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Owyyn
RealPoor Guru
Joined: 11 Oct 2002
Posts: 2900
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 Posted: 08/29/05 - 20:20 Post subject: Electrical troubles
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The blower motor in my car quit working awhile ago. The relay went bad and it was on it's way out. I had the positive running directly to the battery (fused). It switches via the ground on the controls so all I had to do was be sure to turn it off whenever I shut the car off. Then after awhile I had to start smacking the damn thing with a wrench to get it going.
Long story short, I got a new motor and relay to combat the winter months before they come. Plugged the new relay in... my radiator fan springs to life and my blower motors real harness starts getting positive power again. All seems good.
I put in the new motor. Nothing happens. I check the voltage again... 0 volts on the positive wire, but the ground is working. I unplug the harness and check the voltage again... 12 volts now! Plug it back in... 0 volts! So I hook the blower up the way I had the old one just to make sure it even works.... blows better than my old one ever could have dreamed about.
This doesn't make any sense to me. How can I lose voltage only when I plug it in, on a motor I know works perfectly fine?
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gotissues68
RealPoor Sensei
Joined: 21 Aug 2003
Posts: 1866
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Posted: 08/29/05 - 21:54 Post subject:
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sounds like something is shorting out somewhere. It doesn't seem possible but I've seen weirder things happen.
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sinrakin
RealPoor Master of Posts
Joined: 11 Oct 2002
Posts: 7044
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Posted: 08/29/05 - 23:26 Post subject:
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I don't know anything about cars, so this is just a shot in the dark.
There should be a blower motor resistor block somewhere that has resistors that get switched in series with the motor to provide different fan speeds. If those resistors (or the contacts to them) have failed in such a way that you have a very high resistance through them, then you'll see 12 volts, but when you put the motor across it you'll get a tiny current flowing with most of the voltage drop occuring across the faulty resistors, making it look like there's only zero volts across the motor.
You could try putting your meter in current mode and in series with the motor - if you register a current when you turn the fan on, but it's only milliamps, that would be a possibility.
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Rennol
RealPoor Guru
Joined: 11 Oct 2002
Posts: 3741
Location: Charleston, IL
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Posted: 08/29/05 - 23:48 Post subject:
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| sinrakin wrote: | I don't know anything about cars, so this is just a shot in the dark.
There should be a blower motor resistor block somewhere that has resistors that get switched in series with the motor to provide different fan speeds. If those resistors (or the contacts to them) have failed in such a way that you have a very high resistance through them, then you'll see 12 volts, but when you put the motor across it you'll get a tiny current flowing with most of the voltage drop occuring across the faulty resistors, making it look like there's only zero volts across the motor.
You could try putting your meter in current mode and in series with the motor - if you register a current when you turn the fan on, but it's only milliamps, that would be a possibility. |
SOMEONE WAS PAYING ATTENTION IN CIRCUIT ANALYSIS
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principessa
RealPoor Guru
Joined: 11 Oct 2002
Posts: 3149
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Posted: 08/30/05 - 00:12 Post subject:
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If you have 12 volts at the blower motor, then it is likely the motor that is defective--swap it for another one. If you don't have 12 volts, there may be a resistor/fuse/switch to look at as the culprit.
I worked a lot of years in my ex's shop....I absorbed a few things along the way :p No guarantee on what I typed above, but it seems to be what I remember....
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Owyyn
RealPoor Guru
Joined: 11 Oct 2002
Posts: 2900
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Posted: 08/30/05 - 02:06 Post subject:
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I tried hooking up a DC light to the wire. It wouldn't work either... voltage would disappear when I tried to turn it on. CRAZINESS.
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sinrakin
RealPoor Master of Posts
Joined: 11 Oct 2002
Posts: 7044
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Posted: 08/30/05 - 07:42 Post subject:
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| Owyyn wrote: | | I tried hooking up a DC light to the wire. It wouldn't work either... voltage would disappear when I tried to turn it on. CRAZINESS. |
I think it's definitely what I said then. You've got a very high resistance in that wire. So when there's no current flowing you see the whole 12V. When you complete the circuit with a light or motor, you get a tiny amount of current flowing (because the resistance is so high), and the entire 12V gets dropped across the wire.
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Owyyn
RealPoor Guru
Joined: 11 Oct 2002
Posts: 2900
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Posted: 08/30/05 - 13:08 Post subject:
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| sinrakin wrote: | | Owyyn wrote: | | I tried hooking up a DC light to the wire. It wouldn't work either... voltage would disappear when I tried to turn it on. CRAZINESS. |
I think it's definitely what I said then. You've got a very high resistance in that wire. So when there's no current flowing you see the whole 12V. When you complete the circuit with a light or motor, you get a tiny amount of current flowing (because the resistance is so high), and the entire 12V gets dropped across the wire. |
The resistance for different fan speeds is set up on the ground wire. I don't think it is the problem. I can use the switched ground wire with a positive hooked directly to the battery and it will work fine, with fan speeds changing as expected.
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sinrakin
RealPoor Master of Posts
Joined: 11 Oct 2002
Posts: 7044
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Posted: 08/30/05 - 13:19 Post subject:
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Hmm, so the resistor block sounds ok then. So what happens if you hook the light bulb (or the motor) through an ammeter to the wiring harness +12V (or wherever it's supposed to be hooked up but doesn't work)? Do you see the current jump from 0 to a small but nonzero value when you turn the switch on? Or does it stay at zero current?
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