Blower Motor Circuit

3673 Views 9 Replies 5 Participants Last post by Powerglide, May 9, 2011

Powerglide

Discussion Starter · May 8, 2011

The A/C/Heater blower motor fuse gets red hot on my 1976 280Z. With the proper fuse it will blow quickly with the fan switch on 3 or 4. Obviously there is a LOT of resistance in the circuit. I tried two different blower motors with the same results. I'm aware of the fact that there is a resistor in series with the fan connection that adjusts the voltage to the motor for different speeds. There is also a relay under the hood near the fusible link. My question is how do I determine where the excessive draw is? If a relay is functioning, can it create the excessive resistance? Or is the resistor in the "squirrel cage" the most likely culprit? I haven't found excessive corrosion in the wiring or the connections. A lager fuse will substantially delay the inevitable...but I'm completely aware that this is NO solution, is potentially dangerous, could destroy other circuits and even cause a fire. Automotive electronics is not my forte...

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bonfire79

· #2 · May 9, 2011

Do you have an FSM for your Z? if not, get one from xenons130.com
the first section is all A/C and it covers everything.... oh and it's about the thickest section in the whole book, so it should have plenty of reading for you. there is a lot of information about the A/C electrical system and how it all works. I hope you find your problem.

Bon

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steve240Z73

· #3 · May 9, 2011

Powerglide said:
The A/C/Heater blower motor fuse gets red hot on my 1976 280Z. With the proper fuse it will blow quickly with the fan switch on 3 or 4. Obviously there is a LOT of resistance in the circuit. I tried two different blower motors with the same results. I'm aware of the fact that there is a resistor in series with the fan connection that adjusts the voltage to the motor for different speeds. There is also a relay under the hood near the fusible link. My question is how do I determine where the excessive draw is? If a relay is functioning, can it create the excessive resistance? Or is the resistor in the "squirrel cage" the most likely culprit? I haven't found excessive corrosion in the wiring or the connections. A lager fuse will substantially delay the inevitable...but I'm completely aware that this is NO solution, is potentially dangerous, could destroy other circuits and even cause a fire. Automotive electronics is not my forte...

Actually, it is not obvious that there is a lot of resistance. The opposite is true. V=IxR. Therefore I=V/R. Increasing resistance drops the current. You have a short. It could be at the switch or in the wiring. As Bon suggested, download the FSM and look at the wiring diagram. However, look at the link in my signature, since it's for the S30.

If you are looking for a likely culprit, learn how to take resistance measurements. You're looking for when the resistance drops dramatically. That's when you're blowing your fuse.

I hope you're not putting in fuses larger than the rated sizes. That is a great way to burn up your wiring.

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nrkrel

· #4 · May 9, 2011

On my 78z there is a big relay behind the glove box that powers the blower motor. It is old, check it out. Norm K.

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dreamteam

· #5 · May 9, 2011

Had this happen on our van last year.
Turned out it was the blower motor.
I see that you have switched that out. hope the "new" one isn't the problem.

Relay is not a likely culprit, although it can happen. have you changed or added anything
to your electrical system lately? Stereos are the first thing to check.

If you know how to use a multimeter,set it on 0-10volts DC, put the + lead on the + connection to the blower
and the - lead to a good ground. If you see voltage with the blower switch off, you have a short.

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Powerglide

Discussion Starter · #6 · May 9, 2011

I have the FSM in hard form. The circuitry is clear. I'll need to break out the milti-meter. Perhaps it not a resistance issue. Could be voltage. Need to test... There IS an aftermarket stereo in the car. NO aftermarrket amplifier though. The fan (and a/c) relay is under the hood by the fusilble links. The resistor is near the fan itself. The wiring is in good shape.

The higher rated fuse gets HOT---but doesn't actually blow.

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steve240Z73

· #7 · May 9, 2011

There are three components powered off the fuse:
1. The blower motor
2. The magnet valve
3. The compressor relay.

What is happening is that the circuit is pulling a fair amount of current when the fan is at 1 or 2. However, you're below the rating of the fuse, so you're good. When you go to 3 or 4, the resistance in the blower motor circuit drops, increasing the current. That puts you over the edge, and the fuse blows.

So, I suggest that you check 2 & 3 to determine why you have the high current draw.

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Powerglide

Discussion Starter · #8 · May 9, 2011

Update: I cleaned the fusible link and the contacts associated with the blower motor circuit. Applied a little dielectric grease to same. I also cleaned/polished the contacts for the fuse itself. The fuse on high speed is actually a little cooler to the touch, now. The fuse warms up whether the compressor/air conditioner is engaged or not. So it's definitely in the blower motor circuit.

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steve240Z73

· #9 · May 9, 2011

Hmm, in that case, does the FSM have the value of the resistor? Part of it might be burned out.

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Powerglide

Discussion Starter · #10 · May 9, 2011

I have a suspicion the resistor may be the real culprit. Apparantly it functions only on high (4) fan. And that is when the fuse REALLY heats up. There is probably some minor corrosion in the wiring in the circuit and perhaps in the entire harness exacerbating the problem. I WON'T be replacing the entire harness. Again the proper 20 amp fuse doesn't blow. It just gets REAL hot and actually starts to melt the fuse box cover at the a/c fuse location. I will try another motor to see if that helps as well. It IS cooler to the touch since I did the work discussed earlier...

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