I had my cranking batteries both go bad due to them being overcharged from the alternator. I checked voltage coming from alternator 17-19 volts. Replaced alternator and was shown on machine new one was putting out perfect. Replaced all batteries cranking and house and when checking batteries while idling its still overcharging house batteries 17-19 volts and my cranking batteries are getting no charge. I found this paper in the coach it seams like they may have had this problem before, can anyone make any sense of the diagram drawn on the paper?

Back diagram

Your diagram shows an isolator in the charging circuit. The purpose of the isolator is to allow the alternator to charge both coach and start batteries independently without allowing them to feed to each other. If the isolator is working correctly you should see full alternator output voltage at the center terminal, coach battery voltage at one end and start battery voltage at the other end, when the alternator is running. Each end of the isolator is connected through a diode, which allows current to flow in only one direction, ie from center to end. These diodes can fail by shorting or going open.

When the alternator is not running, the center terminal voltage will drop, leaving the coach and start batteries isolated. If the convertor/charger is operating, the coach batteries should show the voltage supplied by the convertor.

If your start batteries are not getting charged I would suggest that side of the diode has failed open, ie. no connection.

Meanwhile, many of the old 12V convertors used for charging coach batteries were notorious for poor voltage control and often overcharged, ie 17 volts.

If you locate the isolator, you can measure at each end. When alternator is not charging, the start batteries should be about 12.5 volts. If the convertor is operating, the coach battery end should be charging, but no more than about 14.5 volts. The center should be 0 volts.

When the alternator is charging, (assuming the convertor is not plugged in) we should see full alternator voltage at the center (about 14.5 volts) and about .6 volts less at each end. (The voltage drop across a diode is usually about .6 volts).

If the alternator has a voltage sense wire, it should be connected to the start battery end, downstream of the diode. That way it senses the actual battery voltage, after the .6 volt diode drop.

In your case, it appears the start side of the isolator may be open and the coach side is being overcharged by the convertor.

Try removing the wires from the isolator. Connect the center alternator wire and the starting battery wire. With alternator running, you should see normal charging. Meanwhile check the coach battery wire for the 17 volts.

When you measured the voltage at the alternator , was the coach plugged in to shore power ?

Not plugged in just with engine running

For reference tha alternator I have only puts out 14.4 volts. Thought I might need to replace when I went with lithium batteries, but was fine. I have a Delco with internal controls.

The alternators with internal regulators are set up to produce a specific output voltage at the output terminal. If the connection from the alternator to the battery is of adequate size the voltage at the alternator and the battery will be pretty close. Very simple system. If the wire goes through an isolator, the voltage to the battery will be .6 volts less.

If the alternator has a sense wire, it is designed to check the voltage directly at the batteries. However, if the wire is grounded, the alternator will run wide open to try to bring the voltage up to the normal level. You could easily see 17V or more. Check your alternator harness wires.

After looking everything over I noticed when I replaced the house batteries I accidentally connected the wire that was suppose to be disconnected. So now I'm no longer overcharging but I'm only reading 12.5 or so coming from my brand new alternator. Also noticed alternator gets fairly hot is that normal?

The alternator can get hot , but it should be putting out around 14 volts.

The wire you disconnected , what is it's function ? Is this the sense wire Mr Steve was referring to ?

I'll have to look at ours tomorrow and see how ours is setup . It should be similar to yours .

In the first diagram I posted there is a wire going from converter to house batteries. It says to disconnect and replace with another wire from converter to a solenoid type deal im guessing, which I'm sure the previous owner did. I'll go over all wiring again to make sure.

Ours looks like a standard GM alternator . A single post on the back and a two wire plug on the top .