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12 volts on interior lights when master switch off

May 06, 2011, 11:45:22 pm

I always thought that on my coach the lights, etc. were run off the engine battery because they would come on even if the master switch was off.  This has been bugging me for some time so I decided to do some checking today.  First I verified that according to the schematic there should be no power to the 12 volt fuses under the bed if the master switch is off.  I then checked the 80 amp circuit breaker under the bed and discovered that when the master switch was on I had battery voltage of 12.8 volts and when the master switch was off I had 12.0 volts!  Something is rotten in Denmark!  Has anyone else experienced this?  If I can short cut the troubleshooting process I would really appreciate it.  Could it be the battery isolator?  Hmm, 12.8-12.0=.8, approximately a diode drop.

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Re: 12 volts on interior lights when master switch off

Reply #1 – May 07, 2011, 08:33:39 am

I'd check the master switch first.  Always best to check the obvious first.

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Re: 12 volts on interior lights when master switch off

Reply #2 – May 07, 2011, 08:49:54 am

Yes, after verifying that the master switch is indeed working properly, disconnect the house battery from the battery isolator.  Remember that the disconnected wire will be HOT, so don't touch any metal with it.  It is possible someone  through the years hooked the wires to the wrong terminals or that the isolator has failed.

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Re: 12 volts on interior lights when master switch off

Reply #3 – May 08, 2011, 07:42:42 pm

An update:  Disconnected the wires on the isolator today and verified which wire was which.  Next I ohmed out the isolator and found a short between the alternator wire and the engine battery.  The alternator to house battery connection read like you would expect from a diode.  Infinity one way and about 400 ohms the other way.  This explains a lot.  I am now chasing wires down because I still do not have an open between the house battery and the isolator when the master switch is off.  It is a high resistance so it will take some chasing to find.

I now need to replace the isolator but I have no idea of what it is since the printing on the tag has faded.  See attached photo.  Note the raised area of epoxy where I suspect the diode failed.

PPL Motorhomes has a 120 amp isolator for $65.00 which I can get tomorrow if they stock it.  Is that enough current capacity?

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Re: 12 volts on interior lights when master switch off

Reply #4 – May 09, 2011, 12:52:40 am

Mine is a powerline model 22-31 [PLI-190-2] rated for 190 amp alternator. It has 3 lugs like the one shown. Hope this helps.

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Re: 12 volts on interior lights when master switch off

Reply #5 – May 09, 2011, 08:06:57 am

Hey John,

If I'm not mistaken, the alternator puts out 160 amps, so the isolator should be rated somewhat higher, like 190 to 200 amps. I'll bet Foretravel has what you need, though you may have to pay extra for overnighting it. They could at least give you the part number for a replacement.

Was looking at mine yesterday. Ugh. Seriously rusty terminals. Another project.

Steve

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Re: 12 volts on interior lights when master switch off

Reply #6 – May 09, 2011, 10:54:38 am

John,
I just thought of an easy test you can do to try to trace down your problem.  If I understand your post, you have power to the fuses under the bed eventhough the master switch is OFF.  With the master switch OFF, pull all the fuses under the bed.  One side of the fuse bank is commoned to the master switch and the other side is the individual circuits.  If everything is wired correctly there should be no voltage on either side.  Where you find voltage is the start point to trace to the problem.  My  thinking is somewhere, someone, at sometime wired the house and chassis batteries together when adding a device or doing a repair.  Look for non-OEM addons or rapairs.  I would guess under the dash where it's easy to get the two voltage sources mixed up.  An example might be the radio that has two power hookups: constant 12V and Ignition.  Just a thought.

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Re: 12 volts on interior lights when master switch off

Reply #7 – May 09, 2011, 11:48:15 pm

Problem solved.  The Hehr isolator was toast.  Measured zero ohms between the stud for the engine battery and the alternator stud.  Measured a high resistance between the alternator stud and the house battery, both directions.  Talked with Hehr Power Systems today and they asked me to check the output specs for the alternator even though they verified that the isolator was a 120 amp model because of the size (6.5"long). It seems they thought that the only way it could fail was by the alternator putting out more current than the isolator could handle. The plate on the alternator says it puts out 105 amps.  On my Oshkosh chassis the isolator is mounted on a rear cross chassis member and there is no room for anything larger. 

I had a great deal of trouble trying to find dimensions for the isolator that PPL Motorhomes sells so I just drove to the store.  I worked with Earl (his extension is 151), the inside counter parts guy and he found the correct Sure Power model 1202.  The first one he found did not have all the parts with it so he went out back and found another that was complete.  The Sure Power is about 1/2" narrower in width than the Hehr and the bolt holes do not match but I was able to get it installed.  Now the lights do not come on if the master switch is off.  I have more checks to run tomorrow but everything seems o.k.  I should also mention that the Sure Power unit was about $65.00, Hehr quoted me $155.05.  BTW, Sure Power now appears to be a Cooper Bussman company and claim to be the inventor of the battery isolator in 1959.

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