Skip to main content ↑ ↓
Topic: Alternator install (Read 818 times) previous topic - next topic

Alternator install

Just had my alternator rebuilt. We found that the voltage regulator was bad. This was my fathers old coach and I found notes in the manual where he had had previous issues with the alternator. Since it had been off before I want to make sure that it was wired correctly when he did it.  Plus (+) and Minus(-) are a no brainer... There is a Voltage Sensing Terminal and an Ignition Terminal.. On my wiring diagram the exciter wire runs from the #4 post of the isolator to the Sensing Terminal and supplies voltage to the alternator all the time( ignition off or on). There is no mention on the diagram of the ignition wire. There is a wire that is hot only when the ignition is on. Is the correct assumption that it goes on the Ignition Terminal?
Any clarification would be appreciated.
94 GV U-280 8.3 
Karl Shurtleff
Kingwood, TX
94 U280 Grand Villa 36'
Build #4487

Re: Alternator install

Reply #1 –
Just went through this. Hope your rebuild goes better than mine.  On my 98 model there are two lugs other than main + and - lugs.  These two lugs were not labeled on my altenator but the sense wire with voltage on it all time went to small lug closest to + side of altenator.  The wire hot only with ignition on went to lug closest to - terminal.

Larry B
1998 U 270
Larry Bradley
SOB
former 1998 U270
Build 5251
Jeep Grand Cherokee

Re: Alternator install

Reply #2 –
Quote from: Larry Bradley –
Just went through this. Hope your rebuild goes better than mine.  On my 98 model there are two lugs other than main + and - lugs.  These two lugs were not labeled on my altenator but the sense wire with voltage on it all time went to small lug closest to + side of altenator.  The wire hot only with ignition on went to lug closest to - terminal.

Larry B
1998 U 270

That is what I did...problem is I am not getting any output from the alternator... I pulled the cable off at the #2 pin of the isolator and I have nothing. I guess its back to the alternator shop!
Karl Shurtleff
Kingwood, TX
94 U280 Grand Villa 36'
Build #4487

Re: Alternator install

Reply #3 –
Karl,

When you say you pulled the cable off the #2 terminal, is that the one from B+ on the alternator? 

You do NOT want to run the engine with the B+ "open".  That will blow the diode(s) in the alternator. Same happens if the diode in the isolator "opens"-- it acts the same as feeding an open circuit.

To test, just leave it hooked up, verify the sense wire has a good connection, put engine RPM at 1000 and check voltage at both the isolator center lug and both the outer ones (that go to the chassis and house battery banks).
Brett Wolfe
EX: 1993 U240
Moderator, ForeForum 2001-
Moderator Diesel RV Club 2002-
Moderator, FMCA Forum 2009-2020
Chairman FMCA Technical Advisory Committee 2011-2020

Re: Alternator install

Reply #4 –
Is there a procedure to check the isolator? Should I have no voltage at the center lug ( Alternator) unless the engine is running? I assume that there is a diode between the start battery  and alternator and another diode between the house and alternator connection?
Karl Shurtleff
Kingwood, TX
94 U280 Grand Villa 36'
Build #4487

Re: Alternator install

Reply #5 –
Voltage with engine off: Two outer lugs having same voltage as their respective battery bank (one chassis, the other house)

With engine running and around 1000 RPM, voltage at center lug in the mid to high 14's.  Voltage at the two outer lugs about .7 VDC less (13.5-14.2).
Brett Wolfe
EX: 1993 U240
Moderator, ForeForum 2001-
Moderator Diesel RV Club 2002-
Moderator, FMCA Forum 2009-2020
Chairman FMCA Technical Advisory Committee 2011-2020

Re: Alternator install

Reply #6 –
I think that I have a bad isolator. I have start battery voltage with engine off at the alternator lug.  I disconnected house and start batteries from the isolator and this is what I have:
From alternator to start batteries 580 ohms....not .58
From start batteries to alternator ..the same thing 580 ohms
From alternator to house batteries 0  ohms
From house batteries to alternator 0 ohms.

Karl Shurtleff
Kingwood, TX
94 U280 Grand Villa 36'
Build #4487

Re: Alternator install

Reply #7 –
Karl,

Post voltages as I asked.  With diodes (isolator) ohm readings can be misleading.
Brett Wolfe
EX: 1993 U240
Moderator, ForeForum 2001-
Moderator Diesel RV Club 2002-
Moderator, FMCA Forum 2009-2020
Chairman FMCA Technical Advisory Committee 2011-2020

Re: Alternator install

Reply #8 –
Quote from: wolfe10 –
Voltage with engine off: Two outer lugs having same voltage as their respective battery bank (one chassis, the other house)

With engine running and around 1000 RPM, voltage at center lug in the mid to high 14's.  Voltage at the two outer lugs about .7 VDC less (13.5-14.2).
Her is what I have:
Engine Off
House 14.4
Alternator 13.0
Start 13.1

