[66Malibu]

1966 Malibu, stock.
With a new 10 amp heater fuse, the blower works when the ignition key is in the accessories position. But if the engine is running and I turn on the heater, the fuse blows immediately. Why? The blower is new, and the wiring was cleaned up and inspected when I did the resto., however this problem has been occurring ever since. Could some wires be hooked up wrong in the connector at the heater switch?

 

[anychevy]

1966 Malibu, stock.
With a new 10 amp heater fuse, the blower works when the ignition key is in the accessories position. But if the engine is running and I turn on the heater, the fuse blows immediately. Why? Could some wires be hooked up wrong in the connector at the heater switch?

Was (or is) this a factory A/C car ?

 

[66Malibu]

Sorry, I should have mentioned that: No air, never had it.

I'm thinking that there must be a short in one of the wires to the switch that is only hot when the engine is running, since that's when the fuse blows. I'm not good with electrical issues, but I'll check the schematic in the manual and see if I can figure this thing out.

 

[anychevy]

I'll check the schematic in the manual and see if I can figure this thing out.

I think you'll find there is no original wiring to the fan switch, blower motor or resistors that's only hot with IGN on (except with factory A/C cars)
Check the fusebox and see if there's anything plugged into the IGN terminal

 

[66Malibu]

I have an aftermarket temp. gauge that's plugged into the IGN terminal.

Here are the wires to the socket at the heater switch:

1) A brown wire that comes from the fuse block for power.
2) An orange wire that goes thru the firewall connector to the blower motor.
3) Another orange wire (on the same terminal) that goes to a resistor, according to the schematic.
4) & 5) Yellow & Lt. Blue wires that come from the same resistor and back to the switch.

What's the purpose of the resistor in the circuit? Does that control fan speeds? Could the resistor be bad and cause the fuse to blow? Can this be tested? I don't even know where the resistor is located.

Also, both orange wires were apparently cut and rejoined with connectors at some time. Maybe one is a bad connection. I should redo the splices.

I have a multimeter, but I don't know how to use it to check resistance.

Please, electrical geniuses, help me! The schematic is on pages 220-222 in the 66 factory assembly manual.

 

[anychevy]

The blower motor works on all speeds with ACC on, but blows the heater fuse only if the IGN is on (engine running) ? Very strange.

All the above wiring sounds right, I doubt it's the gage unless something else has been spliced into it.
The resistor contols the low fan speed by dropping the voltage to the blower motor (orange wire)
The resistor, or resistors (if 3 speed) are located behind a 3 prong plug mounted to the inside of the heater box.

The only thing I can think of is, when the engine is running, vibrations are causing a short to ground in either the power wire to the blower motor (orange) or the power supply
from the fusebox (brown) but you could test that by just switching the IGN on without the engine running ?
So tidy up those splices and re insulate the wiring (can't hurt)
This is a bizzare problem I've never come accross, I wish I could be more helpful

 

[66Malibu]

Still working on this little problem. Luckily the heater won't be needed when the warm weather gets here, but I'd still like it to work.

I removed the heater resistor and took a picture of it. There doesn't seem to be anything visually wrong with it, no burn marks or anything. The middle spade is pushed into the cardboard base a little and can be pushed further. Maybe when pushing the connector on, it shoved the spade completely through and either didn't make good contact, or ?? I need to figure out a way to secure the spade.

With the resistor removed, I installed a new fuse, and the blower works fine with BOTH the engine off AND with the engine running. So that means that the fuse blows with the wiring connector attached to the resistor and the motor running.

Any way to test the resistor? I think I should check the voltage at the connector for each of the three wires. What should the voltage be?

Any other ideas? Are these parts available new? Maybe just easier to replace and then know for sure.

Come to think of it, how does this thing work and what does it do?