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Re: 1990 gmc 1500 heater motor does not turn on
The fan switch itself is a possibility, but there is also a resistor
pack behind the glove compartment. Hi speed doesn't use the resistors,
though, so if it doesn't run on hi, it narrows it down to the switch,
the motor, the motor ground, or the blower relay, or any of the
associated wire connectors.
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Did you disconnect the battery before you disconnected/ reconnected the under dash computer? These devices are extremely sensitive to power surges and most are not "hot swappable" which means that they need power removed before you connect or disconnect them.
Look for a heater control relay in the fuse panel. It may be under the hood. If you see another relay that is the same swap the two around. Will the fan now go off? If it does the relay contacts have welded shut. Take the relay that was bad to a parts store and get a new one. I hope this helps. Please rate my answer so I get points for it.
if it works only on high speed you need a blower motor resistor you will find it close to the blower motor has three wire to it and looks like a small 1 inch by 3 inch piece of computer board material with two screws holding it.once you get it off you will find that one of the springs has lost contact.
Check your cruise control fuse or relay. It will be in your owners manual. If fuse is good than the cruise itself may be faulty. Check continuity on cruise controler itself. If there is power, than the controller itself is bad and need to be replaced. If there is no power than it may be a faulty switch.
sounds like the relay is turning on sending the power out to the blower motor resistor witch is locate dby the blower motor it is held in the case by two screws check it for power when you turn the switch on and off