Engine On 1200 RPM Gallon jug propped against peddle :)
House 14.4
Alternator 2.5
Start 13.1

Exciter 13.1

I have also noticed start batteries draining in a relatively short time. They are brand new AGM's and since they have been draining down I have kept them topped off with the boost. Should there be power to the alternator lug with the engine off?
Karl Shurtleff
Kingwood, TX
94 U280 Grand Villa 36'
Build #4487

Re: Alternator install

Reply #9 –
Just replaced mine if you need a number, no connection just knowledgeable guy and great service.
Jimmy
2003 U295 3620

Re: Alternator install

Reply #10 –
Quote from: Karl Shurtleff –
Here is what I have:
Engine Off
House 14.4    Good
Alternator 13.0    Good (Alternator terminal will have backfed voltage through the alternator windings and the Alternator voltage regulator.  This is a STATIC voltage charge that just "accumulates" there and has no way to drain to the negative side of the batteries, until engine start and run circuits are completed,
Start 13.1    Good, but low if the Boost switch had been closed prior to your taking these voltage readings.  With the boost switch closed, this should quickly become the same as the house battery voltage, as the start batteries accept the house battery charge.

Engine On 1200 RPM Gallon jug propped against peddle :)
House 14.4  Good, but also not good.  It should have changed a bit in reaction to the sensing voltage line being low at 13.1 Vdc, and with the engine running, it should have changed.
Alternator 2.5  Not Good.  With all ground, sense, ignition (excitation), and power lead wiring being correct, this is the classic indication of an Alternator failure or Voltage regulator (inside the alternator) failure.
Start 13.1  Not good.  This should have jumped up because the Alternator should be trying to get to the start battery up to 14.0 (to 14.2) Vdc

Exciter 13.1  If you mean "Ignition" per your first pictures,  this is good as (with proper wiring) it should read the same as the Start Battery voltage.

I have also noticed start batteries draining in a relatively short time. This may be of concern.  Not sure what you mean by "draining in a relatively short time".  Without boost or some sort of battery maintainer being connected (Good battery maintainer is highly recommended), good start batteries should quickly decay to around 13.6 Vdc -- after the coach is run (or the Boost switch is opened). Then they shouldn't decay more than a tenth of a volt dc per day for many days.  More than that and you need to open all dc circuits (pull fuses), install an ammeter on the battery (+) positive lead and then install fuses one by one until you find a circuit that has a high current (anything over a few milliamps).  They are brand new AGM's and since they have been draining down I have kept them topped off with the boost. Should there be power to the alternator lug with the engine off? (Answered above -- not really power but rather static charge that can't drain off).
Karl,
I'm not Brett, but the above is my 2 cent's worth:
HTH,
Neal
The selected media item is not currently available.
Neal (& Brenda) Pillsbury
'02 U320 SPEC, 4200, DGFE, Build #5984
'04 Gold Wing
'07 Featherlite 24'
'14 Jeep Grand Cherokee Summit
MC #14494
Exeter, NH & LaBelle FL
Quality makes the Heart Soar long after Price is Forgotten

Re: Alternator install

Reply #11 –
Thanks Neal! I have pulled the alternator and will run it back to the rebuilder on Saturday. They just replaced the regulator...Not sure what the problem is....
Karl Shurtleff
Kingwood, TX
94 U280 Grand Villa 36'
Build #4487

Re: Alternator install

Reply #12 –
Quote from: Karl Shurtleff –
I think that I have a bad isolator. I have start battery voltage with engine off at the alternator lug.  I disconnected house and start batteries from the isolator and this is what I have:
From alternator to start batteries 580 ohms....not .58
From start batteries to alternator ..the same thing 580 ohms
From alternator to house batteries 0  ohms
From house batteries to alternator 0 ohms.
High current diodes are difficult to test with a standard multimeter because they can have a very small reverse leakage current, which is normal. The best way to test an isolator is to use a small 12v light bulb like a 1156 bulb, which draws a couple of amps of current. Forward voltage should light the bulb, reverse voltage should not light the bulb. If the light either doesn't light either way, or lights both ways, the isolator diode is bad.

The 4 post isolator you have has 3 diodes, all 3 should be checked separately.
Peter and Tammy Fleming
1991 U300 GV 40 - Sold, owned for 4 years
Downsized to Roadtrek Popular 210 class B
 

Re: Alternator install

Reply #13 –
My rebuilder ran the rebuilt unit on a 1930's made test machine.  Bakelite. Made in Chicago. Swing needles.  Big knobs

Full load was dialed in. 

Voltage adjusted running
"Riding and rejoicing"
Bob
1997 U320 40' Mid entry, build 5132,  wtbi ce27, 4th owner
2007 Solara convertible
2 prodeco tech outlaw ss electric bikes

1095 watts solar
08 Ls 460 and a sc430
2000 Ford F-250 superduty 4x4