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Old 12-09-2009, 06:52 AM
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Voltage Issue

I have a trunk mounted battery using a Moroso disconnect switch that has 2 large studs and two small ones. I'm not sure of the part number.

Here's my quandry. My voltage gauge reads a hair under 12 volts when running, and if I turn on my turn signals, head lights, or my electric fan comes on there is a noticeable drop on the gauge. The first thing I checked was the stud on the back of the alternator and got 19.7 volts, when I checked the post on the battery disconnect switch the alternator goes to I also get 19.7 volts. But when I check the other 3 posts on the switch I get 11.92 volts and the same on the battery.

The alternator as a Power Master 140 amp one wire unit that is about two years old. I assume the internal regulator is fried and will address that separately.

But what about the switch? Shouldn't it be passing 14.2 volts or actually the 19.7 approx to the battery? Thoughts?

Also, I have the switch wired as in these instructions.

http://static.summitracing.com/globa...74102_inst.pdf

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Old 12-09-2009, 07:08 AM
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Sometimes the main wire that goes to the alternator could be too small. If you are using a #10 wire, try going to a #8
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Old 12-09-2009, 07:51 AM
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[QUOTE=72Orange] alternator and got 19.7 volts, when I checked the post on the battery disconnect switch the alternator goes to I also get 19.7 volts.
But when I check the other 3 posts on the switch I get 11.92 volts and the same on the battery.

.................you say you see 19.7 at the alternator terminal on the switch... but at the other switch terminals you see an 8 volt drop, that would indicate a high resistance open and dropping 8 volts from one side of that switch to the other .. bad switch
................ As to the reading at the back of the alt, running look for 13.5 14.0 anymore and you need to have that alt repaired, one other indication of over charging would be a hot battery, easily felt with the hand on the side .. you might need to put a mile or two on the car to feel the over temp ..

Take a resistance reading across the switch .. sw open infinity ohms and closed 0 ohms, resistance when closed indicates dirty or burnt internal switch components .. failing.

Not a hard problem to solve it has to be one of the two places I have mention .. I had that very alt go south and overcharged a battery so that can happen , 50 bucks and a diode pack ...... all is well .

Last edited by pepi; 12-09-2009 at 09:05 AM. Reason: clarity
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Old 12-09-2009, 08:08 AM
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I would think you smoked the battery switch . The terminal that the alternator is connected to is only rated for 20 amps [alternator field connections ] and you have connected a 140 amp load on it , thats 700% more than it was designed for . Disconnect the wires from the small terminals ,put an ohm meter across them , open and close the switch and see if you have a circuit . to do what you want with that type of switch and a one wire alternator you will need a relay rated at least 140 amp intermittent the same as the alternator max out put . And this would be controlled by the ignition switch via the battery switch .
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Old 12-09-2009, 09:15 AM
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The alternator is not a 140 amp load, the car circuits are the loads and most of the fuses for those loads are in the range of 30 amps .
If all the circuits were running at the same time, then one would not want to exceed the Alternator's poetical output of 140 amps .. otherwise the alternator would fail or if internally protected go open until cooled down again .

Thinking that an alternator is a load is a misunderstanding of the basic electrical components purpose and the wired operation of those parts .
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Old 12-09-2009, 10:23 AM
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Great, thanks for all the advice so far, I'm going to check the ohms when I get home from work tonight. I have one more thought too ... what if the reason the alternator is overcharging is because it's trying tpo overcome resistance in the switch? Would that be possible?
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Old 12-10-2009, 06:24 PM
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Today I'm thinking that the switch is no good and that the alternator is overcharging trying to keep up with the demand. So I took the switch out of the equation connecting the alternator output wire, the battery cable and the cable that goes up to the starter. I found that the output of the alternator is 14.47 volts and by the time it gets back to the trunk it's 14.06 at the battery. Seems to me the switch is no good.

I'm thinking that the switch is the colprut. Can anyone with one of these switches check their voltage and see what they get at the various posts?

Thanks
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Old 12-10-2009, 07:59 PM
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Quote:
Originally Posted by 72Orange
Today I'm thinking that the switch is no good and that the alternator is overcharging trying to keep up with the demand. So I took the switch out of the equation connecting the alternator output wire, the battery cable and the cable that goes up to the starter. I found that the output of the alternator is 14.47 volts and by the time it gets back to the trunk it's 14.06 at the battery. Seems to me the switch is no good.

I'm thinking that the switch is the colprut. Can anyone with one of these switches check their voltage and see what they get at the various posts?

Thanks
That reading sounds good, and I would say ya have it, good work ...
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