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Topic: Isolator check (Read 787 times) previous topic - next topic

Isolator check

Troubleshooting charging problem I found this.  With engine off I was checking voltage drops on the isolator and found 13 vdc on both left and right terminals and 13vdc on the center terminal which according to what I have read is a sign that the isolator is defective.  Am I doing this right?  Where do I find a replacement isolator?

Re: Isolator check

Reply #1 –
I'm not a expert, but it's probable that your isolator is OK.  I had a very similar question about my (original factory installed) isolator, but after the subject was discussed at length on the Forum, I decided that the center post voltage was not "harmful".  You might be interested in reading through my old thread, to see if you get any insight into your situation.  By the way, I never did replace my isolator, and as far as I can determine, it is still doing its job exactly as intended.  My batteries stay fully charged under all conditions, and the coach never fails to start on the first crank.  However, we have not done any boon-docking with our coach, so we have never pulled the house battery bank down very far...I don't actually know if our isolator would function properly to protect the start batteries in that situation.
 
Bad Isolator?

Re: Isolator check

Reply #2 –
Quote from: CAPEHORN31 –
........With engine off I was checking voltage drops on the isolator and found 13 vdc on both left and right terminals and 13vdc on the center terminal which according to what I have read is a sign that the isolator is defective............
Terry,
Those are normal readings for an isolator with the engine off (alternator not spinning).  As Chuck has already offerred, there is a detailed explanation here:

Bad Isolator?

At response #23, the thread also has guidance from Peter Fleming on how to determine isolator health through current measurements, which is the better way to determine isolator proper functioning.  That way is not recommended (by me, anyway) unless you know what you are doing as it involves either moving wires live or many engine starts and stops to rearrange test equipment. 
That's why most techs just look for (with the engine running) alternator output terminal (+) Vdc on center term, then 0.7 to 0.9 Vdc less on each battery bank terminal (healthy power diodes drop about 0.7 Vdc as they conduct current), and then (with the engine not running) relatively equal diode resistances across the diodes in both the forward current flow and the reverse current flow directions (low resistance values going forward and high going backward).  Absolute values matter less than comparatively equal resistance values because no two diodes are going to be identical nor would they partially or completely fail to comparatively equal resistance values.  When these power diodes fail, it is almost always in the open (high resistance) direction.  They seldom fail to a shorted condition.
Hope this helps.
Neal

Re: Isolator check

Reply #3 –
Terry,

With a couple of digital lighter plug voltmeters, you can read the engine battery voltage on the instrument panel plug and the house on the plug by the air controls. They are very inexpensive and accurate. Easy way to tell if the isolator is working with the engine running or off. With the boost switch off, the voltages should be different.

Our isolator was replaced but cables/wires put back in the wrong places by the last owner or shop and the boost solenoid was also stuck on. Took a while to figure out that one. 

What is the problem/symptoms that concerns you other than the 13V at all three terminals?

Pierce

Re: Isolator check

Reply #4 –
Quote from: Pierce & Gaylie Stewart –
Terry,

With a couple of digital lighter plug voltmeters, you can read the engine battery voltage on the instrument panel plug and the house on the plug by the air controls. They are very inexpensive and accurate. Easy way to tell if the isolator is working with the engine running or off. With the boost switch off, the voltages should be different.

Our isolator was replaced but cables/wires put back in the wrong places by the last owner or shop and the boost solenoid was also stuck on. Took a while to figure out that one. 

What is the problem/symptoms that concerns you other than the 13V at all three terminals?

Pierce


The problem started with the volt meter on the dash going from 13 to 16 vdc intermittently.  I took the alternator to the shop and they found nothing wrong.  I also noticed that with the headlights on the volt meter showed 12vdc. I have had the chassis batteries load tested showed ok.  any thoughts?
Terry

Re: Isolator check

Reply #6 –
Connections and grounds.
Study the prints to find out where there are connections between the back and the front. Shouldn't be that many to check.
It's only 12volts so not much pressure to push the power through a poor connection
JD
 

Re: Isolator check

Reply #7 –
Adding to possible sense wire problem, perhaps the intermittent problem is the reason the alternator shop didn't find the problem right away. Might check the engine battery voltage at the batteries with the engine running and headlights on and compare the voltage there to the dash voltmeter reading. Check connections on ignition solenoid just in front of you under the dash top.

An independent mom and pop electrical shop can usually find the problem quickly if you drive it down to their shop. Usually not much of a charge unless they find something amiss that needs replacing.

Piece