Truck runs low volts when i run my blower and lights.

[Coogan88]

Any ideas to solve a problem with my voltage goin down to almost 12 when I run my blower motor and lights at low rpms. My truck has been swapped with a 302 but I think the alternator is only 70 amps. I recently put a big sound system in the truck that doesn't help to this problem. Will a higher amp alternator help?

 

[RonD]

Any other "working" alternator would help

You have at least one bad Field in your current alternator, maybe two

There are 3 Fields in the case of an alternator, each has a pair of diodes that change the Fields generated AC voltage to DC voltage
So the 3 Fields in your 70amp alternator generate 23amps each, and that's when engine is at 1,800rpms, at idle alternators can only deliver about 60% or rating so in your case about 42-45amps

If a Field fails then you lose 1/3 of the amps BUT voltage regulator will try to increase the amps/voltage to compensate
Most see that as dimming headlights at idle which is not a big deal since we don't spend alot of time at idle
If a 2nd Field fails then you will really notice it like you are seeing now, there is just no way 23amps is going to run all vehicle electrics
Most vehicles require 40amps with all lights on and blower on high

So yes swap in a 90 or 120amp alternator, doesn't hurt to go higher than needed, alternator just doesn't have to work as hard at idle

 

[Curious Hound]

Yes, a higher rated alternator will probably help. It's normal for the alternator to not provide full rated output at idle - maybe about 70%. What voltage do you have without turning on a bunch of electrical loads?

Edit; + everything Ron said.

 

[Coogan88]

Any other "working" alternator would help

You have at least one bad Field in your current alternator, maybe two

There are 3 Fields in the case of an alternator, each has a pair of diodes that change the Fields generated AC voltage to DC voltage
So the 3 Fields in your 70amp alternator generate 23amps each, and that's when engine is at 1,800rpms, at idle alternators can only deliver about 60% or rating so in your case about 42-45amps

If a Field fails then you lose 1/3 of the amps BUT voltage regulator will try to increase the amps/voltage to compensate
Most see that as dimming headlights at idle which is not a big deal since we don't spend alot of time at idle
If a 2nd Field fails then you will really notice it like you are seeing now, there is just no way 23amps is going to run all vehicle electrics
Most vehicles require 40amps with all lights on and blower on high

So yes swap in a 90 or 120amp alternator, doesn't hurt to go higher than needed, alternator just doesn't have to work as hard at idle

Thank you very much! I will be swapping a new alternator in then