1997 GMC Jimmy Pulsating Electrical System

Tiny
DEADWOOD777
  • MEMBER
  • 1997 GMC JIMMY
  • 6 CYL
  • 4WD
  • AUTOMATIC
  • 178,000 MILES
My entire electrical system is pulsating. Dash lights, headlights, fuel gauge, everything. The voltage meter on my dash shows the system charging at 16+ volts, but a handheld meter attached directly to the battery shows a steady 13.5 volts. OK, so my dash volt meter is inaccurate, but what would be causing the system to pulsate? The alternator checks out OK. I just had the fuel pump replaced. Could it be a bad ground somewhere? We also have had sub-zero temperatures which is hard on the battery, but it is very new and load tests OK. I'm baffled... Any ideas would be very appreciated.
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Saturday, January 9th, 2010 AT 4:40 PM

8 Replies

Tiny
BLACKOP555
  • EXPERT
When did you notice all of this happening?

Do they pulse al the way down to all the way to past their higest marks? When lights go out do the guages go on?

Take a volt meter and measure the AC voltage across a battery with it running. Measure at idle and 2000 rpms and get back to me with what you find
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Monday, January 18th, 2010 AT 11:15 PM
Tiny
DEADWOOD777
  • MEMBER
Thanks for the quick reply. It's very odd, but the pulsing has stopped. When it was happening, it was not extreme. The fuel gauge seemed to be most affected and was pulsing about 1/4 of the scale about 3 times per second. It was also very noticable in the dash lights and headlights. The pulsing decreased somewhat at higher rpm's. Today, voltage is a steady 13.5 volts at idle and at 2000rpm. Battery is 12 volts at rest and 11.4 volts under load-test. The pulsating problem began after having the fuel pump replaced, prior to which the vehicle had sat for approximately 2 weeks. During the time that it sat, temperatures were averaging -15F and I had made many attempts to start it, thereby depleting the battery somewhat. My guess is that the pulsating only happens under heavy charging load. Would this indicate the alternator or specifically the voltage regulator? I took it to a local alternator repair shop and the technician tested the alternator and battery and told me that both were OK and that I should not replace the alternator. Although the problem has apparently corrected itself, I am still stumped. I do not want to damage the electrical system or especially my new fuel pump with the fluctuating voltage. What would you recommend?
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Tuesday, January 19th, 2010 AT 10:35 AM
Tiny
BLACKOP555
  • EXPERT
I wanted you to check the AC. Seems that you got dc. But I woudl like AC on the battery.

It can do funky things like that if the ac current is out of wack.

Discharge the battery a bit and let the vehicle charge and measure that ac voltage across the battery!

Shouldnt cause any damage having a bit of ac. Its just turning things on and off fast causing the pulsing
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Tuesday, January 19th, 2010 AT 2:12 PM
Tiny
JIMMY_JAMES
  • MEMBER
Do you have a large stereo with a big subwoofer and many speakers in the truck? If you do, does this problem only occur when the volume is up, and not when the stereo is off? If so, you have too much draw on your charging system/
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Tuesday, January 19th, 2010 AT 2:26 PM
Tiny
DEADWOOD777
  • MEMBER
No. No aftermarket accessories that would be drawing current.
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Wednesday, January 20th, 2010 AT 1:37 PM
Tiny
BLACKOP555
  • EXPERT
Will you please check the ac current across the terminals with the battery charging hard with the alternator.
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Wednesday, January 20th, 2010 AT 1:40 PM
Tiny
DEADWOOD777
  • MEMBER
OK. Give me a day or two. I don't have an ammeter, just a volt meter and load tester. I'll have to stop in at the shop.
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Friday, January 22nd, 2010 AT 9:39 AM
Tiny
BLACKOP555
  • EXPERT
You can test ac voltage with a normal volt meter.

Instead of selecting dc voltage check ac voltage.
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Monday, January 25th, 2010 AT 2:38 PM

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