Home » Public Forums » GMCnet » [GMCnet] The Importance of a Voltmeter
| [GMCnet] The Importance of a Voltmeter [message #368225] |
Thu, 30 December 2021 12:45  |
GMCWiperMan
Messages: 1248
Registered: December 2007
Karma:
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Senior Member |
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I just stumbled across this note from 2006; it's still applicable, so I'm
posting it for everyone's consideration:
Voltmeter to the Rescue
Do you have a voltmeter monitoring your chassis battery voltage? You
should. Here’s an example of why: We returned from Florida last night
after a three week, two rally, two GMC alternator excursion. As we left
Melbourne, FL enroute to Bowling Green, FL, Elaine and I both smelled
something funny, so I stopped and tried unsuccessfully to find the
source. When I restarted the engine, I noticed that my dash voltmeter was
reading 16 VDC and sometimes jumping even higher. Knowing that much
voltage would damage the batteries, we turned on the headlights, A/C
blower, and every interior light we could find. That dropped the voltmeter
reading to about 15 VDC, a marginally high voltage, but not terribly
destructive, so we continued the trip. After about 15 minutes, the
voltmeter reading dropped quickly to 12.5 VDC, which indicated that we were
operating on the chassis battery alone, with no alternator contribution.
With only 130 miles to travel, I decided to continue. Over the next 100
miles, the voltmeter dropped steadily; at 10 VDC, the engine faltered so I
switched on the house batteries using my non-momentary battery boost
switch. The voltage quickly climbed back to 12.5 VDC, the engine ran fine,
and we reached Bowling Green with no further difficulty.
The next day, after failing to repair the dead alternator, I bought a
rebuilt one and installed it. The voltmeter once again read 14.7 VDC, the
exact voltage indicated on Advance Auto’s tester. I kept the old
alternator so I can repair it for a spare and we continued our travels
around Florida. About 100 miles before we got home, the voltmeter again
jumped to over 16 VDC and the inverter I use to run my laptop computer
kicked off because of the overvoltage condition. Even turning on all the
lights would not bring the voltage back down and it would occasionally jump
even higher, so I decided on a temporary repair. Reasoning that the
just-replaced alternator was probably OK, the most likely reason for the
voltage going high was that the voltage sense lead was not providing the
battery voltage to the alternator, causing it to “think” it needed to
produce more voltage. A jumper wire from the #2 lead on the alternator to
the chassis battery corrected the problem and allowed us to continue to
Americus without further difficulty.
The voltmeter alerted me to the original alternator problem so I could take
preventive action (by placing additional load on the alternator) before it
damaged my batteries. Then when the second alternator began to act up, it
enabled me to determine the probable cause of the problem and provide a
temporary fix without damaging the batteries or the rebuilt alternator.
Neither of those problems would have been indicated by the GEN light until
the alternators actually failed. A voltmeter is money well spent.
Ken H.
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