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Topic: Varying Charging Voltage (Read 917 times) previous topic - next topic

Varying Charging Voltage

Yesterday while driving I noticed that the dash volt meter had crept over 14 volts. I checked my Trimetric and noted crazy readings on both the start and coach batteries. The readings were jumping all around between 13.5 and touched 15 a couple of times. After arriving at our campsite I started the generator and noted constant readings on both banks. Started the engine again with the same results as before - wildly varying voltage (noted on Trimetric and with a DVM at the batteries).

On this coach the factory setup attaches the alternator (Leece-Neville 130a) sense wire to a little circuit breaker mounted on the isolator board (photos). I rebuilt that board awhile back with new parts and it has been functioning nominally. Anyhow, I moved the sense wire from that circuit breaker and attached it directly to the chassis terminal on the isolator. That fixed the problem.

Any ideas as to why this would occur? Thanks.
jor
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Re: Varying Charging Voltage

Reply #1 –
Yea, chassis and breaker, not board. Gotta do a bit more proof reading. Thanks.
jor
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Re: Varying Charging Voltage

Reply #2 –
Just a armchair guess:

If that auto-reset circuit breaker (where the sense wire was attached) was opening and closing, then it might cause your alternator output to vary up and down.  You might look at the circuits that come off that relay in the upper left corner of your diagram, to see if there is any problem there that might have been overloading the circuit breaker..
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
"Nature abhors a vacuum"

Re: Varying Charging Voltage

Reply #3 –
Sure, the original voltage sense FT wiring location is problematic.  The isolator panel location needs to be protected from rear tire road debris and water, and if that breaker opens or there are other connectivity issues the alternator response is as if the battery voltage is too low.  BTW John, I checked our inline fuse on our voltage sense wire and found we are using a 1 amp fuse at the battery end of the new voltage sense wire.

Re: Varying Charging Voltage

Reply #4 –
Quote from: jor –
Yesterday while driving I noticed that the dash volt meter had crept over 14 volts. I checked my Trimetric and noted crazy readings on both the start and coach batteries. The readings were jumping all around between 13.5 and touched 15 a couple of times. After arriving at our campsite I started the generator and noted constant readings on both banks. Started the engine again with the same results as before - wildly varying voltage (noted on Trimetric and with a DVM at the batteries).

On this coach the factory setup attaches the alternator (Leece-Neville 130a) sense wire to a little circuit breaker mounted on the isolator board (photos). I rebuilt that board awhile back with new parts and it has been functioning nominally. Anyhow, I moved the sense wire from that circuit breaker and attached it directly to the chassis terminal on the isolator. That fixed the problem.

Any ideas as to why this would occur? Thanks.
John,
It continues to amaze me that there are still FT's out there that have not experienced or had to correct this defective original design issue.  Our 1998 U270 experienced problems with this defect, and the defective design caused various damages practically from day one, when the coach was brand new.  It cost us dearly until I figured it out (slow learner).

Once I figured it out, my problems ceased.  I helped several owners correct the problem on their coaches and eventually I posted the issue and solution (over nine years ago) on the Yahoo site.  I have reposted it numerous times here since then, yet owners still keep discovering the issue.

http://www.foreforums.com/index.php?action=media;sa=album;in=170

On a 1998 U270 at least, as originally designed and built, the E59 Starting Battery Bank voltage sensing wire is attached at the AC Compressor "enable relay".  If wiring or contacts have deteriorated, in any way, such that there is a voltage drop from the true start battery bank positive terminal to the point of E59 wire attachment, then the alternator voltage regulator thinks that the Start Battery Voltage is low in voltage.  In reaction, the voltage regulator in the alternator increases alternator output voltage to try to bring the start batteries back up to the target 14.0 Vdc (or whatever voltage has been dialed into, or fixed into, the regulator).  If the sensed voltage is low enough (or zero), the alternator output voltage will fail to its maximum output which is generally on the order of 17 Vdc.

Now if the contact is just dirty and the voltage, as sensed, jumps around with vibration or some other such disturbance, then the alternator voltage regulator and alternator output voltage jumps around accordingly.  While all of this is going on, the alternator output is being passed on to both banks of batteries through the isolator.  The starting battery bank is smaller, so it will have less resistance to the voltage changes and it will react a bit faster than the house battery bank will.  But both banks can and will be damaged if the condition persists.  And of course, voltage sensitive components that do not have on-board voltage regulation protection will be damaged or destroyed.  Control boards and lighting filaments are particularly prone.

HTH,
Neal
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Neal (& Brenda) Pillsbury
'02 U320 SPEC, 4200, DGFE, Build #5984
'04 Gold Wing
'07 Featherlite 24'
'14 Jeep Grand Cherokee Summit
MC #14494
Exeter, NH & LaBelle FL
Quality makes the Heart Soar long after Price is Forgotten

Re: Varying Charging Voltage

Reply #5 –
Quote
You might look at the circuits that come off that relay in the upper left corner of your diagram,
Good idea. I'll do that.

Quote
we are using a 1 amp fuse at the battery end of the new voltage sense wire.
Thanks. I remember we discussed this awhile back.

Neal, I don't know how I missed your thorough explanation. I will do just that as you, Barry and others have and eliminate this problem once and for all.

Thanks to all.
jor
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Re: Varying Charging Voltage

Reply #6 –
Bottom line:  Make sure that the sense wire is attached to a good, clean and tight location directly connected to the chassis battery.

No question, if there is resistance, an opening/closing breaker, etc, it will feed false "low voltage" to the alternator, causing excessively high voltage output.

 In fact, the vary last Foretravel I inspected had this issue. Yes, on that coach, most of the connections on the board behind the left rear wheel were is very poor condition.
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: Varying Charging Voltage

Reply #7 –
While some have relocated isolator panel from behind rear tire, we just decided to leave all in place, but cover the whole panel with a piece of clear vinyl, held in place with a few cable ties.  This keeps road debris and tire spray from components.
We also ran a new wire from alternator sense connection over to our start battery bank.  Protecting and supporting the wire with a long small diameter PVC pipe run from side to side on top of frame and kept in place with wire ties.  We put a 1 amp fuse at battery end of new wire.

Re: Varying Charging Voltage

Reply #8 –
Quote from: Barry & Cindy –

We also ran a new wire from alternator sense connection over to our start battery bank.  Protecting and supporting the wire with a long small diameter PVC pipe run from side to side on top of frame and kept in place with wire ties.  We put a 1 amp fuse at battery end of new wire.

Good idea to have both the fuse and a KNOWN good clean connection for the sense wire.
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: Varying Charging Voltage

Reply #9 –
Relocating the sense wire from the A/C ckt breaker to the isolator terminal did the trick. When I get back home I'm going to do as others have and run an new fused wire from the alternator directly to the battery terminal. Thanks.
jor
93 225
95 300
97 270
99 320
 

Re: Varying Charging Voltage

Reply #10 –
Going thru similar issues,question,how was everything working OK with original wiring?What changed?
96 U270 BUILD 4810
85 380SL
Drummonds TN.