All lights fluctuating between bright and dim

[Rowland]

2008 Ram Cummings, 3500. The other morning before daybreak I started the truck to go to work and turned on the headlights and noticed that the headlights, gauge lights, and any interior lights that were on would fluctuate from their normal brightness and then dim for a second and then go back to their normal brightness. The dimming and returning to bright happens about once a second. The voltage gauge shows that the voltage is going from a little over 14.4 volts down to around 13 volts and then back again. It does this for about the first 15 to 20 minutes of my drive to work and then goes back to normal until the next time next time I start the truck and it does it all over again.

It reminds me of how the lights would dim for a few seconds when the manifold heater would cycle when it's cold. However, the heater has been removed and is no longer in the circuit.

Any help would be appreciated.

 

[jws123]

test batteries and alternator one of the 2 are failing how many miles on it?

 

[Rowland]

The truck has 135K on it. I'll check the alternator and the batteries. Thanks.

 

[inuyasha-rules]

Not sure about the Cummins engine, but I know the 5.7 gas will cycle off the alternator at idle if the voltage is high enough to squeeze a bit more fuel economy. Since it does the dimming while driving I don't think that's the cause, just something to be aware of. My 5.7 does this with the AC or heat on every 3-5 seconds at idle.

 

[Rowland]

Thanks. We'll add it to the list just in case.

 

[HemiLonestar]

Cummings lol

 

[Rowland]

Beer, then type - Cummings

Type, then beer - Cummins

Will have to post then toast.

 

[Smokeybear01]

I definitely would have suspected the grid heater to, but since you already excorsised that puppy, it has to be a ground that's corroded enough to cause resistance until the batts are back up from the starting procedure. If our alternator has a way of cutting out to save fuel it's news to me haha. Cummins don't need to save fuel, we already get twice the mileage of those gas hog hemi's haha.

 

[Rowland]

The grounds are on our list. Going to be a long slow process. They're hidden everywhere.

 

[Smokeybear01]

The grounds are on our list. Going to be a long slow process. They're hidden everywhere.

I'd start with the big heavy wire where it attaches to the frame on the driver's side. Mine was really corroded there. And if you're going to start pulling and cleaning connections, make sure you got a tube of dielectric grease handy.

 

[inuyasha-rules]

If our alternator has a way of cutting out to save fuel it's news to me haha. Cummins don't need to save fuel, we already get twice the mileage of those gas hog hemi's haha.

It only adds a fraction of a percent to the fuel economy on a hemi, but because of how federal CAFE efficiency standards work, it's worthwhile for manufacturers to chase. It's just software code turning off the exciter coil if the voltage is high enough, so not much cost to copy the code to other platforms if they wanted to.