, say, a square rocker switch hole in a piece of 1/8" aluminum.

Since the controls beside the switch are all cable controls, you could even convert to push pull, akin to what you might see on a big truck (e.g., Kenworth, etc.).

The latter way, when done, all you have to have done was, drill and/or cut a hole for the three position switch, like a toggle, and a couple for the duct (?) controls

When I'm done playing with my truck, the interior won't resemble stock, so a custom plate will be a non-issue anyway.

Inasmuch as I haven't gone down the road of altering heater controls for around fifty years, I don't know what hoops I'd have to jump through to secure the plate, but, where there is no solution, there can't be problem. Right?

AuroraGirl · Friday at 1:43 PM

Dejure said:
Groan.

Fortunately, if you own a drill, some bits and a saber saw/jig saw, you could make your own plate to replace the damaged one.

A good sabersaw (bayonet blade types) will cut nice, straight lines, with practice, for, say, a square rocker switch hole in a piece of 1/8" aluminum.

Since the controls beside the switch are all cable controls, you could even convert to push pull, akin to what you might see on a big truck (e.g., Kenworth, etc.).

The latter way, when done, all you have to have done was, drill and/or cut a hole for the three position switch, like a toggle, and a couple for the duct (?) controls

When I'm done playing with my truck, the interior won't resemble stock, so a custom plate will be a non-issue anyway.

Inasmuch as I haven't gone down the road of altering heater controls for around fifty years, I don't know what hoops I'd have to jump through to secure the plate, but, where there is no solution, there can't be problem. Right?

i plan to put a new control in, the cables were slightly seized but the plastic on the faceplate gave away first. I now can adjust the heater temp and the other slider by playing with the cable , kinda like a slide whistle lol

heater fan - always on?

SirRobyn0

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Ellie Niner

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Dejure

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Novapwr

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75gmck25

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