Voltage decrease on gauge when hot - Chevy Impala SS Forum
 
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post #1 of 6 (permalink) Old 07-26-2017, 02:41 PM Thread Starter
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Voltage decrease on gauge when hot

Weirdness with the charging system that I've been trying to sort for a while. I know that resistance increases along with temps - just wondering if all is normal here or if something else is going on. New 140A alternator

Symptoms: Start the car in the AM. Lights, radio, wipers, AC, ac fan on 2 or 3, both cooling fans whirring away. Volts gauge reads 3/4 or so. Keep driving for a while. Ambient temps climb to 93. Nothing changes outside of shutting the wipers and lights off (so less load). Car is fully warmed up now. Volts gauge on dash reads half. Connect PC to car and see 13.1 volts on Scan9495. I don't think its the compressor because I am not seeing a voltage dip when it cycles. Its just a constant lower reading, but only when the car is hot.

The battery is about 5 years old, so maybe the internal resistance in the cells is climbing out of spec when the battery itself gets hot? Dunno...

Thoughts?


1995 Caprice ERE 383, CIA long tubes, Dynomax VT's, RAISS, Kore3/Z06 brakes, 3.42 posi, Boss 338 20's, 12 way seats, Impala SS console, etc etc etc
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post #2 of 6 (permalink) Old 07-27-2017, 07:16 AM
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I would use a voltmeter and check voltage at the battery . if the volts at battery are 13.75 and the PCM/fuse box reads 13.1 then you have a bad /loose connection from battery to the fuse box.

both fans on and ac on max etc does consume a lot of amps so this does show a poor connection ..also check alternator output wire...

when the air temp is about 100F the alternator should put out 13.7 volts with a hot engine ac on .. this is regulated by the alternator . cold start should be 14.75 then as the heat increases the regulator drops the volts down.. 120F can be down to 13.5volts..
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post #3 of 6 (permalink) Old 07-27-2017, 11:46 AM
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As J Cat says the regulator is designed to prevent "water boil off" by reducing voltage when hot.

Quote:
I am not seeing a voltage dip when it cycles. Its just a constant lower reading, but only when the car is hot.
I think you have voltage regulation and the regulator is normal.

I never recommend the factory gauge as the sole diagnostic tool. Start with a test voltmeter that you know reads correctly.

I would compare the voltage at the alternator (case to stud) with the battery (post to post) when the motor is running and you have the electrical loads running. The alternator may be sending the correct voltage. Cleaning all + and ground cables including the ones to the PCM may result in a higher reading by the PCM.

I replaced my battery, starter, and alternator wires and noticed a big difference. Not all cars need this but if you are losing volts and amps in old cables you can not blame the alternator. The cables can be tested for losses with a voltmeter.(not by resistance measuring)

Z09B4U

Last edited by Z09B4U; 07-27-2017 at 11:51 AM.
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post #4 of 6 (permalink) Old 07-28-2017, 09:53 AM
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[QUOTE=atlantadan;12003033]Weirdness with the charging system that I've been trying to sort for a while. I know that resistance increases along with temps - just wondering if all is normal here or if something else is going on. New 140A alternator

Hi Dan Veronica here. Love your wagon. Mine is pretty rustic, and it shows the same thing with the charging gauge. Thanks for the info. Mine's having some engine trouble now. It's got 318,000 miles on it, and I'm going to search the forum for help. Don't know if it just needs new distributor or if timing chain jumped (maybe both). Laters V
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post #5 of 6 (permalink) Old 07-28-2017, 11:58 AM
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On my pickup, I used to see lower voltage (13.2) when hot but then I doubled up the main power cable and added multiple ground cables. now its steady 14 volts not matter the heat.
I've not done this on my b4u but will get around to it eventually.

what does that get me? hahahahahahaa!

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post #6 of 6 (permalink) Old 07-28-2017, 01:14 PM
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broderick
Quote:
what does that get me?
In the case of the starter cable a better very hot or very cold start.

There is a microchip on the cluster that lights the Check Gauge light when the voltage gets too high or low. It is triggered by the regulator.

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