Community Forums

TheDubs · 07-26-2010, 03:20 PM

Hey guys I have a ? I think the alternator in my MH went out the volts jumped down to 12 when the engine was on. I took it to a auto parts store and they tested it and it was bad. Which leads me to my question. I replaced the alternator with a 78 amp one, and also replaced the isolator cause it was going out too and put a full charge on the battery's. When we went to get on the road today the engine batts dropped back down to 12.65 volts and stayed there. And the coach batts would fluctuate between 12 and 13.5 depending on when the fridge and other appliences kicked on. It seems to me the alternator is working but can't Keep up. Both sets of batts should keep at a continuous voltage of 13.5 if Im not mistaken. Im thinking I need a bigger alternator that puts out more amps like 165 or so. Do you guys think its the alternator or a different problem? On a side note I called around to see if anyone has a higher output alternator and the biggest one I could find was the one I installed. Thanks in advance.Justin

DriVer · 07-26-2010, 03:40 PM

Quote:

Originally Posted by TheDubs

... Thanks in advance.Justin

Justin, I just have to ask ... what is the condition of the battery?

mick · 07-26-2010, 04:03 PM

Either the batteries or get an outside voltage regulator.

Phobos · 07-26-2010, 04:21 PM

If you are talking about one of the Chrysler square back alternators, an alternator shop can bump those up to 100 amp by changing the stator in them. That's about as high as you can go with those alternators on amperage.
I went through the same thing you describe on a pickup truck with these things, I never was satisfied, and eventually went to a GM alternator.

bigskymt · 07-26-2010, 05:03 PM

You didn't mention what chassis or engine. If it's a P-30 Chev. you can go at least 100amps. Whitney may be a good place to research.

TheDubs · 07-27-2010, 11:49 AM

Driver the battery's are 6 months old and hold a charge. Hey Mick what would a voltage reg do? Bigskymt its a p30 chassis with a chevy 6.2 diesel in it. I drove it from Peoria IL to Madison WI today the coach batts stayed at 13.25 and the engine batts would stay at 12.65 unless I kicked on the a/c and it would drop to 12. But when I turn the a/c off it would jump back up to 12.65 again, so I know the alternator is working. Do you think a 100 amp alternator would be enough? or should I try and find a bigger one?

InfinityJim · 07-27-2010, 12:18 PM

Bigger! But don't forget basics, check all connections and replace any relay's that are caring supply current. Contacts in them do wear out, and their inexpensive.

bigskymt · 07-27-2010, 05:21 PM

If you want to know if a larger alternator will have any difference try hooking the alternator direct to first the coach side of the isolator then the motor side and measure the voltage at both(or disconnect one side). This will also direct you to any possible problems either coach or engine.
Seems strange the voltage drop from coach to engine.
Can you compare the output from the alternator to the isolator then outputs from isolator to batts?
After running for awhile feel the battery cases for heat, excessive heat will indicate a bad battery.
I seem to remember the diesel version of your P-30 came standard with a 100 amp. alternator.
FWIW: the 78amp alternator was famous for loosing about 30% of its output after significant use (like a long day driving in heat). It was never designed to run "full field" which it does in a M/H with extra batteries.
I purchased a 100 amp from Whitney several years ago, it outlasted all the 78amp Pep Boys w/lifetime warranty.

TQ60 · 07-27-2010, 07:33 PM

First, the alternator voltage regulator is going to try to keep the voltage between 13.6 and 14 or so volts

The isolator is a couple diodes to isolate the batteries, each has a 0.6 volt drop, so the alternator leg is 0.6 VDC above each of the battry connections.

So the regulator needs to be adjusted to match, if internal to the alternator then batteries both will be too low, if external to alternator and connected to the engine battery electrical then it will compensate.

Next, you indicate the two battery systems have different voltages, one close to normal and the other too low.

First turn off all loads and run the engine, the voltage should be the same on both terminals if both batteries are charged, if not then one battery could be really dead, and the isolator may not be rated for the amount of current required to bring it up.

To save gas, place charger on each battery to fully charge them and re-test, if voltage still different then isolator may be bad, reverse the isolator to see if the problem follows.

Turning on the AC is another clue, the clutch and fan loads are not very large, if the engine RPM slows this could drop the voltage, but if the engine RPM is steady then look for voltage drops across the isolator, any switches or connections, the drop in voltage across a connector indicates bad connection, voltage drop across the isolator will always be very close to 0.6 volts.

Once all of this has been confirmed as good working order then consider your power needs, you canot have too large of an alternator within reason...

There may be simple bolt on upgrade where the mounting arrangement of the alternator is the same but the output is much larger, remove your alternator and make a trip to a local REBUILDER, these folks have forgotten more than most counter parts folks will ever know, they will be aware of any other units that could easily be fit in the same mount or used with little modification, they can swap the pullies to insure correct one.

Also confirm while you are there the regulator type and adjustment procedure and any wires that need to be upgraded, the output wire should be upgraded to #4 minimum, the rebuilded can provide this as well.

TheDubs · 07-28-2010, 11:49 AM

I was talking to my uncle who owns a auto shop. He said he can find me a 140 amp single wire alternator used for ambulances. my only concern is the wiring harness that plugs into the alternator is there for a reason. Do you guys for see any problems bypassing the harness? My uncle seems to think there wouldn't be a prob. The wiring harness looks like it has a positive and a negative wire coming out of it but I haven't tracked them to see where they go yet.

RedneckExpress · 07-28-2010, 04:36 PM

How many batteries you running Dubs? I dumped my lower amperage alternator when I upgraded to 4 batteries (3 deep cycles, 1 starting) and went up to a 130 amp (highest I could afford at the time).

I believe Chevy Alternators, at least with the 85 350s, had a little flat blade plug that connected to the alternator as well as the one big single wire terminal.

The little blade plug was how the Diodes were energized, I believe the voltage regulator was internal to that particular generation of alternator. The big lug was the charging connection.

bigskymt · 07-28-2010, 06:08 PM

Whitney also made a one wire alternator that supposedly worked good.
The flat plug redneckexpress is talking about provides 12 vdc to excite the fields on one side, the other side would feed your "charge" light to indicate no charging...........you probably don't have that with a voltmeter but the wire harness includes it.
I've seen many one wire alternators in Marine use.
There is no negative to the alternator as it grounds through the case to the engine block.
You may want to make sure the one wire alternator doesn't require a separate regulator.
The single wire on the rear with about #10 wire is output.
I believe the Ambulance alt. is also a low cut in, that being it will produce more output at a lower RPM........same as Police Package.

TheDubs · 07-29-2010, 02:14 PM

Hey red I have two engine batts, and two coach batts. Im going to get to my next destination and get the alternator there. I'll keep you guys posted. Thanks again for everyones info.

RedneckExpress · 07-29-2010, 02:26 PM

You're definitely gonna need the bigger alternator. The ambulance one would be my vote, along with an upgrade to 4 gauge for the primary charge wire leads going to and from the batteries.