1997 GMC Jimmy Pulsating Electrical System

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?

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/

No. No aftermarket accessories that would be drawing current.

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.