To have heat, would be nice.......

[thedudeabides86]

Good evening, gearheads!

I’ve got a ‘72 C20 with no heat. It does not have factory air. I’ve had this truck for 25 years and never has the heat worked. Where would you start to figure out the problem? I just replaced the slide switch assembly and when I had it started a couple days ago I played with the switches and had no fan or air pushed out so the blower motor might be bad? Or maybe fuses? I’m still learning the way to fix things on this that I’ve never had to fix before. Any suggestions would be appreciated. Thanks everybody!!

[Astro-Balls]

Does the heater core leak? are the hoses even hooked up to it? or is it bypassed?
Start with the heater core, make sure its in good shape.

[68 P.O.S.]

Do you recall anything from the PO about the heat when you bought it? My limited experience with these trucks is the heater core is usually bad. As you said, it could be a fuse, bad blower motor. Deteriorated vent lines. How's the electrical looking down there?

[Getter-Done]

Pictures would help.

I think the first thing I would do is Pull the fan motor and bench test it.

When you pull the fan out look for Critter Nest also







.

[gmc684x4]

Are the heater hoses swapped ?
I had a service van years ago went to the shop many times still no heat i got stuck with it on a long cold road trip reveresed the heater lines better flowing heat

That said blower motor needs to work heater door needs to be open vent tubes needs to be in place
How much leaves and trash has built up in the cowl area ?

[pjmoreland]

There should be a ground wire attached to the blower motor housing on one end and to one of the heater box screws on the other end. Assuming the ground wire is good, you could unplug the power wire from the blower motor and connect a test wire directly from the battery + terminal to the blower motor power terminal to see if the motor will spin. Don't leave this wire connected if you don't hear the motor start to spin, as there could be a short inside the motor, and this test wire is unfused.

The blower fan could be totally clogged with leaves.

[BigBird05]

Start with the basics. If the heater hoses are connected, do they both get hot? If not you have a coolant flow problem.

[BRL]

Originally Posted by thedudeabides86

Good evening, gearheads!

I’ve got a ‘72 C20 with no heat. It does not have factory air. I’ve had this truck for 25 years and never has the heat worked...

You sound like me, (that's a complement) and some of the little projects I've put off on my truck forever.

But even if the fan itself isn't running, shouldn't some heat still be coming through the vents at cruising speed if the switch is slid to the hot position and everything else but the fan is working properly?

[LockDoc]

Originally Posted by BRL

You sound like me, (that's a complement) and some of the little projects I've put off on my truck forever.

But even if the fan itself isn't running, shouldn't some heat still be coming through the vents at cruising speed if the switch is slid to the hot position and everything else but the fan is working properly?

Not if you have something like this in the heater box.... It's a common occurrence.

LockDoc

[BRL]

Originally Posted by LockDoc

Not if you have something like this in the heater box.... It's a common occurrence.

LockDoc

Yea, once I opened up a kick panel vent while going 50 and my cab became an instant movie theater pop corn machine with little cotton ball sized fluffy stuff flying everywhere!

Dang vermin!

[thedudeabides86]

Originally Posted by LockDoc

Not if you have something like this in the heater box.... It's a common occurrence.

LockDoc

I'm guessing the silver piece in the picture is the heater core? I've never torn into this myself so I'm in unchartered territory. I'll post the pics of where I'm at in the thread.

[thedudeabides86]

I remember back in high school 23 yrs ago, a coworker wired up a variable speed switch to blow heat but that's been removed. I noticed an orange wire with a boot on the firewall that wasn't plugged in. I tried that. but it didn't make anything work. Here's pics of where I'm at. You'll see the orange wire that's not plugged in just off the center of the photo.

[LockDoc]

Originally Posted by thedudeabides86

I'm guessing the silver piece in the picture is the heater core? I've never torn into this myself so I'm in unchartered territory. I'll post the pics of where I'm at in the thread.

Yes, but that is an A/C truck, heater only is easier. The orange wire is the power for the heater. Turn the heater control for the fan all the way on and see if there is 12 volts to that wire.

LockDoc

[LockDoc]

-
Looking at your under dash picture again it looks like your connector for the blower circuit is not plugged in to the switch on the right side of the heater control panel. In the first picture below you can see where the connector is supposed to be plugged in. the other end of that harness plugs into the blower motor resistor located on the top side of the heater box.

LockDoc

[franken]

Jump across the fan switch contacts before tearing things apart...

[HotWheelsFan]

That fuse that is listed at 25A for the A/C is for heating - A/C system:

Use a 25 amp for A/C vehicles or use 10 amp for non A/C vehicles...

It powers the blower motor and A/C compressor if equipped.

[Happy_dan]

I guess I would be wondering why the fuse blew in the first place?

[LockDoc]

Originally Posted by Happy_dan

I guess I would be wondering why the fuse blew in the first place?

The way I read it is there wasn't even a fuse in that spot. Not sure....

LockDoc

[thedudeabides86]

Originally Posted by LockDoc

The way I read it is there wasn't even a fuse in that spot. Not sure....

LockDoc

Correct. I didn’t have a fuse in there at all since I didn’t have a/c. It didn’t dawn on me that that slot would power the blower motor. So, since I don’t have a/c, I should drop a 10amp fuse in that spot, not a 25amp?

[Happy_dan]

Great to hear it's resolved! Love it when things get fixed? Wonder why there was no fuse in there. A mystery.

[thedudeabides86]

Originally Posted by Happy_dan

Great to hear it's resolved! Love it when things get fixed? Wonder why there was no fuse in there. A mystery.

Yeah, not sure why there wasn't a fuse or why over the years we never thought to put one in. I'm glad it was a major overhaul to get it fixed. Now that snow has settled in, I won't be driving it much farther than the end of the driveway.