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My son's car was putting out 15.7v at the alt terminal. I checked the battery and it was roughly 13.5 IIRC. This is for a '91 Toyota Cressida. I was wanting to install a #4 wire from the terminal to the battery, as I read on a couple of sites that had articles on "The Big 3". Isn't this 15V+ a little too much for the battery?
 

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1949 Ford Coupe RESURRECTION
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I can't answer your question as a fact but I had my girlfriend jump off my car the other day and when I hooked up the jumper cables, my digital voltmeter on the dash said 14.6... late model Honda
 

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Bad gauge, cluster, etc will often show higher then actual voltage. In most cases the "key off powered" lighter will show true voltage as a way to check the gauge.

Make sure to ground to the alternatior casing or GRD post when checking regulated voltage. Using the block or battery as a ground may give false readings with some tools.

Regulatiors do wear out causing high voltage. But the alternatior may just be overcharging to maintain a battery with a bad cell.

Test the battery and alternatior at your local auto parts store. Good chance one or both have a fault.
 

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Could be correct if the alternator voltage signal is getting the voltage reading from a remote source. The alternator may have to put out 15+ volts to get 13.5 at the source due to bad connections, bad wiring resistance etc.
Check where the voltage regulator or alternator is getting its signal from and check the connections and corrosion.
 

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Check grounds. Engine to frame, body to frame, battery to engine. If there is none, add them. Ya gotta have them. Bad or nonexistent grounds will cause the alternator voltage to spike up dramatically. I've seen burnt out head light bulbs, ignition modules, ballast resistors, and more from voltage spikes. Use ground straps like below on street rods and customs. Newer cars have tons of grounds. Some are in areas of corrosive attack from road salts in snowy areas. Google ground locations for your vehicle and clean them up and slobber grease on the connections when done. My 2006 Trailblazer had the same problem. Cleaned up grounds, problem gone. I talked to a pro auto mechanic friend of mine and he said "they all do that". I said "BS, not on my car".

 
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