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Old 09-02-2008, 01:36 PM
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Heater fan problem

I have a 1999 chev pickp and the heater fan only works in high, checked under the hood for relays but couldn't find anything, Any ideas on what is going on?

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Old 09-02-2008, 02:18 PM
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There is a fan control module, some are made up of resistors and some are completly electronic. Either way it is most likely your problem. Some vehicle recently have also had a problem with the harness melting near the switch. I think that was Fords mostly.
The unit is usually located near the fan motor and will generally have 5 or 6 wires attached to it. The reason high works is it usually bypasses the module.

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Old 09-02-2008, 02:20 PM
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First off check the fuse #12 25AMP in the inside fuse box.

The high blower relay is located on the front top of the blower housing, behind the glove box.

There is also a blower resistor on top of the blower housing.

The blower high speed relay get's it power from the blower fuse 50AMP in the main fuse box under the hood.

The #12 25AMP fuse supplies the blower control module with 2 power leads.

Hope this helps.
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Old 09-02-2008, 04:12 PM
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In addition to what has been said, there is a fan wiring plug above the glove box that is known to melt, check that.

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Old 09-02-2008, 04:17 PM
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on a much older truck/car, there is the resistor installed in a duct (near the heater box under the dash) to keep it cool. If the multi-wire plug gets knocked off, then it only has high. No idea what a '99 would have
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Old 09-03-2008, 10:48 PM
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A buddy of mine had the same problem with a Blazer about that vintage. On the same module that contains the coiled resistor wires for the lower speeds there are also high temperature switches designed to go open circuit if things get too hot. One of these had failed open circuit, leaving him only with high speed.

I bypassed the failed thermal switch (somewhat bullet-shaped) by soldering in a jumper wire - worked fine afterwards.

This is the module that lives in the heater box, accessible from under the hood and near the blower motor.

Good luck. It sure is annoying to only have one speed!
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