Size of Alternator

Hi,

I think you may find that is 4 amps at 120 volts, not at 12 volts.

thriftydutch wrote:
It says that the fridge on 12v draws 4.0 amps.

Clean all the terminals and cable ends as a first step. Then see what happens.

They did offer a 90A and a 120A in 1990. Take your old alternator down to your auto parts store and compare them. The 90A should be a simple swap, but I'm not sure which 120A would bolt up. Also, as mentioned, check for excessive Voltage drop between the alternator and batteries. Even cleaning the terminals and cable ends can help.
Good luck.

It will take a very long time to fully charge the batteries at less than 13 volts. I would hook up a voltmeter to the cigarette lighter and keep an eye on it while driving. It should show at least 14 volts most of the time. A DC clampmeter would allow easy measuring of the current flowing into a battery. If only 1 amp it would take 115 hours to charge the 230 amp-hour battery from 50%.

Could be a tired alternator, battery or trouble in the wiring. Try disconnecting the house batteries and see if the engine battery gets higher voltage and more current. Try a known to be good battery, have the battery load tested, compare voltage at the engine to voltage at the house batteries - voltage drop larger than half a volt: try to find the poor connection.

Some rigs like you have uses a diode based isolator. Some have as much as a half of volt drop across them.
I would suggest checking the one out that you have.
I would change it to a relay based style.
One like this with the control wired to a point that is hot in run.
Battery switch

More likely you have excessive voltage drop between the alternator and the house battery.
May need to get a different isolator and/or enlarge the cable.
Need to locate the components in this circuit and give them a poke with your voltmeter to find where the issue is.

75amp output rated when cold and moving at our near maximum rpm. Almost all alternators are cold rated. I have a welding alternator that's hot rated. When the alternator warms up to operating temperature it produces less power. When warm it will never produce 75amps.



On average the engine takes 20amps to run. Headlights 20amps. Hvac 20amps.

Your alt should charge them OK but an even bigger one may charge them faster.
While the engine is running, is the voltage at your house batteries close to the chassis battery voltage? And what is that voltage?
I ask because it sounds like the house batts aren't getting much if any charge. You may have a bad diode in your alt. and it's limiting how high the voltage gets.