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Old 06-20-2021, 05:27 PM   #1
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Bad battery isolator ??

I have been checking everything out taking volt meter readings ! With reading various threads here and taking notes from everyones advice i kept plodding along. I just won't let it go, plus i really don't want to take it to the rv dealer in 2 weeks !!!

I had the alternator load tested this week. Repair shop said it was good. So today i went out and meter tested it for voltage. I had to bump up the engine to 1600 to get a good reading. At idle ( 500 rpm ) the reading is erratic

Alternator is putting out 14.3

I then traced the positive alternator cable to the rear electric panel alternator stud ( 14.3 )

Then from there to the battery isolator ( the middle connection ) it read 14.3

Then down to the bottom connection that supplies the chassis battery, it read 13.2

From there to the boost solenoid and it read 13.2 as well.

The reading at the chassis battery was 13.2

Now my troubleshooting leads me to think the battery isolator is bad ?

Is this why with the engine running the chassis battery discharges slowly ? It never goes above 13.2

Thanks for any input !
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Old 06-20-2021, 05:48 PM   #2
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Quote:
Originally Posted by Migkiller View Post
I have been checking everything out taking volt meter readings ! With reading various threads here and taking notes from everyones advice i kept plodding along. I just won't let it go, plus i really don't want to take it to the rv dealer in 2 weeks !!!

I had the alternator load tested this week. Repair shop said it was good. So today i went out and meter tested it for voltage. I had to bump up the engine to 1600 to get a good reading. At idle ( 500 rpm ) the reading is erratic

Alternator is putting out 14.3

I then traced the positive alternator cable to the rear electric panel alternator stud ( 14.3 )

Then from there to the battery isolator ( the middle connection ) it read 14.3

Then down to the bottom connection that supplies the chassis battery, it read 13.2

From there to the boost solenoid and it read 13.2 as well.

The reading at the chassis battery was 13.2

Now my troubleshooting leads me to think the battery isolator is bad ?

Is this why with the engine running the chassis battery discharges slowly ? It never goes above 13.2

Thanks for any input !
Take your middle and lower connection off the isolator and bolt them together. Start the engine and check the voltage at the chassis battery. Make sure you rev the coach to 1000 rpm then let it idle. Just to make sure your alt is putting out current. If your battery is now charging at 14.3 then it's the isolator. It sounds like you have it figured out.

Tim
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Old 06-20-2021, 06:00 PM   #3
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No, that kind of voltage drop is normal thru isolators.

The alternator is supposed to have a battery sense wire coming from the chassis battery. It should be one of the small terminals on the back.

That way the voltage drop is overcome.

Here is a simple diagram.Click image for larger version

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Old 06-20-2021, 06:42 PM   #4
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Yes there is a 'sense' wire ( wire with the orange tape ) on the back of the alternator and a small wire that runs with the positive cable running to the positive chassis post. I assume this is all the same wire ?

The small wire is intertwined with the positive cable at the battery lug.

If so how do i check this wire for a fault ?

Pay no attention to the positive alternator cable missing. I was in the process of taking the alternator out for test and just took a pic to remind me of where that small wire went...
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Old 06-20-2021, 07:28 PM   #5
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Make sure its on the sense terminal of the alternator.

Remove it from the alternator and confirm it has battery voltage on it.

Is that a Delco alternator ?
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Old 06-20-2021, 08:00 PM   #6
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This is whats on the engine right now https://www.prestolite.com/pgs_produ...tem=A0014884JB

I will check for voltage tomorrow.

After looking at the rear photo from the website my alternator doesn't look like this.. So i will look tomorrow and see if it is a different alternator. I am just going by the maintenance records and it showed it was replaced 14 yrs. ago with the prestolite model as described above. ugh !

Luckily its not to difficult to remove !!!

Thanks
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Old 06-22-2021, 01:28 PM   #7
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Ok..got out to the coach today. Yes...it is a Delco Remy SSI22

Pics will show the wires ( 3 ) that are attached to the alternator. The negative cable is off and only has the the attach bolt for ID.

One pic is of the chassis positive cable. Not sure where the red wire goes to that comes off the inline fuse connection ? Could this red wire be the 'sense' wire ?

Don't know where the gray wire that is on the 'R' terminal of alternator goes to ? Maybe the ignition ?

The terminal to the right of the positive stud has no wire on it but it shows 'I' on the case.

No idea why the the red and black wires are connected and taped ?

The red black and gray wires all run in the same protective wrap.


Thanks
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Old 06-22-2021, 01:49 PM   #8
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The Delco is a self exciting alternator, so its not seeing actual battery voltage. That explains the chronic under charging.

There is a plug on the side that needs chassis battery voltage to terminal # 2. The plug is sold everywhere.

One of the taped up wires may be it.

Here is a diagram.Click image for larger version

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Old 06-22-2021, 01:58 PM   #9
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If that dont work out, check this website for the 28SI.

https://www.qualitypowerauto.com/ite...lternators.htm
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Old 06-22-2021, 02:03 PM   #10
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I am not used to seeing a 1.1 VDC drop across a diode-based battery isolator (14.3 to 13.2 VDC). 7/10th VDC drop, yes.


Anyone else experiencing over a full volt drop across their isolator?


Agree, this is not the prime issue, just one of curiosity.
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Old 06-22-2021, 04:10 PM   #11
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Yes..i found the plug ! Nothing is plugged into # 1 or # 2

I did check for voltage on the red wire that was cut off. It read 12.40..same as chassis battery voltage. Not sure what the black goes to, or do i need to hook it to the alternator negative post ?

Now i did take the red wire that goes through the inline fuse at the battery and disconnected it, then went back to the cut off red wire and that wire still reads 12.40. That tells me that red wire off the inline fuse is not a 'sense' wire ?

I will go to the alternator repair shop tomorrow and get the needed parts to hook up to the #2 terminal on the alternator.

So do i need to run a new 14 gauge wire from the cut off red wire to the #2 terminal ? Or run a new wire from the #2 terminal to the chassis battery ?

I am thinking that the gray wire runs to the ignition ? Whoever wired this Delco up must have elected to hook it up at the 'R' stud instead of hooking it up at the #1 terminal ?

I have just about run out of options in tracking down the voltage drop when the engine is running.

Thanks so much for all the help you have given me so far !!!
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Old 06-22-2021, 04:32 PM   #12
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Use the chassis battery voltage wire ( red ) to #2 terminal and see what happens.
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Old 06-22-2021, 04:40 PM   #13
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Quote:
Originally Posted by wolfe10 View Post
I am not used to seeing a 1.1 VDC drop across a diode-based battery isolator (14.3 to 13.2 VDC). 7/10th VDC drop, yes.


Anyone else experiencing over a full volt drop across their isolator?


Agree, this is not the prime issue, just one of curiosity.
I believe voltage drop across a diode depends on current flow.
The OP has chronically undercharged chassis and maybe house batteries.

Never seen a diode wear out. They short closed or burn open.
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Old 06-22-2021, 06:34 PM   #14
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Use the chassis battery voltage wire ( red ) to #2 terminal and see what happens.
Will do ! I will leave that black wire alone.
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