[GMCnet] Testing the battery isolator

[Kerry Pinkerton]

I split this from my "BATTERY OR STARTING ISSUES" thread because the isolator is a different component. The original thread is at:

http://gmc.mybirdfeeder.net/GMCforum/index.php?t=msg&th=24037&start=0&rid=2555

[quote title=Mike Miller wrote on Sat, 03 November 2012 11:22]Quote:
> ... An isolator is just a pair of diodes. To explain the system: Think of "check valves" in an air system. Electrical juice can only flow one way and like a check valve restricts some air flow, the isolator does drop a little voltage as it works. SO... The center post is normally the input and the outer posts are normally the outputs. So, under normal operation, the center post should have higher voltage than the outer ones.
>
> This would be cause battery charging problems if you had an alternator that regulates based on the voltage at the alternator output. (AKA: single wire alternator systems.)
>
> Our alternator system is designed to sense the voltage on the engine battery side of the isolator, it will increase the voltage applied to the center post to make the output posts high enough to charge the batteries.
>
> So... now that you "know" something about the system, how would YOU check the isolator?
>
> :twisted:


Dang Mike, you're not supposed to put me on the spot and open me up to ridicule. I do that enough by myself. :lol:

Seriously, based on what I THINK I know (as opposed to what is the actual facts). This is how I would test the isolator.

1- Disconnect from shore power.

2- Set my meter to 25VDC (over 13 V) range

3- Put the negative lead (black) of my meter on a good ground.

4- Verify the ground by putting the positive lead of the meter (red) to the Positive terminal of the engine battery. Should read 12+ volt depending on battery state.

5- Start the engine. The voltage on the battery should increase by about 3/4 Volt. Perform same test on house battery.

If voltage does not increase at the battery (s) and the alternator is working, either the isolator OR the wiring is suspect. To test the isolator:

1- With engine running, check voltage at center lug of isolator. It should be 13.5 V or so (alternator output V)

2- Check voltage at the other two lugs. If voltage is not the same on the top and bottom lugs, then the corresponding diode has failed and the unit should be replaced (repaired??)

3- If voltage is present on the output lugs but not at the battery, the wiring or connection is bad.

To verify the shore power 120 to 12V power supply is working, shut off engine, plug into shore power (or start generator), and measure voltage at center terminal. Again, should be about 3/4V+ more than battery voltage.

OK, that is what I think I know. Until confirmed, please don't run out and rip apart your coach based on this process.

Now a question. How does battery condition effect measured voltage? Again, what I think I know is that the original BUZZ box converter will always put out the same voltage to the battery regardless of battery state (fully charged or not). I have a new 'smart' converter that is supposed to charge at lesser and lesser rates as the battery comes up to full charge. I THINK that the alternator will also charge at different rates but am not sure.

Ok, pass, fail, what?
--
Kerry Pinkerton

North Alabama, near Huntsville,

77 Eleganza II, The Lady, 403


76 Eleganza being rebodied as an Art Deco car hauler
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[Matt Colie]

Kerry Pinkerton wrote on Sun, 04 November 2012 09:04

> I split this from my "BATTERY OR STARTING ISSUES" thread because the isolator is a different component. The original thread is at:

> <snip>
> Dang Mike,
> <snip>

> If voltage does not increase at the battery (s) and the alternator is working, either the isolator OR the wiring is suspect. To test the isolator:
>
> 1- With engine running, check voltage at center lug of isolator. It should be 13.5 V or so (alternator output V)
>
> 2- Check voltage at the other two lugs. If voltage is not the same on the top and bottom lugs, then the corresponding diode has failed and the unit should be replaced (repaired??)
>
> 3- If voltage is present on the output lugs but not at the battery, the wiring or connection is bad.

**=> Here is a problem <=** See below Quote:

> To verify the shore power 120 to 12V power supply is working, shut off engine, plug into shore power (or start generator), and measure voltage at center terminal. Again, should be about 3/4V+ more than battery voltage.
>
> OK, that is what I think I know. Until confirmed, please don't run out and rip apart your coach based on this process.
>
> Now a question. How does battery condition effect measured voltage? Again, what I think I know is that the original BUZZ box converter will always put out the same voltage to the battery regardless of battery state (fully charged or not). I have a new 'smart' converter that is supposed to charge at lesser and lesser rates as the battery comes up to full charge. I THINK that the alternator will also charge at different rates but am not sure.
>
> Ok, pass, fail, what?

Kerry,

The converter is not typically connected through the isolator.
When charge the house bank with the AC power, the measured voltage of the house bank will go to charge level (13+), the alternator stay at Zero and the engine bank will see quiescent (~12.6).

Answers to questions:
A discharged battery should not be below 11.0V, and a resting full charge will get you to 12.6V~13.0 (depending on battery electrolyte density - put 12.6 in your head).

Typical automotive alternators maintain a constant output voltage of around 14.5V.

That help?

Matt
--
Matt & Mary Colie
'73 Glacier 23 Chaumière (say show-me-air) Just about as stock as you will find
SE Michigan - Twixt A2 and Detroit

[Ken Burton]

The simple way to test the isolator, if the alternator is working, is to connect the negative lead of your meter to the plate that the isolator is mounted on and probe the 3 terminals of the isolator.

The center terminal should be around 14.7 volts and the other two should be .7 volt lower or around 14.0.

Note: If you also have a combiner installed, disconnect the damn thing before taking any measurements.


--
Ken Burton - N9KB
76 Palm Beach
Hebron, Indiana

[Mike Miller]

Kerry Pinkerton wrote on Sun, 04 November 2012 06:04
> ... Seriously, based on what I THINK I know (as opposed to what is the actual facts). This is how I would test the isolator.

