Need advice/help 1974 Grand Prix heater blower

Oliverd642

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Hello! I need help with some wiring diagrams or some sort of indication on how the wiring works on the ac control panel..

My blower motor stopped working yesterday. The motor works fine, I wired it directly to a 12 volt source. It just does not turn on when it’s turned on In the car.

I checked the fuse, it is fine. Checked the positive line at the Blower with a multimeter, no power.

Any advice?
 

melsg5

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All speeds or high still works?
Check the blower motor resistor.
Check if there is a high speed blower relay.
 

Oliverd642

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All speeds or high still works?
Check the blower motor resistor.
Check if there is a high speed blower relay.
All speeds = nothing. I don’t know where the resistor is, I know very little about this car. It’s my first “old skool” vehicle
 

melsg5

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Do you know what the heater core/evaporator housing is under the hood? The resistor typically would be there or try Google for an image.
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Oliverd642

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Do you know what the heater core/evaporator housing is under the hood? The resistor typically would be there or try Google for an image.
I really don’t know, I found the resistor under the hood, three wires go to it. the resistor clicks on and off if I put 12v power direct to it. So the resistor is working as far as my testing goes.
 

melsg5

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So you applied 12 volts to a relay and heard a click, how did you determine 12 volts came out of the relay?
How do you know it is the correct relay and not for instance the horn relay?
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Oliverd642

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So you applied 12 volts to a relay and heard a click, how did you determine 12 volts came out of the relay?
How do you know it is the correct relay and not for instance the horn relay?
Hi, I know it’s the correct relay because when I removed it I jumped the two connections with a piece of wire and the blower turned on.
 

melsg5

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You can also try looking for the resistor under the dashboard passenger side on top of the heater case.
 

Oliverd642

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You can also try looking for the resistor under the dashboard passenger side on top of the heater case.
Thanks for the quick responses! I’m going to check over everything again now. I’ll report back :)
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Oliverd642

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Well. The relay is ok, power is running through it. I don’t know how to check resistors but it physically looks fine and I tested it for continuity which was a success. I also wired 12v to each of the pins on the resistor, low medium and high fan speeds worked.

When I bypass the relay, the blower fan runs on full speed, moving the fan speed switch inside the car does nothing. Perhaps the AC control panel is broken?

Also, before when turning on the ignition (without starting the engine) the fan would blow at its lowest speed, it does not do that anymore. Now it just makes a buzzing sound from the dashboard. Before it would only make the buzzing sound with the engine on and the gear selector in Park.

I found something behind the glovebox that I have no idea what it is for, it is not connected to anything. I’ll post a picture of it.
 

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melsg5

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Try removing that, might be markings on it that would help ID it.
Try to get your hands on a 74 factory service manual for the wiring diagram.
The fan switch may have failed based on your testing.
 

Oliverd642

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Try removing that, might be markings on it that would help ID it.
Try to get your hands on a 74 factory service manual for the wiring diagram.
The fan switch may have failed based on your testing.
Okey I’ll remove it today and see if I find anything and search for a service manual. Now I’m wondering if there’s a bad ground somewhere, this morning my stereo was dead and would not turn on. Thanks for the help, I’ll continue to dig around.
 

melsg5

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Can you see if there is a connection on the stereo where that plug would fit?
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Oliverd642

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Can you see if there is a connection on the stereo where that plug would fit?
Perhaps it was for the original stereo. I bought the car with an aftermarket stereo.

Well, going to spend my evening the garage again tracing wires, wish me luck Haha :D I will report back of I find something.

Ooh one more time thing, do these cars have just the one fuse box near the handbrake release? Or is there another fuse box somewhere?
 

Oliverd642

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That generation of cars had 1 fuse box
It seems I found the problem, although I don’t feel I solved anything, but now it’s working so I guess thumbs up!

I took out the fuse, sprayed some contact cleaner and put it back in. (The same old fuse) now the blower works!? Well I wish I’d have done that a few days ago, I’ve been hours testing and tracing wires.

Just goes to show such simple things can make a big difference.

A big thank you to - melsg5 - for helpful information and support during these last few days, if only everyone was as helpful as him! Cannot say how grateful I am. Quick helpful responses. Thank you.
 
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melsg5

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You may already have done this but it sounds like you should pull all the fuses, one by one, and clean the contacts.
Welcome to the world of older cars!
 
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