The relay can be closing just fine, but the house batteries still not hardly charging when driving.
The has happened to us a couple times in the last 10 years. The problem was corroded contacts on the relay such that a too-high resistance in the contacts was drastically reducing the charging current going into the house batteries.
Our solution was to install a new relay to solve the problem both of those times.
I now have a meter right on the cab dash that shows me when this is happening so that I don't get surprised by this happening any more.
