ALTERNATOR WITH EXTERNAL REG. TEST???? - Off-Road Forums & Discussion Groups
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post #1 of (permalink) Old 01-20-2000, 06:31 AM
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Having some problems with the charging ciricuit in my '77 3/4 ton 4x4 with auto. Have checked grounds and added extra ground straps from battery to firewall, frame, body. New alternator (best rebuild sold by Advance Auto), regulator, and battery. When have on all pwr hogs (lights, heater, etc.) come to idle speed, everything goes dim. With lights off only get about 12.9v at idle in neutral. When switch everything on voltage drops to 12.3 or less. Have checked connnections on the amp guage and they are tight and clean. Ohmed out wires between voltage regulator and alternator, they look to be good.

I have done the "open field test" on the new alternator. When I start up the engine the voltage reads at about 14v to 14.6v. I have to rev engine to get the voltage up above 15 and it will slowly go up to 16v. Should the voltage in this test mod go directly to 15v or better at idle(in neutral) without having to rev the engine or wait for it to build up. I'm thinking I got a lemon alternator from the parts store???

Problem started when son went 4 wheeling with a battery with one shorted cell and covered engine with water and in turn fried alternator. Anything else to look for??

Thanks for help on earlier post and for any help here.

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post #2 of (permalink) Old 01-21-2000, 10:19 AM
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By open field, you mean you have 12 volts at one field terminal and ground at the other? Ok, at roughly 1250 RPM, you should have 13.9 to 14.4 volts. Test the wiring: unplug the voltage regulator.....looking at the connector of the harness, connect a ground from the right-most connector (not the center pin). This effectively gives you the open field test via the wiring. If the alt puts out the voltage, then the regulator is bad. If not....the wiring has a problem.

If you only have 12.3 volts at the battery with the engine running, you are not getting any charge.

post #3 of (permalink) Old 01-22-2000, 09:28 PM
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Here's a quick and dirty way to check if alternator is working. With engine running see if the center of the back of the alternator is magnetized i.e. does it suck a wrench to itself. It won't be strong enough to support the wrench but you should definately feel the attraction. Also, where are you checking the voltage? AT the battery or the alternator? Double check the Ammeter these are notorious for inducing excessive resistance into the system. I usually end up just putting both wires an the same terminal and installing a voltmeter. To answer your question re. revving to get voltage Yes, you should have to rev above hot idle speed to accurately check output the 1250 suggested in prev. post is a good RPM to begin looking at output voltage.

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