I have been working on my nephews 95 S-10 Blazer,When it came to me it had a bad trans and a busted heater core.I pulled the 4L60 and had it rebuilt,put it back in and drove it around for a day or so before tackling the heater core.

Since I put the heater core back in it will not keep a battery charged.I have had the alt tested and it tested good (14.5 volts) but it would'nt keep the battery up.Bein skeptical I put on a used on that also tested good(14.5 volts) and it was no better.It had a red top Optima on it that looked fairly new and I felt it was good but I was tryin to cover all the bases so I put a battery that I knew was good and it did the same thing.With the engine running you can put a meter on the battery and watch it drop voltage as it runs.I undestand that it will do that since it's pullin voltage out of the battery and none is goin back in.It will run for 15-20 min before goin dead.The voltage on the alt post is/will be the same as the battery everytime.

Is there anything behind the dash that I could have missed that would keep the alt from charging???Seems awful coincedental that it happened after I had the dash apart.Any help is appreciated as he is coming Fri to get it.I did a search but none of the results seem to fit my symptoms.

 

If you put a meter on the battery, it should read 14.5 volts that the alternator is feeding into the battery to charge it. If it reads less than that, you know you have a faulty connections.

Is your alternator/battery light on in the dash? If it is, that would probably be the only thing you could have screwed up. Because voltage does go from the alternator to your cluster to light it up if there is a short in the circuit. The way it works is that there is voltage going from both sides of the lamp load that lights up in the instrument panel to tell you that there is a problem with the charging system. Since the same voltage is going to the load from both sides, they hit a wall because they have the same force, and since there is no difference in potential, no current flows, and the light does not light. It only lights if there is unequal voltage on either side of the load because it allows current to flow. It may be possible that there could be a short to ground on the alternator side of that lamp that could be affecting the charging system.

Hopefully this will help steer you in the right direction.

 

did you check how AMPS the alternator is putting out? you should check that first....

 

That opens another can of worms.When I got it none of the gauges worked.The only thing in the cluster that works is the gear indicator.

 

I did not,how can I do that???

 

You can't do it with a meter because you will blow your fuse cuz it only goes to 10A. The only ways I can think of is to get a VAT or a microVAT and do a load test.

 

you can pull out the alternator and take it to a alternator rebuilder and they will check the amp output for free. just make sure you ask if they charge to test that for free just incase they dont. the good alternator rebuilders do it for free. it should take about 15-20 minutes to pull out your alternator and is pretty easy... if you have a vortec motor you should have the 100 amp alternator....