95 TopKick battery drain problem

This truck just had a new alternator installed and replaced the 8 year old batteries. The alternator charges properly.

Because we previously had times when the charged batteries would go dead overnight, I checked the current to ground with the key off. The ammeter immediately read 28 ma. About 20 seconds later it dropped to 18 ma. and then finally settled at around 8 ma. Is this normal? Shouldn't the current drain be zero when the key is off? Is the computer on all the time and draws that amount?

I pulled fuses at the fuse block mounted on the firewall and found two 40 amp circuits where the current was going...around 4 ma each. FYI it has a Cat diesel engine. I'm just wondering how long two big 12 volt batteries will hold a charge with 8 mils of current going to ground.

Thanks for any ideas.

8ma is about what the ecm will draw. That shouldn't make a dent on a pair of fully charged group 31 batts in parallel. What other accy's on the truck? Lift gate, refrigeration unit? Lift gate only being used while truck is running?

Rule of thumb is anything less than 50 ma draw is fine.

That's some driveway you got there.