Heater/defroster fan question.

[Oregoon]

For some reason, the fan on my '74 doesn't turn off. When the ignition is in the on position, the fan runs on low, despite the heat being off, and the temperature selector being slid over to the cool side.

I have no a/c, if that matters.

Any ideas what might be the culprit? I pulled the fuse for the time being, since I can't imagine the fan was designed for constant service.

[rich weyand]

Actually, it was designed for constant service. They run all the time on at least Low, and have no Off position.

As a work vehicle, the gubmint figured people might basically live 8-12 hours per day, every day, in their trucks, so there is a requirement for positive-pressure ventilation to keep exhaust fumes from getting into the cab. Buddy of mine was a GM mechanic, and whenever they worked on the heater/defroster/AC, they had to check for positive cabin pressure. I think they had a gauge on a widget they put at the top of the window.

[77K10Silverado]

Same here. I've replaced the heater resistor and the ait conditioning relay and still runs..

[Oregoon]

I'll be damned. Must be a seriously robust unit...

I'm considering tapping into the power supply and wiring in a simple toggle somewhere unobtrusive.

I assume it will last a lot longer if it's not running perpetually. And if memory serves me correctly, the fan-motor is a pain to replace, right?

[77 350 Shorty Wide]

Afternoon, no the blower motor is relatively easy. If I remember three or four small bolts. self tapping I think and unplug the wiring. Reverse order to install.

God Bless, Terry

[Oregoon]

Thanks, Terry!

I must've been thinking of the heater core. I helped a buddy do one forever ago--either in his '85, or his '93--and I recall it being a serious headache.

[rich weyand]

The heater blower is very simple to replace.

If you wish to put a defeat switch on the blower:

1) Take the bezel off the dashboard.

2) Pull the fan plug from the heater control panel. If you have A/C, you will need to pull the heater/AC control panel out a couple inches. If you have no A/C, you can get the fan plug off through the opening above, where the little glove pocket in the dash bezel is.

3) Two brown wires are crimped into the same connector pin. Identify which comes from the fuse block and which goes out through the firewall to the blower motor resistor on the firewall. The one from the fusebox will be a lone wire, the one to the blower motor resistor will have the other two wires from the blower speed control traveling with it.

4) Cut the brown wire from the fusebox a couple inches back from the blower control connector. Solder a couple wires on the stubs, cover the connections with heatshrink tubing, and take those two wires to your new switch.

5) Reinstall blower control connector, heater control panel, and instrument panel bezel.

[bob_o]

Originally Posted by rich weyand

The heater blower is very simple to replace.

If you wish to put a defeat switch on the blower:

1) Take the bezel off the dashboard.

2) Pull the fan plug from the heater control panel. If you have A/C, you will need to pull the heater/AC control panel out a couple inches. If you have no A/C, you can get the fan plug off through the opening above, where the little glove pocket in the dash bezel is.

3) Two brown wires are crimped into the same connector pin. Identify which comes from the fuse block and which goes out through the firewall to the blower motor resistor on the firewall. The one from the fusebox will be a lone wire, the one to the blower motor resistor will have the other two wires from the blower speed control traveling with it.

4) Cut the brown wire from the fusebox a couple inches back from the blower control connector. Solder a couple wires on the stubs, cover the connections with heatshrink tubing, and take those two wires to your new switch.

5) Reinstall blower control connector, heater control panel, and instrument panel bezel.

Looks like I know what I'll be doing this weekend. Thanks Rich!

[rich weyand]

Originally Posted by bob_o

Looks like I know what I'll be doing this weekend. Thanks Rich!

Oh, and use #14 AWG wire for the switch connections.

You can do this mod without disconnecting the battery, as long as you leave the ignition off. The brown wire (all the heater wires) are only hot with the ignition on.

[77K10Silverado]

So let's see some ideas for a switch spot! Thx for the clinic Rich!!!

[Oregoon]

Thanks, Rich!

I appreciate the thorough explanation. Electricity and I have a fiery relationship.

[Bigstevex4]

Safety thing replace cabin air.