Fan Speed issue on a 66 AC underdash unit - Chevelle Tech
Electrical & Wiring Troubleshooting electrical problems.

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post #1 of 6 (permalink) Old May 24th, 18, 4:12 PM Thread Starter
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Fan Speed issue on a 66 AC underdash unit

Just getting ready to install the compressor and add the refrigerant. I have a question on the fan speed for the AC Unit. High seems fine and Low is low; but Medium seems to be the same as low. What would cause this or is this normal ? Thanks
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post #2 of 6 (permalink) Old May 25th, 18, 12:20 AM
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Re: Fan Speed issue on a 66 AC underdash unit

Generally, high is run directly off of 12 volts. Medium and low are run through resistors to lower the voltage into the blower. So quite possibly a bad resistor in one of the circuits.
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post #3 of 6 (permalink) Old May 25th, 18, 9:35 AM Thread Starter
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Re: Fan Speed issue on a 66 AC underdash unit

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Originally Posted by DUTCH MAX HEADWORK View Post
Generally, high is run directly off of 12 volts. Medium and low are run through resistors to lower the voltage into the blower. So quite possibly a bad resistor in one of the circuits.
So that thing with the funny looking thing with two springs?
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post #4 of 6 (permalink) Old May 25th, 18, 11:39 AM
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Re: Fan Speed issue on a 66 AC underdash unit

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Originally Posted by Jim_66SS View Post
So that thing with the funny looking thing with two springs?
On those type of blower motor resistors, do not operate the fan motor long with those out of where it was mounted when testing your low or medium speed as they are meant to be in the flow of air. They can get VERY hot so watch yourself.

Jim
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post #5 of 6 (permalink) Old May 25th, 18, 3:48 PM Thread Starter
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Re: Fan Speed issue on a 66 AC underdash unit

The resister in the custom underdash unit I just installed came off the air duct from an old used 64 Chevelle custom (underdash) unit. So It could be bad. I just read from the 66 service manual that if the low or medium fan on the custom AC does not work properly you should replace the resister.. I will wait until I get the refrigerant installed to see if everything works on high. If so, then buy a new resister and install that. Thanks everyone.
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post #6 of 6 (permalink) Old May 25th, 18, 10:26 PM
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Re: Fan Speed issue on a 66 AC underdash unit

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So that thing with the funny looking thing with two springs?
That's it. When they go bad either a coil is broken, no continuity or came loose at the solder joint. You can test them with an ohm meter. If they show resistance they are usually good. You should get a low resistance value on the medium speed and a higher resistance on the low speed. If they show an open circuit, your resistor is bad and the fan wont work at that speed.

What you described, both low and medium at the same speed is hinkey. You should come up with nearly the same resistance reading on both coils if you test them. If this is the case, then the blower resistor is the culprit. But if the readings vary, you may have other issues. If that's the case, you probably want to test the fan speed switch and if that functions correctly, the wiring between the switch and the resistor.

When they are in the engine compartment like on some stock a/c systems the terminals and plugs get corroded. You clean them up and they work again.

Like Jim said, don't run power through the blower resistor without it being installed in a functioning a/c system. They can overheat and burn out without the air blowing over them as a coolant.
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