Size of Alternator

Okay, I have solved the problem of no charging of the house batteries. It turn out that I did not have a proper ground. When I installed a proper ground the charging read 14.4 volts. Did the job myself and saved money. It did not need a new alternator or isolator.
Thank you all for your help.

My bad. I was unaware that some double digit IQ RV OEM would dare fit a tiny 75 amp Mitsubishi alt to an RV. 75 amperes is barely enough for a car never mind an RV. Today's fuel injection pumps suck 12-18 amps. Deduct headlight, and running lights, throw in a working chassis A/C unit and a person would be fortunate in itself to have -any- spare amperage. The 75 amp Mitsubishi does not have the same mounting as the 40/90 Bosch or Mitsubishi. The 50/120 Mitsubishi can replace either the Bosch or Mitsubishi 40/90 amp but the pulleys cannot interchange. I bought a drawerfull of 100 watt 10 ohm power resistors to fool the ECU regulator. But my eye is on a Ford 175 amp alt. I will strip off the reg and convert it to an external reg.

NONE of the above is do-able for the home mechanic.

Find a large automotive electrical rebuilder shop and let THEM puzzle out how to fit a more powerful alternator. Is everyone here offering advice SURE this alternator has an ECU regulator? It amounts to a bunch of hunches and guesswork until the job has been surveyed by a professional. Alternator, isolator and wiring need to be eyeballed by someone who knows their business. But I will offer this without hesitation - rectifier based isolators have lots of fins to contribute to global warming. They are a loser. Integrated one-piece electronic relay isolators are the way to go.

A 75 amp alt is a loser as well. Like a Fire Truck using a garden hose.

Are there competant rebuilders out there any more? Or is it a dying art?

Double post

thriftydutch wrote:
If I install a 120 amp alternator can I install a 90 amp isolator or does the isolator have to be the same amps as the alternator.

That would depend on how low your batteries get
If they ever get discharged enough to accept over 90 or even close and you Riv up the engine to full alternator output, you will burn out the 90 amp diode isolator

It is always best to have an isolator, or relay with contacts
Of a greater ability than the alternator
This way there can not be any awe ##### oopsie moment requiring repair

I don't think you need a bigger alternator.
I hope that is a relay based isolator not diode based.
Voltage is the issue more than amps.

If I install a 120 amp alternator can I install a 90 amp isolator or does the isolator have to be the same amps as the alternator.

Does your fridge not work on propane? If it could use propane instead of electricity it would save a lot of the electricity requirement.

You can easily find out if the battery isolator (preventing the engine and house batteries being connected while the engine is not running) is reducing the charging voltage at the house batteries. Just connect a wire from the input terminal to the output terminal of the isolator and see if the charging voltage increases. I have a small class C that uses the Ford auxiliary battery relay in the engine fuse box as the isolator. Last summer I pulled it out and replaced it with a piece of wire. I put it back when boondocking for a few days. It's purpose is to prevent the house from discharging the engine battery. This system only contributes about 5 amps to charging house batteries.

Thank you all for the good advice everybody has given. I will start using this advice when the weather gets a little warmer. At least now I am not looking at the problem blindly.

I see in the OP that the Vehicle is a 1990 dodge, essentially the same as my 89 in terms of alternator and voltage regulation.

If the circuit between your alternator/ isolation method, and house battery checks out good, which I do not believe it will based on your reports, Tricking the engine computer and using external voltage regulation is an option I can detail.

The higher rated alternator with the same voltage regulation/isolation method/product will only improve performance in a very narrow field, Like when cold at idle speed.

Not sure what a ford 3g alternator has to do with this discussion.

Much depends on the circuit thickness/distance itself and the isolation method, but cheap and dirty with a old solenoid hooked to engine battery is a recipe for poor house battery charging.

Also the house battery is likely grounded to frame. Frame grounds become problematic and highly resistive all too quickly, and dodge are notorious for ground issues. Stock, there is one 6 awg ground cable from engine battery to engine and one 10awg ground from engine battery to firewall. At 28 years old if this 6awg cable is original it insults each electron it passes.

If house battery is grounded to frame, then all alternator current to house battery has to flow over this 10 AWG ground wire, then down 6awg cable to engine block.

Upgrade grounds, or add a cable between frame and alternator mounting bolt and let those amps flow, instead of gettng bottlenecked by the OEM circuitry.

I did upgrade my original alternator from the lesser rated model when it failed, but the weak point is the voltage regulator and the poor ground path that a frame grounded house battery has to deal with. The 10AWG engine battery to frame ground is forcing the frame grounded house battery... is forcing the marathon runner into breathing through a cocktail straw.

The OEM voltage regulator in the engine computer will seek either 14.9 or 13.7 too. Not much charging happens at 13.7v through old inadequate copper and poor grounds. 14.9v is a bit ridiculous too, and mine would just decide that 14.9 was desirable, when it was not, and 13.7v was the call, when it was not.

There is all sorts of room for improvement of the charging system as there are so many incredibly inadequate areas on this chassis in regards to alternator charging. it might have been fine for the original starting battery that was never depleted and with few additional loads, but is extremely inadequate when more batteries are tacked on to the ned of the Circuit, and that circuit is nearly 30 year sold too.

Beef up your copper and grounds if you want alternator juice to make it to distant house battery.

The VR modification is not Nasa grade engineering, but not for the electrically timid either.

Start with te copper and some experiments with a voltmeter on engine battery and on depleted house battery with engine running, at idle and at higher rpm. Cold and when hot. Expect to find inadequate numbers on depleted house battery.

Simple test. Start with the house batteries partially discharged. While driving at highway speed, check the voltage at the house battery. It should be >13.5, >14.0 would be better. If it is low, either your alternator is not putting out enough voltage or you have too much voltage drop from the alternator to the house batteries is too great.