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Bob, Auto Tech
Category: Chevy
Satisfied Customers: 4087
Experience:  Plus 40 years GM, over 30 certified with Chevrolet, ASE certified Master Tech. (Expired, Retired)
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1991 S10: The AC/Heater fan motor..pickup..intermittently..resistor

Customer Question

The AC/Heater fan motor on my 1991 S10 pickup comes on and turns off intermittently when set on high. I've replace the fan relay and the resistor with no change in the problem. The low and medium speed on the fan don't work at all. Is there relationship with the cooling capability of the compressor with the fan switch. I can't imagine it does. Thoghts?
Submitted: 9 years ago.
Category: Chevy
Expert:  Bob replied 9 years ago.
Your thought is correct, there is no relationship between the cooling capability of the compressor and the fan speeds or switch. If there were, the heater would never have adjustable speeds. I would suggest after considering the repairs you have already made that you may want to remove the control panel and check the blower switch itself for out put on different speed settings. Also check to make sure it has constant voltage to it with the key turned on. Especially look for any sign of burn marks from excessive voltage at the connectors on the back side of the control panel, more at the blower speed switch than any place else. Seems if I remember right, they also had some wiring harness issues from the control panel to the blower motor resistor, etc. You may want to also check the harness real close. If you can do a wiggle test and get the power into the speed control to shut off, we may want to back track the wiring to the ignition switch and fuse panel to see if we can find where the break down occurs, same with the power out to the blower resistor etc. Thanks
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Customer: replied 9 years ago.
I think the response of "check for a short" in the harness is something I will have to take to a mechanic. I'll respond with an "accept" but the information given was pretty generic feedback. I'll end up at a mechanic to verify the response from "Justanswer".
Expert:  Bob replied 9 years ago.
Hello Bruce, thanks for the accept and the response. The problem is that because of the type of condition you have, the answer has to be pretty generic. If you can isolate where the system has voltage and where it does not, when it is acting up, then we can be much more specific. But, as it stands now all we know is that there is an open someplace in the circuit and it could be anyplace between the ignition switch and the blower motor. As long as the system is working correctly, everything is going to test out, as ok. But, if we can run some tests when the system is in its not operating state, then it is just a matter of starting at the blower motor and testing for voltage and working our way back from the motor to the ignition switch to find where the open is in the system. As an example if you test at the blower motor when it won't work and you have no power at the motor, we would work from there back to the resistor and relay, if we have no voltage there and then we test the blower switch and find we have voltage going in and not out of the switch, then we know the blower switch is bad and simply replace it. Same with the rest of the system, if no voltage to the switch, then we test at the ignition switch, if we have voltage going in and out of that, then we know the wire is open between the ignition switch and the blower control switch. But, because there are so many components involved we have to start someplace and just work our way back to the beginning, eliminating things one at a time as we go, until we locate the problem. Finding intermittent shorts or opens (as your case) is never easy unless you can get the system to stay inoperative long enough to complete the tests. If you think it might help, I would be happy to try to locate a wiring diagram of the system for you. But I have no idea of your mechanical abilities and have to rely on you for the help in diagnosing the system since the truck is with you and not me. My only way to help is through the information you give me and any test results that you obtain in the process. If I can be of further assistance, just post me a note. Nothing we would rather do than help you fix the problem. Thanks
Customer: replied 9 years ago.
The reason I don't subscribe to the "short" theory is that the low and medium speed don't work at all. The fan works intermittently when on "high" but intervals don't happen with jolt or jiggling of harnesses or the fan switch.

I also could not find an inline fuse. Removing the switch assy from the console will require a mechanic and that looks like where I need to end up.

Even testing the fan controls for power requires the controls be removed from the console to get to them.

Thanks for your time, I agree that electrical is a more elusive issue and much harder to detect.
Expert:  Bob replied 9 years ago.

If you had not already replaced the resistor, that would be the first thing I would have checked. If you can access it without too much trouble, you can test right at the resistor and see if you are getting voltage to all of the terminals on it when you move the fans speed control switch. If you do not, this would be a further indication of a bad blower switch. As far as the fuses go, there is no in line fuse for the blower motor. The only fuses that would affect it would be the 20 amp fuse that says ign. gages or the 25 amp that says heater ac, both in the fuse block. I will enclose a copy of the wiring diagram just in case. Good luck with it and thanks.