When the vehicle is running I will get a drop in my volt guage from 14V to 10V(The "check gauges" light will come on but not always). When this happens it puts a charge on the passenger side battery of 18V and only 10V on the driverside battery. I have tested all connections, both batteries are new and tested good, the alternator tested good as well. I was told that I need to test both terminals going into the ECM and if those check out ok then the ECM needs to be replaced.

This had only happened a couple times for less then a day previously...Now it is constant and I almost blew a battery yesterday. After a few miles of driving the battery got so hot it was pouring out vapor and spitting out acid from the vent hole.

 

Typically, it is caused by a weak battery, or a bad connection between batteries. You said both batteries were new? Check your jumper wires ( the ones going from first to second battery) for loose or dirty connections.

 

Ive checked all connetions, checked resistance on all cables, run full diagnostics on both batteries and the alternator and everything checks out fine. A troubleshooting checklist I was using gives me one more option to check, which is to diconnect the terminals going into the ECM and check voltage readings on each pin and if that checks out the ECM needs to be replaced

 

Your batteries are in parallel!!! There is NO WAY the ECM can over charge on and under charge the other.

When the battery tests are done, is it removed from the truck electrically?

If not you are testing both together.
You have either, a bad battery, or a bad cable, or a bad connection.
I found the passenger battery ground on my truck was very loose(at the block) after some trouble shooting.

 

Your logic is the same as mine man...when I had the batteries tested I mentioned removing them but the guy said no and it did give a printout for both batteries individually. And being that they are in parallel I cannot comprehend how it works but in my research there is a voltage regulator(which is located inside the ECM and not servicable ofcourse) that assigns 14V from the generator to each battery. This makes no sense to me though, considering the alternator, though it does have a connection to the ECM, has a direct connection to the adjacent battery only. How could it possibly send two different voltages through the same connection?!!!

I'm going to try swapping the batteries tomorrow...Right now I get an 18V reading off the passenger side and a 10V reading off the driverside. So if it's a bad battery those readings should stay with the batteries when swapped right?

 

Dead cell on the drivers battery is my guess. Make sure all the grounds are good and clean as well.

 

Well I Disconected both batteries, charged them both up individually. Removed, cleaned, and coated every connection with dielectric grease and so far so good guys. Thanks!