97 350 voltage briefly dips

[kadenhaugen]

While at idle or driving the voltage goes down by about a volt and the lights dim about every 30 seconds give or take. Replaced alternator about a thousand miles ago and just started developing this issue. Battery less than 6 months old and no battery light

 

[Road Trip]

While at idle or driving the voltage goes down by about a volt and the lights dim about every 30 seconds give or take. Replaced alternator about a thousand miles ago and just started developing this issue. Battery less than 6 months old and no battery light

There are a couple of possibilities that spring to mind. Either the new alternator is the perpetrator
of the symptoms or a victim of a fault in the wiring harness. But in order to better assess which
path to follow, we need some additional context surrounding the symptom you described.

* Why was the original alternator replaced? (Noisy bearings? Complete failure with a battery light? Or was it also dropping voltage once every 30 seconds?)
* Does this 'every 30 second voltage sag' start right from a cold start? Or do you have to drive
for 10-15 minutes and everything gets fully warmed up before the voltage cycling begins?
* Is there any way you know of to avoid the voltage cycling? That is, if you turn off all loads will it settle down, only to start back when you turn the A/C on,
fan on high, high beams, etc?
* What brand is the new alternator? Sourced over the internet or from a local parts store?
* With the hood up, does the sound of the alternator change while the voltage is cycling up and down?
* Do you have a voltmeter and can tell us the actual voltage at the battery while this is going on?
(14.5v > 13.5v > 14.5v is one thing, but 13.5v > ~12v (battery alone) > 13.5v is another.)
* ANY aftermarket gear on this vehicle? Kilowatt audio amps? CB radio with a huge linear? (I'm sure the answer is no but shame on me if I don't ask. :0)
* IF you have a load tester, does the voltage remain steady at 10/20/30/40/50 amps, then start cycling when you draw 60+ amps from alternator?
(Bonus points for having an o-scope where you can see 1 or more diodes start to break down under load and give you an ugly-looking output.)
*Are all the grounds in the engine bay clean & tight? If the alternator bracket/engine block isn't on the same ground plane
as the battery then the internal voltage regulator may be trying to cut back due to no true/intermittent reference to the state of the battery.

****

I realize that these are a lot of questions, but ultimately I'm trying to get a feel for if you are
experiencing a thermal failure inside your alternator, or if your truck has a latent fault in the
wiring harness that is somehow setting up your alternator for failure?

Verify all your grounds are in new condition, recheck your connections at the battery and alternator,
and answer whatever questions above that you can. Depending upon your answer, you may need
to take it to the store you bought it from and have them test it on their stand. (Of course, a 30 second
run on the stand isn't going to prove anything if the truck has to run 10+ minutes before the cycling starts.)

Let us know what you discover.

 

[HotWheelsBurban]

Sounds like the way the diode trip in the old GM 10SI delcotron's would act, when going bad. Dad showed me how to change out the common fail items in them, and we did quite a few "quicky rebuilds" on the bench, our vehicles and customers of the parts store. The newer alternators seem to be better built, and last much longer. I don't think we had to replace it on the other 99 Burb, haven't done it on the current Burb either.
It's probably not this, but I have noticed that if the battery terminals are not firmly tight, it does affect where the voltmeter reads; even an eighth turn loose, retightened, makes a difference. Yes everyone says side terminals suck, but I've had plenty of issues with the top terminals coming loose on our mopars. Houston area roads are rough, unless they have just been replaced. And a one ton with no load in the bed, rides like a tank, so that doesn't help. So I keep a wrench in the passenger door pocket, and when I notice the gauge is reading lower, time to pop the hood and check those babies.

 

[kadenhaugen]

There are a couple of possibilities that spring to mind. Either the new alternator is the perpetrator
of the symptoms or a victim of a fault in the wiring harness. But in order to better assess which
path to follow, we need some additional context surrounding the symptom you described.

* Why was the original alternator replaced? (Noisy bearings? Complete failure with a battery light? Or was it also dropping voltage once every 30 seconds?)
* Does this 'every 30 second voltage sag' start right from a cold start? Or do you have to drive
for 10-15 minutes and everything gets fully warmed up before the voltage cycling begins?
* Is there any way you know of to avoid the voltage cycling? That is, if you turn off all loads will it settle down, only to start back when you turn the A/C on,
fan on high, high beams, etc?
* What brand is the new alternator? Sourced over the internet or from a local parts store?
* With the hood up, does the sound of the alternator change while the voltage is cycling up and down?
* Do you have a voltmeter and can tell us the actual voltage at the battery while this is going on?
(14.5v > 13.5v > 14.5v is one thing, but 13.5v > ~12v (battery alone) > 13.5v is another.)
* ANY aftermarket gear on this vehicle? Kilowatt audio amps? CB radio with a huge linear? (I'm sure the answer is no but shame on me if I don't ask. :0)
* IF you have a load tester, does the voltage remain steady at 10/20/30/40/50 amps, then start cycling when you draw 60+ amps from alternator?
(Bonus points for having an o-scope where you can see 1 or more diodes start to break down under load and give you an ugly-looking output.)
*Are all the grounds in the engine bay clean & tight? If the alternator bracket/engine block isn't on the same ground plane
as the battery then the internal voltage regulator may be trying to cut back due to no true/intermittent reference to the state of the battery.

****

I realize that these are a lot of questions, but ultimately I'm trying to get a feel for if you are
experiencing a thermal failure inside your alternator, or if your truck has a latent fault in the
wiring harness that is somehow setting up your alternator for failure?

Verify all your grounds are in new condition, recheck your connections at the battery and alternator,
and answer whatever questions above that you can. Depending upon your answer, you may need
to take it to the store you bought it from and have them test it on their stand. (Of course, a 30 second
run on the stand isn't going to prove anything if the truck has to run 10+ minutes before the cycling starts.)

Let us know what you discover.

Can't get through the whole list rn but old alt was a total failure out of nowhere with no battery light, battery light did come on for maybe 3 seconds yesterday though.

Sag starts right up from cold and voltage draw doesn't seem to make any difference

Ill listen for change in sound in the morning

no multimeter, sleeping at a truck stop pretty far from home rn however it seems to dip just higher than battery alone

Ill look over grounds in the morning. Could be caked with mud

The truck does have subs but amp is only something like 800 watts if I remember right

New alt came from O'Reillys and was one of the mid price ones

 

[Road Trip]

Sag starts right up from cold and voltage draw doesn't seem to make any difference

Given the failure is right from the get-go it should duplicate on the test stand, which is a good thing.

If you don't find anything obvious while double checking your connections it looks like pulling the
alternator and having it tested on the auto store's bench will solve the mystery. Fingers crossed
this is still under warranty.

Let us know what the final solution was.