Heater/AC Blower Problems

[landy]

'72 K5 with factory A/C. The fan switch was broken when I bought the truck, so replaced the switch with a new one from LMC. The blower motor now worked. So onto the A/C. Installed a belt, and the compressor would engage, but the blower motor no longer worked. Well, the system is likely out of refrigerant, so understandable why it doesn't blow cold.

But now the blower motor no longer works at all. There's 12 volts at the brown wire where it comes into the switch. The fan switch has three positions. There's 12 v on the blue wire into the switch at positions 2 and 3, and 0 v in position 1 (the upper position). There's 0 v on the orange wire at all positions. There's 12 v on the yellow wire at positions 2 and 3.

At the wire to the blower motor (in the engine compartment), there's 0 v with the switch in position 1, and only 7 v with the switch in positions 2 and 3. Is there a resistor in the circuit between the switch and the blower motor? My manual seems to suggest that the switch should have four positions, off, lo, med, and hi.

Any thoughts?

Thanks.

[Daddy'O]

Have you checked the AC fuse in the fuse block? If it is good then I suggest you post this question in the electrical section. Some sharp guys in there that I'm sure could get you going again.

[landy]

Fuse is good. I'll post it in that section too.

Thanks.

[mrein3]

A few years ago I converted a non-AC truck to AC using all factory components. One thing I found when looking at junked trucks is that, like the headlight circuit, 30-40 years of lots of current flowing through marginal sized wires leads to alot of melted wires. Espcecially at junctions where corrosion leads to higher resistance in the connection which leads to heat which leads to melted connectors.
Long story short, EVERY used AC wiring harness I looked at had melted plugs at the switch end. I'm not saying GM used too small of a wire but with added resistance from the green fur that grows on the connector behind the switch you get too much heat and the connector can melt.

I too purchased a new switch. Since I was in no hurry I also purchased a new plastic connector for the back of the switch. (I found mine in the LMC catalog but I'm sure there are other sources).
Then go to NAPA and get a handfull of part number 725147. This is the female packard type 56 connector used in quite a few of the connections on this vintage of GM vehicles. The wire can be removed from the conntector with a little flat blade screwdriver inserted to push down the tang then pull the wire out the back.
If you need to replace wire, use the same color so you can trouble shoot in the future.

If I remember correctly the orange wire goes to a relay that supplies a full 12 volts via another circuit. On my Chevelles this circuit has a fuse in the gutter behind the engine along the firewall. I don't recall if the truck does that but lots of GM vehicles of this vintage have this inline fuse holder there. Check that after you rebuild your wiring harness.

I hope this helps. It cured all my similar problems with the blower fan.

[ole dollor]

some parts supply places have really cheep replacement switches....might try another brand....

[landy]

Got the blower working again by replacing the relay. Where is the factory mounting location for the relay? Mine was just hanging by the wires and I don't see any obvious mounting holes.

Thanks!