seavolt88

Hello all! I have a buddy who has a 99 tahoe 5.7 with a pain in the butt voltage issue. to try and make this as short as I can, so far I have checked for a parasitic draw on the battery (it had one, I took care of that) so now the battery doesn't drain within a day of not starting. The truck does have a brand new battery, alternator, serpentine belt, and starter. I have done the ground check and replaced both ground straps (engine to frame, firewall to frame) with new ground straps. I have metered the truck while running and it stays within 12 volts and fluctuates but never drops below unless you were to roll a window down, or turn the blower motor on, then it goes back up to 12V. It did however the first start after changing the ground straps start and stay at 14V for about 30 seconds then it stalled, and after that back to the 12V issue. Unfortunately it hasn't thrown a code either to check for a bad sensor. I am kind of at a loss with this tahoe right now. Any other areas I should look at? and other common denominators I should be checking?

tech2

unplug the alternator brown wire...it should have battery voltage with the key in run...if not check the gauge fuse.in the instrument panel fuse box
-check the fuse off the positive battery cable. depending on the build it may be a fusilbe link or 175amp mega fuse.

doing voltage drop tests of the insulated and grd circuits are the next step but try the above first and report back

seavolt88

Awesome!! Thank you for your response! I will def check that next, i wont be able to check the truck until tomorrow but i will post back as soon as i do!

seavolt88

Ok so, im at the tahoe right now checked the brown cable to the alt while the key is on it reads 12v. Any other ideas?

kevinkpk

Is this running? I am questioning the voltage readings you posted with it running, and only 12 volts.

kevinkpk

I'd have the alternator tested, if no loose plug ins, and I KNOW it's new, new dosen't mean good.

seavolt88

Yea im going to take the alternator in tomorrow and have it tested. While the truck is running it only puts out 12.5 volts. I did leave the truck runni g today for about 10 min and during that time it got up to 13.5 volts. I had my meter on it while i gave it some gas and it would drop down, and start to climb back up letting off the gas. I noticed like my suburban i have looking at the parts store they offer different amperage alternators. Currently the truck has a 105amp. Did they come from the factory with the higher amp? I wasnt around when he got the new one so i didnt see the original one.

tech2

do the voltage drop tests
-check the insulated (power) circuit
with a voltmeter put one lead on the big positive terminal on the alternator. put the other meter lead on the positive battery post. start the vehicle and turn on all electrical loads. reading should be 0.3 volts or less.
-check the grd circuit
one lead on the alternator case (its case grounded), the other on the negative battery post. start the engine and turn on all electrical loads. the reading should be less than 0.2volts