This Florida summer is brutally hot. Just retired and I'd like to drive my 68 GTX around.
Here is the problems.... Turn on AC and it will blow cold air from 15 to 45 seconds. One time out of ten it won't blow at all.
Ran power directly to blower motor and it will run all day.
Is there a resistor somewhere that controls fan speed on AC cars?
Where will I find it and is it fixable or replaceable.
If replacement is needed who can supply one?
Does the dash switch just turn things on and off or is there magic inside?
THANKS for any and all help.
Posted in two different forums hoping more will see it.

Is the blower intermittent in all speeds? There is a blower motor resistor attached to the rear side of the A/C box. Could also be the switch.

I have a 70 Sport Satellite with AC . . . I get similar situation . . . where the blower motor gets nothing from the switch - I did find a NOS switch (spendy !! $$$), as the original was very worn - the blower motor pulls a fair bit of current. Didn't change things, sometime would work, sometimes not. When I restored the car, I made sure ALL connections at bulkhead were clean and tight ! . . . wondering if it is the "vacuum switch"- which controls where you get the airflow (defrost, heat, ac vents) - as the blower motor circuit does go through the 'Max AC" position . . . when I put 12volt direct to blower motor - full output for sure. Time to upgrade and build a relay panel to be controlled by the OE switch, and then the relays take the current draw . . .

Oh, and have to make sure if you change out the vacuum switch unit - you get the one for your year !! It has been superseded over the years, and while it "looks" the same, the functionality was changed over the years . . .

Cheers,

The resistor can be seen by looking above the gas pedal and it is in the air box. Mine had melted down the wires going into it. Had to get a used one. You can rewrite it for a relay. 1wildrt posted details on fbbo on doing that. You can boost up the fan speed by doing that as the full 12 volts isn’t delivered even on high by the factory setup.

Did that on my '70 Charger and the motor got really hot.

The safe way of boosting the high fan speed would be with the use of a high speed relay in the circuit. This would help reduce the risk of melting the blower wiring.

The issue is whether or not the motor will survive running on straight battery voltage. There have been other instances noted where a motor won't survive at full voltage.

Thought this was in the A/C air flow duct work to cool it?

The problem with increasing fan speed is that the flow increase directly with speed but the power increases as the cube. As an example, if you double the speed of a fan, the flow doubles. The power required is eight times the original. This is all from the fan affinity laws. That's why a modest increase in speed results in significant power increases with the resulting motor heating.

My 77 truck with factory ac had a switch wired in for the fan when I got it. The ac worked for about ten miles the day I bought it, enough to run out the charge they out in at the lot. I had to manually turn the fan on every time I got in and the motor never slowed down or caused any trouble in the years I owned it. I don’t doubt it got hot, it just didn’t didn’t matter.

The resistor coils are in the air flow, the connector terminals in my pic above are exposed outside the box.