Loaded up ie. headlights, AC w/blower on High and a satelite radio receiver and the voltage is around 12.9-12.7 and the Alt. was replaced 8 months ago. This is while running down the road at normal speeds. Same variables as above and in Reverse (back up lights on) voltage drops to 12.5-12.6 volts but eng. at idle. Fog lamps tend to drop it down another .2 or.3 volts from where ever it starts. All electrical draws off and idle voltage is around 13.3 to 13.6 volts.

My question is whether or not the Volt Regulator is stepping up like it should to cover the load. I do see it peak right after engine start when the one minute or so has expired and the Alt. comes online around 13.8 or 13.9 to recharge the battery.

I understand voltage is pushed and amperage is pulled and out of a 90 amp alternator I would expect the voltage to find more of a constant around 13.5 volts and you would see the amperage increase to cover the load. It seems the voltage should not pull down until the amperage can no longer keep up with the load.

Thanks to all those that lend an opinion.......MD11Drvr

 

I think you are fine.

Here are my voltages:
<TABLE style="WIDTH: 674pt; BORDER-COLLAPSE: collapse" border=0 cellSpacing=0 cellPadding=0 width=899 x:str><COLGROUP><COL style="WIDTH: 674pt; mso-width-source: userset; mso-width-alt: 31776" width=899><TBODY><TR style="HEIGHT: 12.9pt" height=17><TD style="BORDER-BOTTOM: #d4d0c8; BORDER-LEFT: #d4d0c8; BACKGROUND-COLOR: transparent; WIDTH: 674pt; HEIGHT: 12.9pt; BORDER-TOP: #d4d0c8; BORDER-RIGHT: #d4d0c8" class=xl24 height=17 width=899>Voltage After Resting Overnight......... 12.3 vdc</TD></TR><TR style="HEIGHT: 12.9pt" height=17><TD style="BORDER-BOTTOM: #d4d0c8; BORDER-LEFT: #d4d0c8; BACKGROUND-COLOR: transparent; WIDTH: 674pt; HEIGHT: 12.9pt; BORDER-TOP: #d4d0c8; BORDER-RIGHT: #d4d0c8" class=xl24 height=17 width=899>Voltage During GP Warm-up............... 11.7 vdc</TD></TR><TR style="HEIGHT: 12.9pt" height=17><TD style="BORDER-BOTTOM: #d4d0c8; BORDER-LEFT: #d4d0c8; BACKGROUND-COLOR: transparent; WIDTH: 674pt; HEIGHT: 12.9pt; BORDER-TOP: #d4d0c8; BORDER-RIGHT: #d4d0c8" class=xl24 height=17 width=899>Lowest Voltage during Start-up.......... 11.7 to 11.8 vdc</TD></TR><TR style="HEIGHT: 12.9pt" height=17><TD style="BORDER-BOTTOM: #d4d0c8; BORDER-LEFT: #d4d0c8; BACKGROUND-COLOR: transparent; WIDTH: 674pt; HEIGHT: 12.9pt; BORDER-TOP: #d4d0c8; BORDER-RIGHT: #d4d0c8" class=xl24 height=17 width=899>Voltage after Start-up (before alt)...... 11.9 vdc</TD></TR><TR style="HEIGHT: 12.9pt" height=17><TD style="BORDER-BOTTOM: #d4d0c8; BORDER-LEFT: #d4d0c8; BACKGROUND-COLOR: transparent; WIDTH: 674pt; HEIGHT: 12.9pt; BORDER-TOP: #d4d0c8; BORDER-RIGHT: #d4d0c8" class=xl24 height=17 width=899>Voltage after Altenator kick-in at idle.. 13.3 vdc</TD></TR><TR style="HEIGHT: 12.9pt" height=17><TD style="BORDER-BOTTOM: #d4d0c8; BORDER-LEFT: #d4d0c8; BACKGROUND-COLOR: transparent; WIDTH: 674pt; HEIGHT: 12.9pt; BORDER-TOP: #d4d0c8; BORDER-RIGHT: #d4d0c8" class=xl24 height=17 width=899>Voltage at 1500rpm.......................... 13.5 - 13.8 vdc</TD></TR><TR style="HEIGHT: 12.9pt" height=17><TD style="BORDER-BOTTOM: #d4d0c8; BORDER-LEFT: #d4d0c8; BACKGROUND-COLOR: transparent; WIDTH: 674pt; HEIGHT: 12.9pt; BORDER-TOP: #d4d0c8; BORDER-RIGHT: #d4d0c8" class=xl24 height=17 width=899>Voltage after 30 minutes of driving...... 13.6 vdc</TD></TR></TBODY></TABLE>

High idle operation does not seem to impact the voltage in these tests.

My truck is 3 years old and I have already changed out the factory batteries. I am currenty shopping for a high output alternator.

 

Mark,

Thanks for the lightning fast response! I had to replace batteries last winter during a drive home from ATL at 0200 in the morning. Walmart was my only option and I have not been happy with the 750 CCA 65's I bought and have been debating replacing them with an 850 CCA from interstate or another premium battery.

It was an interesting expierence to watch as the batteries failed. To condense the story so it won't be too long I'll just cover details.

Truck sat at airport hotel for 20 +/- days, batteries stone dead. Put another battery (wrong size) in to start and excite the field expecting to be able to charge batteries on the 3 hour drive. I should have known they were sulfated but after being out for many days I wanted to get home and deal with it there. Well after about 40 miles there was not enough voltage to maintain the field and within 7 miles to the next exit the truck was a dead stick. It was interesting to watch the computer shed the load in an effort to save itself. The ABS was first to go followed by radio lights then dash instruments and the last half mile was inertia as I coasted off the ramp and into a parking lot off of exit 28 from 85 south. A cab ride at 0300 to Walmart one exit down was the only option or sleep in the truck until morning. Needless to say I learned alot on that night.

The whole time the idiot light for voltage never came on. So much for the idiot light.

 

You are lucky you didn't frye the FICM.

 

I was without a doubt lucky on that one. I should have played the lottery that night but I was too distracted by the lack of sleep and the problem at hand.

 

what makes the alt. start charging is there something in the alt. are is there a computer that senses low voltage and thin starts charging

 

Voltage regulator built into the alternator.

 

also keep in mind that the alternator will not charge with the gpcm timer still on. max on time for gpcm is 120 sec. so basicly start the truck wait 2 min then test output

 

Just another follow up note. I had to replace the Alternator today. The Voltage regulator went south as I had undercharge voltage and over charge voltage. Got the truck home without incident had the Alternator changed in less than an hour and all is well once again. Thanks again to all those who contribute to this forum.

 

Just curious what numbers you are getting now as compared to your earlier post? Thanks.

 

truck should chage at 13.8-14.2
that being said I see a lot at 13.4-13.6 on the low side as norm.
much less then 13.4 I would be looking a little harder.

for those that dont know how to test a diode in an alternator with a dvom...
Set the dvom to ac voltage and check the battery with the engine running for ac voltage. you should see .3 ac volt or less more is a suspect bad diode.

If this requires an explaination let me know.

 

I like that. Funny, in 23 years in electronics, never heard it put thay way.