strange alternator

[KParsons]

new batteries today,the truck gauge reads 12v. at the alternator 15 to16 volts. seems like its over charging, i took the alternator off and had it tasted at the parts store and it looks ok? what am i missing?

 

[AH64ID]

Have you checked the crossover cable?

The alternator feeds the passenger battery, but the ECM is power off of, and reads, the drivers side battery.

 

[KParsons]

i have a universal cable that i will put on tomorrow, no parts store had one to fit.

 

[KParsons]

i put the cross over on and no diff. the pass side battery is getting warm after 4 min. of run time.could the test the store did be faulty?

 

[AH64ID]

i put the cross over on and no diff. the pass side battery is getting warm after 4 min. of run time.could the test the store did be faulty?

Sure sounds like a crossover cable. The passenger battery overcharging is a dead giveaway.

Have you checked/cleaned the connections on the drivers side battery?

The next thing to check is grounds.

 

[KParsons]

all connections very clean. the new cable is on . the engine ground is clean too.

 

[AH64ID]

What does the voltmeter say on either battery with the engine running?

 

[DavidC]

New battery's does not mean good battery's. Things happen.

 

[KParsons]

almost 16 volts. its an old volt meter, not digital

 

[AH64ID]

almost 16 volts. its an old volt meter, not digital

On both sides?

 

[KParsons]

this is where it gets odd. the driver battery is showing 12volts, but the cross over shows 16volts

 

[Ozymandias]

Gauge showing 12V can also be a faulty temperature sensor under the driver side battery.
Shy 15V at the Alternator is ok.

 

[AH64ID]

this is where it gets odd. the driver battery is showing 12volts, but the cross over shows 16volts

That’s why it is acting like a crossover cable. A crossover cable makes the batteries act as one, so they shouldn’t ever have different voltages.

If you’re confident that the cable is good than swap the batteries side to side, and see if the issue follows the batteries or remains on the same side.

 

[KParsons]

is the voltage regulator in the alt or external.volts is 16 maybe20? right side bat is over charging big time.

 

[AH64ID]

is the voltage regulator in the alt or external.volts is 16 maybe20? right side bat is over charging big time.

The ECM is the voltage regulator.

The alternator hooks into the passenger side battery, the ECM reads the drivers side battery.

Based on what you have described the alternator and ECM are doing what they should.

Really only 3 options for what you are seeing.

1) crossover cable, 99.99% of the time it’s this.
2) bad battery on the drivers side only
3) bad ECM ground.

Based on you seeing different voltages between battery’s it’s option 1 or 2.

Since you’re confident it’s not 1, you need to have the drivers side battery tested.

 

[KParsons]

new batteries and new cable. where is the ecm ground. thanks k

 

[AH64ID]

new batteries and new cable. where is the ecm ground. thanks k

New doesn’t mean good... or we wouldn’t have factory warranties and the lemon law.

There should be a ground at the block by the ECM, but again the different voltages between batteries while running doesn’t really point to anything but crossover or batteries.

If the crossover and batteries are good there would be a very small, if any, voltage difference between batts... not 4V. The ECM is doin get what it should bases on drivers side voltages.

 

[KParsons]

i do have a load tester, i will try it and see .

 

[KParsons]

both batteries tested good. how can the alternator put out 20 volts or more,and the ecm is good?

 

[AH64ID]

both batteries tested good. how can the alternator put out 20 volts or more,and the ecm is good?

Because the ECM isn’t seeing what the output voltage is. The ECM only reads the drivers side battery, which you have said is low on voltage. It’s doing what it’s supposed to. .. I know you’ve checked it but this issue is VERY rarely anything but a crossover cable.

Swap the batteries side to side, if the problem remains on the same side it’s a crossover cable/connection... regardless of what you’ve tested.