Helping my parents with their 2003 U320, and have a problem
Coach has been plugged into shore power for about one month. Unplugged, and when the ignition key is turned, the volt gauge shows 0 volts. Turned the Boost switch on, it turned red, but the volt gauge still show 0 volts. The house battery display inside the coach shows 12.9v. I tried getting to the starting batteries, but since the coach has lowered, I am unable to get under. Something has stopped the starting batteries from taking a charge and/or stopping the coach batteries from assisting in the engine start.
OE, there was no means of charging the chassis batteries from shore power/generator. Did they or a previous owner add a device (several viable choices)?
I don't know if the boost solenoid "signal" is from chassis battery. If so and chassis battery dead, it will not work.
But, a short, small gauge jumper wire from the house bank lug of the boost solenoid to the signal terminal may connect them short term. I would disconnect the signal wire to the chassis battery when doing this, so the jumper would not be trying to charge the chassis battery directly.
If you can get hold of a good quality high-amp portable battery charger, you could try charging the start batteries by connecting the charger to the start battery post on the isolator. That is, if the the battery isolator is easily accessible.
A "smart" portable battery charger might not want to charge the batteries if they are totally discharged. For completely dead batteries, a "dumb" old school battery charger often works better.
Of course, if the start batteries are completely dead and have been sitting in below freezing temps, then they are now most likely junk.
If the chassis batteries are really dead, you may be able to revitalize them with a charger with a "smart" de-sulphation setting. The smart charger won't work on a totally dead battery so you will have to use a jumper to the house batteries as Brett suggests, a high output regular charger or bring the voltage up by jumping from a car running for at least 30 minutes. The smart charger, over a long period, may dissolve the sulphation. Battery temperatures as high as 140 degrees may aid the process but with a slow charge, these temperatures can't be achieved with the smart charger alone. This may not work on AGM batteries.
Others have been successful with a chemical solution on conventional batteries but this is a last resort.
Pierce
Foretravel wired diodes to the boost switch, so boost will work if either battery bank is dead.
(http://www.foreforums.com/MGalleryItem.php?id=4757)
Boost relay could be bad also. Brett as a good working solution with the jumper. Jumper cables work good for this also.
Scott
If Peter's schematic just above is correct for THIS coach, it could get 12 VDC positive from either bank.
RE: "Foretravel wired diodes to the boost switch, so boost will work if either
battery bank is dead. "
. . . ON SOME COACHES. WITH FORETRAVEL THERE ON ABSOLUTES WITH CHANGES BETWEEN YEARS AND MODELS AND MONTH OF PRODUCTION
May have to jumper small boost solenoid positive terminal so it is powered by house battery. Can also use car battery jumper cable to jump between big terminals on boost solenoid. But even this because of dead start battery bank will immediately drain good house battery into start battery bank even before engine starts.
Start battery bank is usually only charged with running engine, and will always deplete if there is no after-market device to charge start batteries when engine is off.