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Low voltage

Had a check engine light and low 10-11 volts. Turned on generator and voltage went up to 12. Vmspc shows 12.5 at idle. Fleet service checked alternator and batteries all checked ok. Thoughts?
David and Dana Pendleton
2001 U320 40'
2018 Jeep Rubicon
College Station, TX

Re: Low voltage

Reply #1 –
With engine running at idle (700-800 rpm), using a good quality multimeter:

1.  Check voltage at B+ large HOT alternator output post.
2.  Check voltage at alternator (input) post on battery isolator.
3.  Check voltage at chassis battery post on isolator.
4.  Check voltage at coach battery post on isolator.
5.  Check voltage at POS post on chassis battery bank.
6.  Check voltage at POS battery post on coach battery bank.

While doing these voltage checks, look for loose, corroded cable connections or frayed cables.

Let us know the results.
1993 U280 SE 40' WTBI, Build: 4359
C8.3 300hp, 6-Speed, Exhaust Brake
960 watts on the roof (6 x 160)
Sorento (or BOLT) on a Kar Kaddy SS
"It goes without saying..."

Re: Low voltage

Reply #2 –
A 2001 has the dreaded blue box that sometimes gives bad info. Look for threads on it.
1999 40 ft. U-320 wtfe build 5563 Chuck & Lynda's "Rollin' Inn"  2030 watts solar
prev. mh's 71 GMC 5 yrs. 73 Pace Setter 1 yr. 78 Vogue 5 yrs 81 FTX 40ft all electric 18 yrs. 1996 Monaco Signature 3 yrs.
2014 Grand Cherokee Overland
Dream as if you will live forever. Live as if you will die today.  James Dean

Re: Low voltage

Reply #3 –
When the tech checked the voltage at the alternator 14.5 at the batteries 12.5 v.  Would older weak house batteries contribute to low voltage?
David and Dana Pendleton
2001 U320 40'
2018 Jeep Rubicon
College Station, TX

Re: Low voltage

Reply #4 –
House voltage is independent of chassis voltage unless the boost switch is on.
1999 40 ft. U-320 wtfe build 5563 Chuck & Lynda's "Rollin' Inn"  2030 watts solar
prev. mh's 71 GMC 5 yrs. 73 Pace Setter 1 yr. 78 Vogue 5 yrs 81 FTX 40ft all electric 18 yrs. 1996 Monaco Signature 3 yrs.
2014 Grand Cherokee Overland
Dream as if you will live forever. Live as if you will die today.  James Dean

Re: Low voltage

Reply #5 –
Quote from: DP2 –
When the tech checked the voltage at the alternator 14.5 at the batteries 12.5 v.
Rather than guessing, can the "tech" (or someone) give us the readings I asked for in Reply #1?

The goal is to find the "weak link" in the charging circuit.  This requires a systematic approach to trouble shooting.
1993 U280 SE 40' WTBI, Build: 4359
C8.3 300hp, 6-Speed, Exhaust Brake
960 watts on the roof (6 x 160)
Sorento (or BOLT) on a Kar Kaddy SS
"It goes without saying..."

Re: Low voltage

Reply #6 –
Will do. I don't have access to the voltmeter at the moment. Will perform volt test and will report back. Thank you.
David and Dana Pendleton
2001 U320 40'
2018 Jeep Rubicon
College Station, TX

Re: Low voltage

Reply #7 –
As you have voltage at the alternator, the first thing I would check the voltage at the isolator and then follow
follow Chucks chart..\
Peter and Frieda Morin
1999 36ft. U320 Foretravel
Build # 5436
1998 Suzuki Sidekick Sport

Re: Low voltage

Reply #8 –
Article in current motorcader magazine.  Suggests it could be isolator.

2000 GV320 4010 build #5712  2019-?
1999 Bounder 2000-2008
Bardstown, KY
🥃The Bourbon Capital of the World🥃

Re: Low voltage

Reply #9 –
Thanks everyone. At the coast and do not have the ability's to check at the moment. However, question, I have 13.5 v when running the generator?  Is it ok to run the generator to keep the voltage up until I get home?
David and Dana Pendleton
2001 U320 40'
2018 Jeep Rubicon
College Station, TX

Re: Low voltage

Reply #10 –
Don't think that should be a problem.  Where/How are you reading 13.5 volts? On the backup monitor?  Do you have a 12v socket that shows voltage?  On my 2000 the two 12v plug in sockets on the dash, the top socket shows voltage on the house batteries and the bottom shows voltage on chassis batteries.  I would probably run the generator and turn the boost switch on so the generator will be charging both sets of batteries.

2000 GV320 4010 build #5712  2019-?
1999 Bounder 2000-2008
Bardstown, KY
🥃The Bourbon Capital of the World🥃

Re: Low voltage

Reply #11 –
Headlights on or off?  I've seen the dash reading drop by over 0.5V when the lights come on especially if you have a trailer or toad with lights as well.
Keith
2003 U320 38' #6197

Re: Low voltage

Reply #12 –
Many owners have ran their generator for many days when their isolator has failed when going down the road.
Richard & Betty Bark & Keiko our Golden Doodle
2003 U320T 3820 PBDS
Build # 6215
MC # 16926
2016 Chevrolet Colorado 4X4 diesel

Re: Low voltage

Reply #13 –
Yes, the generator will "power" the inverter/charger or converter to charge the HOUSE battery bank.

On some, it will also charge the CHASSIS battery bank-- but that is more the exception than the rule.

If the BOOST SWITCH/RELAY is activated (and working), it will also charge the chassis battery bank, which is critical for driving.

If boost switch/relay is not working (easy to tell, as both banks will have the same voltage if it is working), easy to work around-- usually as easy as moving one wire and adding it to another.
Brett Wolfe
EX: 1993 U240
Moderator, ForeForum 2001-
Moderator Diesel RV Club 2002-
Moderator, FMCA Forum 2009-2020
Chairman FMCA Technical Advisory Committee 2011-2020
 

Re: Low voltage

Reply #14 –
One way to test the isolator if you don't have a voltmeter is to take the leads of the isolator and
hook them altogether and see if you voltage goes up. If the voltage goes up, drive that way and
at night disconnect them.
Peter and Frieda Morin
1999 36ft. U320 Foretravel
Build # 5436
1998 Suzuki Sidekick Sport