My first post to this forum... my 2001 GMC K3500 Duramax, Allison is giving me some worries. One evening about 6 months ago I noticed the headlights and dashlights appeared to be pulsing. A check of the voltage meter confirmed the voltage appeared to be a bit high and the needle was wavering a bit. Then I noticed that once in a while (not every time) when I let off the throttle that the voltage would drop suddenly (lights dimming, blower fan slowing), then it would "catch" and return to a steady higher voltage. So, I did the obvious, I replaced the alternator with a brand new one (140 amp). The problem didn't go away. In fact, sometimes the voltage will climb to an indicated 16.5 volts (it hits the mark between the 14 and 19 on the gauge) and will pulse dropping a half volt or so about every half second. If you are driving at night it looks like you have one of those pulsers on the headlights like some motorcyclists use. The darn problem is intermittent... sometimes it will pulse for a few minutes and then come under control, other times it will keep doing it for a half hour or so. Then other times it will maintain a regulated voltage without problem.

One final thing, I searched the archives and note people requested info regarding the delay in voltage rising after startup... mine takes anywhere from a minimum of one minute (warm days) to three minutes (cold days) before the voltage will come up above 14 volts. Both batteries were replaced two years ago with 7 year batteries.

So, what is this noob missing? Is there an external voltage regulator on these trucks? Or is my new alternator also bad? The truck appears to be running okay. I'm at 92,000 miles and the truck is stock other than a 55 gallon SuperTank.

Thanks in advance for any advice!
Steve

The voltage regulator is inside the alternator. It is not a user repairable item. You might want to got to AutoZone and see if they can check it for you. If it is bad, you'll have to replace the alternator. (If the alternator is new, check to see if it is still under warranty)

You may want to give them a call and see if they can check the alternator out of the truck. This will at least determine if the problem really is the alternator or maybe something else is loose/bad in the electrical system.

Obviously, another alternative would be to have your "friendly" GM dealer check it out.;)

BTW - Welcome to the Diesel Place! :)

The problem didn't go away. In fact, sometimes the voltage will climb to an indicated 16.5 volts (it hits the mark between the 14 and 19 on the gauge) and will pulse dropping a half volt or so about every half second. If you are driving at night it looks like you have one of those pulsers on the headlights like some motorcyclists use. The darn problem is intermittent... sometimes it will pulse for a few minutes and then come under control, other times it will keep doing it for a half hour or so. Then other times it will maintain a regulated voltage without problem.

One final thing, I searched the archives and note people requested info regarding the delay in voltage rising after startup... mine takes anywhere from a minimum of one minute (warm days) to three minutes (cold days) before the voltage will come up above 14 volts. Both batteries were replaced two years ago with 7 year batteries.



Few things,

Have you had both batteries load tested seperately. It is possible to loss a cell in a battery, and test full charge with little to no amperage and with two batteries may not show up as being very noticable. I have had an instance where a vehicle would start, run but the alteranter was running wide open trying to charge the battery and run all the accessories leaving the lights to pulsate and the like.

I too had wondered why my truck wouldn't charge right off the bat, to the extent that I was watching with a VOM and would see only 11.2vdc. This is nothing more than the glow plugs and intake heater running to get things warmed up but it doesn't seem to run much more than a minute at the most, not that I live in the coldest climate either. It does seem to last longer when it's cold out although I'm not sure just how long the max time is for this feature.

Don't take the dash gage for gosspil at least not till you compare it to a good quality meter. They very easily can be off by a volt or two but you probably know what looks normal if you've owned it very long.

Get those batteries double checked for load and also have the alternater rechecked. It maybe rebuilt but that isn't always a sure thing. Hope this helps.;)