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High tempature in blower switch wire-HELP!

Hello everybody.

I have a 1990 S-10. When I use the air conditioner while in the medium setting, the switch and the wire leaving the switch becomes very hot. (this wire is blue in color, and runs from the blower motor switch to a blower resistor. But, if I use the HIGH setting, I no longer feel or smell heat.

I have a wiring diagram, and on that diagram, I see that on the HIGH setting, all current is from the battery straight thru an enerized relay to the blower motor, bypassing the resistors altogether. At the medium, or low settings, the relay is not enerigized and current must flow thru the blower resistors to get to the blower motor.

I think that there is something wrong with those blower resistors. I have had basic electronics. I checked all resistors OUT of the circuit and all of them read ZERO resistance. It looks like just a coil of naked wire. It seems that under DC current ( which is what you have flowing to the motor) you will not have any DC resistance thru a coiled wire.

I thought there may be a problem with the relay, but I have replaced that but still have the problem. also, I have replaced the blue wire without success.

Can anybody tell me what the problem is ? I am very frustrated ! !!

Reply to
Dan
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Most likely the problem is either bad contacts in the switch for that speed, or a bad connection to the back of the switch. It only takes a very very small amount of resistance at a connection or switch contacts to generate a lot of heat. I've seen switches melt from the heat caused by bad contacts.

Reply to
AZGuy

AZGuy:

Thanks for your input.

What I did not tell you was that I have replaced the blower swithch a few years ago, but now the new one looks like it has had heat damage. At that time, I also had to replace the plastic "housing" that separates the different contacts.

Now, when I can find time, I guess I will replace the switch again, and also find new contacts, and replace them at the same time.

Thanks for your input. I did not know that such a small amt of resistance could cause such heat, but since you told me, I now thought of the storage battery's high amperage capability.

One question: What purpose do the blower resistors serve ? I read "zero resistance" with my ohmmeter across these resistors. They look more like an AC inductor to me. I assume that these resistors drop some of the 12volts so that the blower moter gets a lower voltage, therefore it is at a lower setting (low or medium). But where IS the resistance ?

Reply to
Dan

Well, remember V=IR, so if R is very small, I is very high.

The resistance of the wires goes up quickly with heat. Reading it cold will only give you a go/no go answer.

I'd probably look at the switch, specifically where the wire to the medium circuit hooks into it. The connection or wire might be bad. The wire is probably partially broken near the connector (if I had to make a guess) since the wire is getting hot there.

Reply to
John Alt

The other poster answered most of what you asked as follow up. I would add this to give you a better hands on idea of how much heat that switch could generate with bad contacts. Think about how hot your brake light bulb gets after it's been on for half a minute. Too hot to even touch.

It draws at least 2 amps. Using the V= IR formula v=12 volts I = 2 gives

12=2xR solving for R gives a resistance of 6 ohms.

The blower draws a lot more current then 2 amps. At high speed it draws around 15 amps and the formula says

12=15xR R of the blower motor will be 12/15 or 0.8 ohms.

If you want a medium high speed that only uses 12 amps of power, then

12=12xR and the TOTAL R will be 1 ohm. There is already 0.8 in the motor so you have to add another 0.2 from the resistor pack. That's why when you try to measure those resistors you seem to be finding such low resistance... it is low.

Since the voltage drops are proportional to the resistance, the 12 volts is used up as

12x0.8 in the motor = 9.6 volts lost in the motor and 12x0.2 = 2.4 volts in the resistor. So your 12 volt motor would be only getting 9.6 volts when you run it on medium-high and that's why it is slower.

You might also recall that watts are = volts times amps. The total amps was set as 12 earlier. So 2.4 volts x 12 amps = 28 watts. That's close to the wattage of some fog lights !!!! Those resistors get HOT - that's why they put them in the air duct.

Reply to
AZGuy

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