Do these alternator voltages look ok?

[east302]

Truck is a 1998 Tahoe with a new 105A alternator (Oreilly Ultima new) and battery cables (Delco). These are my voltage readings taken from the alternator post to battery negative.

Do they look ok? I'm seeing a noticeable drop in a/c blower fan speed when I come to a stop then back to normal when driving. The dash voltage gauge gives a corresponding dip, but the gauge reading never corresponds to the meter readings at the battery or alternator.

Battery voltage: 12.55

14.23V at 2,000 rpm, accessories on
14.36V at 2,000 rpm, accessories off

12.98V at idle, accessories on
14.33V at idle, accessories off

Accessories on is headlights, a/c, radio, rear defrost and blower on high.

Comparable battery readings (positive to negative posts) were nearly the same as the readings from the alternator (maybe 0.1V, if that) so there doesn't seem to be a huge drop between them.





Sent from my iPhone using Tapatalk

 

[RawbDidIt]

mine reads 14V+ at idle with blower, radio, and a/c on. Headlights are always on for me (DRL). I don't have a rear defrost, and my blower doesn't have to move air in the rear, but I have a hard time believing those 2 would make such a big difference. In my daily driver, that's the difference between having my 1500W+ audio full blast or off. Not sure what would cause that. Are these trucks regulated at the alt, or are they externally regulated?

Sent from my SM-G930V using Tapatalk

 

[MrAdam]

No, your idle voltage is a bit low I think. Your gauge will never be accurate, it's there to indicate a problem not to diagnose it.

When you put the new alt on did you swap your pulls or did it come with one? If it came with on was it the same size?


Sent from my iPhone using Tapatalk

 

[east302]

The new alternator came with a pulley, and it matched the diameter of the existing one. The truck came with an RPO code for a 100A alternator, and that's what was on it.

RawbDidIt- pretty sure it doesn't have an external voltage regulator. Thanks for the voltage comparison, mine should be comparable to your truck.


Sent from my iPhone using Tapatalk

 

[MrAdam]

Anything over 100 amps is sufficient for most cars and trucks in stock form. My Grand Marquis came with a 65amp which I just upgraded to 140amp.

Based on what you said above and the fact that it's a store brand alt, I would question the Alternator. My GMs only get Bosch or delco electronics; even OEM JY units seem more reliable and that is from someone who used to do reman work.


Sent from my iPhone using Tapatalk

 

[east302]

Yeah, my rule of thumb is Delco except when you're stuck and have to overpay at a parts shop. You know how it is.

Checked the numbers again at idle:

With all accessories on and blower on high, it's 12.7V.

Turning only the blower off, and it goes up to 14.3V.

Blower on low: 13.8
Fan speed two: 13.2
Fan speed three: 12.8

Turning off the lights gave me barely one more volt, so it looks like the blower is the issue?

They draw straight 12V on high, right?


Sent from my iPhone using Tapatalk

 

[df2x4]

Seems a bit low to me, but nothing too horrible. I usually see similar numbers to that in my Suburban with everything running maxed out, but it has everything you listed plus a few more lights and an extra blower motor.

The blower motors and rear defrost seem to be the big current draws as far as factory options.

Based on your last post I'd say you've pretty much narrowed it down to the blower.

 

[east302]

Yep, it was the blower. I had heard it squeal a few times at startup over the past few weeks but had ignored it.

Swapped it out today, now have 14.3V at idle with everything on and blower on high.




Sent from my iPhone using Tapatalk