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Topic: 1999 U320 Alternator reading 10 volt on gauge  (Read 302 times) previous topic - next topic

1999 U320 Alternator reading 10 volt on gauge

I'm just off a nice vacation coach trip all went well but began noticing a 10v reading on dash gauge at near end of trip. We pulled into the building at home plugged into 50 amp and unloaded. About hour later decided to test start and it cranked sluggish but started.  I have scrolled through the forum and it seems the first steps are tightening and cleaning all connections.
No problem I can do that first.
Question I have, should I disconnect house batteries? Honestly don't know if it even matters. 
Appreciate all the contributors on the forum.
Invaluable resource to Foretravel owners.
Thanks for your input.
John

Re: 1999 U320 Alternator reading 10 volt on gauge

Reply #1 –
Note that the engine and transmission ECMs won't like low voltage.  Run the generator and use the boost switch to keep the chassis voltage up while driving (be sure to shut off the boost switch when the genset isn't running) if all other signs point to an alternator issue.

Learn every day, but especially from the experiences of others. It's cheaper!  - John C. Bogle

2000 U320 36' non-slide / WildEBeest Rescue
2003 U320

Re: 1999 U320 Alternator reading 10 volt on gauge

Reply #2 –
While driving, the alternator should charge both house and chassis batteries. If the dash gauge reading was 10 volts chances are the house batteries were low too.

When plugged into shore power only the house batteries will be charged, this is probably why starting was still sluggish. Use of the boost switch will get the chassis batteries recharged on shore power.

This seems to be a sudden change in voltage. Before going to the trouble of retightening a bunch of wire connections, I would suggest taking some voltage measurements. With engine idling measure voltage at the alternator B+ post and at the isolator center post. These should both be around 15 volts. Then turn on headlights to add some load on the alternator and measure again.

If voltage is low under load you likely have a failing alternator. If alternator voltage is good at both points under load, then more troubleshooting will be needed.
Peter and Tammy Fleming
1991 U300 GV 40 - Sold, owned for 4 years
Downsized to Roadtrek Popular 210 class B

Re: 1999 U320 Alternator reading 10 volt on gauge

Reply #3 –
John C.
And
Flemings
Thank you for those detailed replies, the transmission was indeed acting odd at about time i noticed the 10v read on the amp gauge.
I will test first as suggested. 
Much appreciated!

Re: 1999 U320 Alternator reading 10 volt on gauge

Reply #4 –
Yes, with your digital voltmeter and engine around 1,100 RPM.

A the battery isolator:

Center lug (from Alternator B+) to good clean ground should be in the high 14 VDC.

Each outer lug to good clean ground should be about .7 VDC lower but high 13's to low 14 VDC.

This will help identify if you have an alternator issue or battery isolator issue, yes or wiring issue which could also cause the same issues.
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: 1999 U320 Alternator reading 10 volt on gauge

Reply #5 –
Just a heads up, my voltage gauges started showing less than 12 volts on the way home from Sturgis last year. Ran the generator a few times to get home. Tested at the Isolator 9 volts at center post. Replaced the alternator and no change. Disconnected the isolator and had 14.5 volts. So now I have a new isolator and new alternator. Isolator tricked me. Jim. 
Jim and Dona
2002 U295 36'
Lily, blue healer. Our fury person
Manx mid travel Ecotec turbo
2022 Bronco badlands Sasquatch
Trail 90 and 110cc
 

Re: 1999 U320 Alternator reading 10 volt on gauge

Reply #6 –
Remember when alternator fails, running generator with boost solenoid connected, one can drive for weeks until there is time or place to fix alternator.