>
> 1- Disconnect from shore power.
>
> 2- Set my meter to 25VDC (over 13 V) range
>
> 3- Put the negative lead (black) of my meter on a good ground.
>
> 4- Verify the ground by putting the positive lead of the meter (red) to the Positive terminal of the engine battery. Should read 12+ volt depending on battery state.
>
> 5- Start the engine. The voltage on the battery should increase by about 3/4 Volt. Perform same test on house battery.
>
> If voltage does not increase at the battery (s) and the alternator is working, either the isolator OR the wiring is suspect. To test the isolator:
>
> 1- With engine running, check voltage at center lug of isolator. It should be 13.5 V or so (alternator output V)
>
> 2- Check voltage at the other two lugs. If voltage is not the same on the top and bottom lugs, then the corresponding diode has failed and the unit should be replaced (repaired??)
>
> 3- If voltage is present on the output lugs but not at the battery, the wiring or connection is bad.
>
> To verify the shore power 120 to 12V power supply is working, shut off engine, plug into shore power (or start generator), and measure voltage at center terminal. Again, should be about 3/4V+ more than battery voltage.
>
> OK, that is what I think I know. Until confirmed, please don't run out and rip apart your coach based on this process.
>
> Now a question. How does battery condition effect measured voltage? Again, what I think I know is that the original BUZZ box converter will always put out the same voltage to the battery regardless of battery state (fully charged or not). I have a new 'smart' converter that is supposed to charge at lesser and lesser rates as the battery comes up to full charge. I THINK that the alternator will also charge at different rates but am not sure.
>
> Ok, pass, fail, what?

I finally had time to read again...

I would say "pass." But with the following comments:

-- The measured voltage on the center post should be a little higher than 13.5v as you should be around that (as charging voltage) on the outer posts.

-- I would also check the center post for zero volts with the engine not running to ensure that the isolator has not been by-passed (relatively common) or shorted (not so common).

-- From the factory the converter only charges the house battery. So you should NOT have anything on ethe input to the isolator if the engine is not running... pugged in or not. Some have added a "battery combiner" to connect the house and engine batteries when either battery has a charging voltage, but I suspect there are more coaches without than with combiners.

-- "How does battery condition effect measured voltage?" A bad battery can cause lower measurements, mostly noticed when no other voltage is being applied but can draw down even normally good charging voltages.

-- A good modern converter/charger is always a good idea over the old buzz box.


--
Mike Miller -- Hillsboro, OR -- on the Black list
(#1)'73 26' exPainted D. -- (#2)`78 23' Birchaven Rear Bath -- (#3)`77 23' Birchaven Side Bath
http://m000035.blogspot.com

[Kerry Pinkerton]

Thanks Mike, I think I understand now.

--
Kerry Pinkerton

North Alabama, near Huntsville,

77 Eleganza II, The Lady, 403


76 Eleganza being rebodied as an Art Deco car hauler

[sgltrac]

Ken and others, is there a simple way to test an un installed isolator to verify if it is good?

Thanks
--
Sully
77 Royale basket case.
Future motorhome land speed record holder(bucket list)
Seattle, Wa.

[Emery Stora]

Just take an ohmmeter. Touch one terminal to the center connection. Then reverse the leads. There should be no resistance one direction and infinite the other direction.

Repeat the test from the center connection to the other outer side.

Emery Stora
77 Kingsley
Frederick , CO

[sgltrac]

Thanks Emery!

[Thomas Phipps]

Cheap or free Harbor Freight Multi-meter, set for diode, test from center terminal to outside posts. Meter buzzes, good diode section. No buzz, no good. Switch leads to double check. Bad diode section equals bad isolator.
Tom, MS II
--
1975 GMC Avion, under forever re-construction
Vicksburg, MS. 3.7 miles from I-20

[Todd Sullivan]

What is the symbol for diode?

Todd Sullivan

Sully
77 royale
Seattle

[Hardie Johnson]

sgltrac wrote on Fri, 15 November 2013 18:29

> What is the symbol for diode?
> Todd Sullivan
> Sully
> 77 royale
> Seattle
> <>

-|>- ??

--
Hardie Johnson "Crashj"
1973 26 foot Glacier, White Thing
Raleigh NC

[Thomas Phipps]

Diode =. --I<--

Tom, MS II
--
1975 GMC Avion, under forever re-construction
Vicksburg, MS. 3.7 miles from I-20

[Ken Burton]

sgltrac wrote on Fri, 15 November 2013 17:29

> What is the symbol for diode?
>
> Todd Sullivan
>
> Sully
> 77 royale
> Seattle

https://www.google.com/search?q=diode+symbol&ie=utf-8&oe=utf-8&aq=t&rls=org.mozilla:en-US:official&client=firefox-a

--
Ken Burton - N9KB
76 Palm Beach
Hebron, Indiana

[Johnny Bridges]

Sure, with your ohmmeter.  You should see a front to back ratio of a thousand or more.  If you have a cheeeep digital meter, it may not be capable of biasing the diode in question enough to turn it on.  Use a 12V bulb and put each half of the isolator in series wioth the bulb and battery.  One direction it should light, the other stay dark.  If it kights both ways, that diode is shorted.  If it doesn't light at all, that diode is open.
 
Basically: chassis + I<-alt->I+house, the ->I being the diodes.
 
--johnny

[John R. Lebetski]

Another quick test ....meter on DCV engine running meter center to house side and center to chassis side. You should see about .7 in both cases. No voltage means a shorted diode on that side and greater an open.
--
John Lebetski
Chicago, IL
77 Eleganza II
Source America First

[Todd Sullivan]

I said simple Johnny ;)

Todd Sullivan

Sully
77 royale
Seattle