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Old 08-02-2005, 05:15 PM
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Blower Motor Stops on High

I have a 98 GMC that I replaced the fan switch and the blower motor still stops on high. Could this be a bad motor? I checked the voltage and I get the same volts all settings. What makes the speed change anyways?

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Old 08-02-2005, 06:33 PM
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I am not familiar with that vehicle but there are a few "common methods" of doing that. First is a resistor network. Its a set of wirewound resistors located in the air plenum often. Your switch (the part you just replaced) switches between the different resistance windings. If one of those is burned out (open) it does not work on that setting. Some cars use electronic modules and some use multi winding motors.

You need to find out what kind of method yours uses. Call your parts house and ask for a speed control for your vehicle. More than likely they can tell you what it is.

The resistor network is usually easy to diagnose. You take it out and one of the windings is burned open.

If you can find an online schematic, I can tell you which yours uses more than likely.

Rich
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Old 08-02-2005, 08:49 PM
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PS: Just found this post.

Here is the part that controls the motor speed.

http://www.hotrodders.com/forum/atta...achmentid=6064
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Old 08-03-2005, 03:10 AM
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Great. I'll take a look after work today.
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Old 08-03-2005, 03:14 PM
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I found it. I removed the glove box and there it was sitting on top of the air box. Thanks for the help.
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Old 08-03-2005, 03:35 PM
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Well just look at the coils and make sure they are not burned open.

Got thinking after making another post helping someone with a Camaro if your truck has A/C it could be the relay that runs the blower on high. In his case they use a relay to run 12 volts to the blower motor in the high fan position so it does not go through the switch.

Check that to because its likely.

Rich
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Old 08-03-2005, 06:49 PM
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I was wondering if the current bypasses the resistor on hi. I did find a relay next to the resistor, it seams to be working as I can hear it and feel the coil click.
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Old 08-03-2005, 07:14 PM
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Well it doesnt really "bypass" the resistor but it does feed a higher current 12volt to the output of the resistor which is the same as the going direct to the motor.

Assuming we are talking about the correct relay and you hear the click, the relay is pulling in but does not mean the contact is making. Relays fail in two ways. Either in the coil section or the contacts.

An easy way to check this is to put the blower on high and listen for the relay to click in. If there are four wires going to the relay two of them are probably small (one is ground and one goes back to the switch on your dash) and two larger. One is hot with the ignition turned on and the other one goes to that common point (output of resistor to blower motor).

Once you know which two are the contact ones, you can jump them across and see if the blower kicks up to high. If you have a test light or volt meter you can also check both contact wires with the relay engaged. If the relay is engaged and you have 12v on one contact and nothing on the other, you have a bad relay.

Rich
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Old 08-04-2005, 06:12 AM
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I did forget to mention that I did hear the relay click when I switch to high. I'll give it the test as you described with a volt meter. I now Radio Shack has a nice 30 amp relay for around $5.00 which I've used before on a backup light circuit on the Camaro.
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Old 08-04-2005, 03:22 PM
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I checked voltage at the relay before and after. With the blower unplugged I get 12v. Plugged in I get nothing. I pulled out the resistor panel and I can't see a broken wire. Almost sounds like a bad connection.

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Old 08-04-2005, 04:03 PM
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So you are saying this happens on both sides of the relay? The feed side and the blower side?

If so, you are right there is a connection problem somewhere. You will just have to keep moving back the line until you find it. Could be a bad fuse connection, etc.

The 12volts you measure with the motor removed is just residual voltage.

Rich
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Old 08-04-2005, 04:05 PM
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high speed blower relay. esp on a camaro. replace.
if not (it worked on mine, had all other speeds but high) then try this: http://www.nastyz28.com/ubb/Forum9/HTML/003458.html

Last edited by bullheimer; 08-04-2005 at 04:11 PM.
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Old 08-04-2005, 05:07 PM
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Yes that does happen on both sides of the relay. With the blower plugged in I was getting....low-4.78v, med.low-7.96v, med. hi-9.90v, HI-3mv. With the blower unplugged 12.7v on every speed including hi. But like you said "It is residual voltage" I'll start tracing this weekend.
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Old 08-04-2005, 05:13 PM
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The lower voltages are correct for the speeds going through the resister with the blower plugged in.

The resistor only has a voltage drop when there is current flowing. The actual voltage is dependent on the amount of current the "load" draws. When there is no current draw, the voltage is unchanged across the resister.

So at this point, your lower speed circuits are fine. The high has its own high current feed which is independent from the lowers. So on the input side of that relay, start tracing backwards.

Sorry, but dont have a schematic for that vehicle or could give you places to look. If you took a jumper wire and ran from the battery + to the feed side of relay you will find everything works as it should.

Rich
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Old 08-04-2005, 05:16 PM
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Keep us posted. I have a similar problem in a 1999 GMC that I havent had time to get to. I hope that your solution will help me fix mine.

John
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