I recently replaced my alternator. Was having issues with it not charging. I replaced with the truck alternator 140 amp. I'm running underdrive pulley and the overdrive alternator pulley, electric water pump also. According to HP Tuners cruising at 70mph its charging 13, 13.10 volts. When cruising around town when I cut on my AC it drops to 12.6- 12.7 volts. Got home and got my voltmeter out. At idle HP Tuners is seeing around 13 volts, the voltmeter is showing 13.8 volts. At idle when AC was cut on HP Tuners is seeing 12.6-12.8 volts, voltmeter is showing 13.6 volts.

This doesn't seem normal to me. I would think they would both be seeing the same voltage roughly. My concern is if the computer is seeing lower voltage its going to effect the charging and how the car runs. Anyone have any experience with this type of thing. TIA.

 

I see some of these same voltage differences whenever I measure around the vehicle at different places. Due to the relatively small thickness of the wire and the different lengths of the wire to each place being measured, there will always be some voltage drop no matter where we measure. As long as my PCM voltage stays above 12.7 VDC and I'm seeing around 14.2 VDC at the battery with the vehicle and both fans running, I'm not concerned at all. I also have the same high amperage truck alternator that you possess.

I do have a very good set of thick wire grounds throughout my vehicle and I'd suggest that you might perform the "Big Three" ground upgrade that the big wattage car stereo people so often need to do. This might raise your measured voltages and tighten up the variances you see at your different measured points.

Rick

 

I've never seen 14 volts on my car. Not even with this higher amp alternator. In fact it charges the same as the 105 amp as far as I can tell. I have the smaller alternator pulley. So its spinning faster so as not to lose amperage from the underdrive pulley. I never had a problem until after the underdrive crank and overdrive alternator pulleys were put on. Not as soon as I swapped them. But it was pretty soon after. That could of course be coincidence.

What "Big Three" are you referring to? Which grounds? What gauge wire? Thank you.

 

Hello,

As with most things, a Google search will reveal many, many hits for the "Big Three Stereo Upgrade". Lots of words and You Tube videos. And I'd suggest that to save time, you can do these ground wire upgrades temporarily without tidying up the installation to see if anything improves.

Remove your installed alternator and your previous alternator and take them both to a well equipped auto parts store for an alternator "load test". These are typically free tests because they want to sell you a replacement. This will spin up and bench test your equipment for voltage levels, amperage production and alternating "ripple current" production. Make sure they at least briefly "full field" it to fully load your alternators to max capacity but most places probably won't be able to fully load a big 140 amp truck alternator - that generates a lot of heat if done for very long. You might have to go to a specialty alternator and starter rebuild shop for a heftier test.

Get the printouts of the tests and hope to see only tiny amounts of alternating current - think around .05 volts AC here. Much more than this indicates one or more blown or leaky diodes in the bridge rectifier section of the alternator. And then around 14.2 or so for volts DC. Don't accept simply seeing the words "PASS" or "FAIL" on their test bench screen - we need the real values here. Some failed components in an alternator are repairable with higher quality parts / bearings etc. if you desire but probably not by an auto parts store.

Testing your alternators in this manor will at least remove any doubt about them and allow you to focus on vehicle wiring.

Rick

 

Gotcha. Should have just done a search on Yotube. Brain doesn't work as good at night. LOL. Thank you for your advice.