heater fan - always on?

[SirRobyn0]

@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]

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]

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]

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]

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]

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]

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.