87 3/4 ton heater fan

[Superduty09350]

Hey all,

I just bought a 87 3/4 suburban and it has some little things wrong with it and the one thing I'm not educated on is the heater sys. The heat/ac fan is always on high and it wont switch to any other position then blowing on the floor. It will switch from hot to cold. Also the fan stays on when the key is off. Any ideas?

Here is a PIC of it cuz I know you guys like PIX.

 

[dyeager535]

If it stays on with the engine off, take a look under the dash and in the engine bay, see if someone has screwed with the wiring. If they have, you'll need to fix that and see if the problem goes away.

I can't think of how the fan could run with the key off, it's ignition switched, somehow power would have to be getting to the fan motor with the engine off. Maybe the relay is bad on the heater box. Look for melted connectors/wires if you don't see evidence of someone monkeying with the wiring. GM wasn't stupid, they didn't use anything like scotch-locks or butt connectors. If you see any, you know someone has been messing with it.

 

[campfire]

Hey all,

I just bought a 87 3/4 suburban and it has some little things wrong with it and the one thing I'm not educated on is the heater sys. The heat/ac fan is always on high and it wont switch to any other position then blowing on the floor. It will switch from hot to cold. Also the fan stays on when the key is off. Any ideas?

Here is a PIC of it cuz I know you guys like PIX.

You have 3 different systems at work here, IIRC. The fan speed is controlled by an electrical switch (look for gremlins there). The position switch operates a series of vacuum lines. If you don't have functional vacuum pressure, you can't change which ports the air comes out of (even though you can still move the switch). The temperature is controlled by a cable.

It looks like you have an electrical issue and a lack of vacuum pressure. Follow the vacuum line through the firewall and see if there are any cracks or loose connectors. Post pictures of what it looks like, and folks should be able to tell ya if it looks funny.

 

[Superduty09350]

Thanks for the info. Sounds like a good place to start.

 

[Rmy660r]

I just went through the same trouble with air only blowing out at floor. The main culprit ended up being the Hvac vacuum check valve. It's on the intake near the firewall. Easy to get at with air cleaner off. I bought a generic one at O'reillys. Works fine now