A/C blower always running when key on

Good evening all, and happy fathers day! While trying to find a switched 12v source, I noticed that my blower runs the entire time I have the key turned on (2nd notch, just before it cranks). Anybody know what could cause that? 1977 GMC K15 with factory air. I looked at all the connections and grounds and they all seem good, but I didn't really know what I was looking for.

That is how it works from the factory. The blower is always on, doesn't matter if you switch to the "off" position. It's "a fresh air" system.

Yep.

Truck feature.
New to square bodies r ya? What made you switch?

TELL THE STORY

OK, good to know. I'm new, but I'm not. My dad had an '83 Sierra Classic he bought just before I was born. Light blue/white 2 tone. Beautiful truck. I Grew up riding in it, learned to drive in it. Many happy memories in that truck. But he sold it back in 2003. Finally found a nice one for a steal of a deal, so I bought it. I don't recall the blower being on all the time, but then again I was never really poking around under the hood with the key on back then either lol.

It was called "Astro Ventilation" and it's designed to keep stale air moving out of the cab --- either through the vents in the edges of the doors where those strange "grills" are with the rubber flaps inside them --- or later versions have an exhaust system built into the back of the cab.

On a few of the late '70s C/P-30s and K/P-40s I worked on, those with manual heat and no AC had a passive Astro Vent, and those with air conditioning (the Frigidaire kind from the factory, not the dealer add-on) had active systems that ran all the time the key was in the RUN position.

You could immediately tell if the system was passive if the word "VENT" was on the heater control panel.

Diesels had a different approach in that there was a metal sensor - usually on one head, that measured the block temperature --- not the coolant.

This was to allow the cooling-side of the HVAC to not run the blower until the head got to a minimum temperature so you could always modulate the temperature at least to keep from freezing yourself in the cab until there was enough heat to warm things up.

GM played with a lot of nice creature-comfort systems in those days.

Silver Square said:

OK, good to know. I'm new, but I'm not. My dad had an '83 Sierra Classic he bought just before I was born. Light blue/white 2 tone. Beautiful truck. I Grew up riding in it, learned to drive in it. Many happy memories in that truck. But he sold it back in 2003. Finally found a nice one for a steal of a deal, so I bought it. I don't recall the blower being on all the time, but then again I was never really poking around under the hood with the key on back then either lol.

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It was only 70's trucks. In the 80's the off on the switch would actually turn the fan off. I can't remember if it was an a/c only thing though.

PrairieDrifter said:

It was only 70's trucks. In the 80's the off on the switch would actually turn the fan off. I can't remember if it was an a/c only thing though.

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I think they all had them. My ‘80 K25 is a base model truck with no A/C and the blower always runs. This was my grandpa’s truck he bought in ‘88 and he hated that bc it would blow hot air around in the summer so he installed a toggle switch in line with the blower motor power wire so he could fully shut the blower off.

dsteelejr said:

I think they all had them. My ‘80 K25 is a base model truck with no A/C and the blower always runs. This was my grandpa’s truck he bought in ‘88 and he hated that bc it would blow hot air around in the summer so he installed a toggle switch in line with the blower motor power wire so he could fully shut the blower off.

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For all intents and purposes, 80 is the last of the 70's models. 78-80 is the model year divisions for most trim options and wiring. 81-up is the new body style front end and major electrical/systems changes. My 80 K25 is a Sierra Classic w/AC and the blower always runs also. I'll probably modify that a bit with all the other changes happening with the motor swap by moving the power feed to a relay in the new under hood fuse box. AC compressor activation will already be switched to being run by the computer anyways, so might as well fix that too. It's also nice to get a heater core bypass valve installed in the heater lines to help keep extra heat out of the cab in thew summer. I was planning on using one of the AC Delco valves that are used on GMT-400 SUV's and using a T into the heater activation vacuum lines to activate it.
https://www.summitracing.com/parts/...MI9rfvhr_qhgMV5CytBh33sQtkEAQYAyABEgIcsfD_BwE

Just to add: my ‘80 is an a/c truck and it has an always on fan.

Trucksareforwork said:

Just to add: my ‘80 is an a/c truck and it has an always on fan.

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An 80 is supposed to. An 81 will not.

PrairieDrifter said:

An 80 is supposed to. An 81 will not.

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Hmm... so my 87 shouldn't be doing that? (Which it does.)

RaysnCayne said:

Hmm... so my 87 shouldn't be doing that? (Which it does.)

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They may have switched in 87. I'm not well versed in 87-91. 87 was the big changeover year.