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Old 07-04-2014, 09:08 PM
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Voltage drop, rough idle, blown fuse; Alternator?

I'm having some electrical problems with my '94 Corvette LT-1 engine in my '37 Plymouth.

About a year ago, the alternator bearings went bad and I replaced the Delco 12-Si that was part of a package from Street & Performance that the guy who built the car installed in 2001. I used AC-DELCO # 321-269, a 94-amp unit in 3-wire configuration, #8 wire to the starter; sense wire to the fuse block.

I have driven the car very little since then, but have been experiencing odd problems. Rough idle, low vacuum at idle, but runs fine at highway speed.

Then the ACC power line that runs through the column-mounted starter switch melted the switch (the fan power was run through that circuit), so I replaced the switch and ran the ACC circuit through a relay plus the fan was now powered directly through a dedicated fused circuit from the battery.

Next up, the mechanical thermostat for the radiator fan had its contacts ruined (the builder wired it that way without using a relay), so I replaced it with an electronic thermostat and a relay. Then the fan fuse would blow after about 20 minutes of driving; i did notice the voltage dropping from 13.8 V to 12V at that time.

I installed a 105-amp 12-Si alternator I had for another car under construction and that seems to have solved a lot of the problem, but I'm still getting some voltage drop at idle. The battery may be suspect since it has gone through nearly a dozen deep discharges and is about 5 years old.

What should I be looking at next? Is the alternator still under-powered? Battery causing the whole problem now?


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Old 07-05-2014, 10:43 PM
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. Some alternators are kind of lazy and don't want to kick in or stay kicked in and charge at idle RPMs... also, newer engines usually idle a little fast and the alternators may be designed for that...

. Alternators are usually wired straight to the + battery post now...

. Fan should be running little or not at all while driving... unless stopped or crawling...

. Maybe you have two going bad alternators... should be about 14.2 volts at battery with engine running...
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Old 07-06-2014, 11:47 AM
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Nothing wrong with running the ACCY circuit thru the ign switch....... however all your fan "running" circuitry should be run thru a relay. All your fan controlling circuitry should be run thru the ACCY circuitry.

Hook the fan controller to the ACCY circuit, the fan controller should control a 70 amp (that's what I use) relay, the fan relay should be hooked directly to a home run wire to a slo-blow fuse or a 12 volt circuit breaker (I use the circuit breaker) then to a 12 volt buss or wire to the battery and the feed wires should be as short as possible (and still look good with the routing).

Basically use the low amp ACCY circuit to turn on the hi-amp relay, protecting the hi side with a circuit breaker and the low side with the normal low amp fuses that exist in the fuse block.

I typically connect the sense wire about 6-8 feet of wire from the pig tail on the alt on the wire feeding the fuse block.

Battery may be suspect.
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Old 07-07-2014, 06:38 PM
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The 20-amp fan is powered through a 30-amp relay with a 30-amp fused lead directly from the battery. The relay is powered from the ACC circuit and the ground side is controlled by the solid-state thermostat in the radiator. All of that works well now.

That is until I'm at idle with he engine warmed up and the fan kicks on. Then I see a voltage drop and experience a rough idle. Not as bad as before and the fan fuse does not blow, but it should not behave that way. I'm thinking 1) not enough current at idle from the alternator or 2) battery capacity has been compromised sine it's been discharged deeply so many times and the battery is "old" in battery years.

Since the battery is old, I'm replacing it which I can do later this week when and if I get that damn DeSoto running and off the lift (I have to remove the right front tire to access the battery).
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Old 07-07-2014, 06:53 PM
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That would be my next step, I think you are on the right track. Might make sure your grounds are clean and tight.
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Old 07-08-2014, 03:55 PM
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Running rough could be low voltage to the ignition system. i would check the feed for teh ignition to be sure it has battery voltage on it. With all the stuff you found done wrong it would not be surprising if you had a bad connection on the ignition system or a resistor somehow made its way in there
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