Topic: Blower Fan wiring issue - can someone please help?

[hepkat63]

Hi All,
ok, trying to get my blower fan working.  I have taken the following steps, but need some help as it still is not working:
1.  Checked the fuses - all good.  Replaced them anyway just in case
2. Taken off the front guard to expose the blower motor.  I disconnected the two wires and hooked up 12v positive to larger connection and negative to smaller one - fan turns at high speed.
3. Connected the battery and turned the heater blower switch on dash to high - test light at large connection shows power, test light at small connection shows power.
4.  Connected the battery and turned the heater blower switch to dash to low - test light at large connection shows power, test light at small connection shows NO power.
Given 3/4 above, I assume that the wiring is correct.   However, when I connect it all back up again, blower motor does not turn.  I am thinking maybe that the blower motor needs an earth - or perhaps the earth (where ever that is) is not connected.
I have all the pontiac books here and have checked the wiring diagrams, but can't make any sense of it.   Also, my car (originally) had Air Conditioning and Cruise control and maybe when the previous owner removed those, there were wires unplugged that were integral to the blower motor too that they had in common?  Is anyone able to help please?

[Wallington]

If your air con has been removed then there's the issue. Try looking for a ground wire unplugged for a start, or any cut or removed harness. The harness normally grounds on the compressor brackets, when removed you lose your fan.

[hepkat63]

thanks for the reply.   I have found the other harness that used to be connected to the A/C setup.  it has a black wire with a ring connector which has a positive connection (with ignition on).  There is another black wire spliced into that as well which has been cut off and then a white plastic connector with a dual plug, but only one green wire attached - negative connection.

I would have thought that if I simply ground the black wire coming from the blower motor, it would be fine but not so. 

I am a little perplexed as when I remove the purple and black wire, if I test them with the ignition on, they BOTH have a positive power (when fan speed is set to high), when fan speed is set to low, just the purple wire has power.

From what you are saying, the black wire should be a ground?

The other thing is, I put the purple wire on (whilst it had power) and then used a jumper lead from the terminal where the black wire is connected straight to earth and expected the fan to turn, but it didn't?.
Also, when I put the purple wire on the Fan, and leave OFF the black wire and put a test light on the terminal where the black wire goes - there is a positive there - almost as if there is a feed through somehow?

Then, to totally confuse things (as I mentioned before), if I take both wires off (purple and black) and use my portable 12v power pack - positive to the purple wires connection point and negative to the black wires connection point, the fan fires up and spins very quickly.

Hence, my confusion.  What am I missing?   

[Stringer]

Here is the ac wiring diagram for a 1978. There may be some minor changes through the years (color of wires, etc.) but I believe they are mostly the same.  Hope this helps.