Skip to main content ↑ ↓
Topic: Alternator overcharging? (Read 1030 times) previous topic - next topic

Alternator overcharging?

Shortly after pulling out this morning I looked down to see my voltage was zooming over 16 volts. I was able to pull over and shut the rig down. Checked my start batterys and chassis batterys and found they where cool to touch and showing 12.8v and 13.4v on a digital multimeter. Started the coach back up, voltage increase again at dash gauges. Prosine said charge high.  Turned on dash ac and the voltage dropped to about 14.5v on my return to the cg I had just left.
So, Is there a way to safely disconnect the alternator and drive the coach with the generator running to charge the batterys?
Kathy
05 36' U270

Re: Alternator overcharging?

Reply #1 –
You can use the boost switch and the generator if the regulator has died or turn on the lights and the blower fan and see if you can get the voltage down for a short trip to a repair place to have it either rebuilt or replaced.
2025 Wanderbox Outpost 32 on F600 Expedition Motorhome
2015 Born Free Royal Splendor on Ford 550 nonslide version  for sale
Former Coaches  covering. 360,000 miles
1999 34 U270
2000 36 U320
2001 42' double slide U320
2018 Jeep Rubicon

Re: Alternator overcharging?

Reply #2 –
You may have a broken sense wire, bad sense wire connection, or bad breaker in the sense wire that tells the alternator to go to work. If any of the above happen it can cause the alternator to run away.

To answer your question you can remove the wires off the back of the alternator and tape them up so they don't short out if your alternator don't have a belt that only drives it. If your alternator has a dedicated belt then just take the belt off.    Then use the boost and the generator to go on down the road. 

Mike
Pamela & Mike 97 U 320

"It's not what happens to you, but how you react to it that matters."

Re: Alternator overcharging?

Reply #3 –
High voltage from alternator is often a poor / broken battery voltage sense wire connection. This is one of the two small wires on the gen that if connected directly to the start battery bank positive could likely permanently fix your problem.

Yes as many have done with a broken alternator, run generator which will supply 120volts to power battery charger and charge house battery, then turn on boost switch to close boost solenoid which ties house and start battery banks so start battery will be charged to run engine.

Only problem your alternator is working but at too high voltage, so by disconnecting alternator big positive wire at alternator or at center isolator may damage alternator by not being connected to a battery. But it may not damage the alternator and it will not damage anything else.  If you need to get going, worth doing.

Re: Alternator overcharging?

Reply #4 –
Another possibility, the battery isolator may have failed, and if the sense wire is attached to the isolator on the failed side, the alternator may not be seeing the battery voltage and is therefore defaulting to maximum output. I am not sure how the 05' may be wired differently than our 99', but this happened to ours and burned out a few expensive components by the time I could get off the hwy. If this is the problem, you can bypass the isolator temporarily by moving the alternator positive from the center post to the chassis battery post. Our isolator had just three lugs on it. if yours has four the principle is the same but you will have to look at the diagram carefully to determine the correct bypass. Hopefully, others will corroborate or refute my assertion as  appropriate ???
Don
The selected media item is not currently available.
Don & Tys
1999 U270 3602 WTFE #5402
Xtreme Stage 1 w/Headlight, Step Conversion, etc.
2009 Honda Fit Sport with Navi
Freedom is NOT "just another word for nothing left to lose"... with apologies to Kris Kristofferson

Re: Alternator overcharging?

Reply #5 –
Thank you for the responses.
The isolator has 3 posts. The sensor wire is reading battery voltage at the alternator. I started the coach and checked voltage at the isolator. The center post was 17v.
It has a serpentine belt on it I don't think can be removed.
Kathy
05 36' U270

Re: Alternator overcharging?

Reply #6 –
The alternator belt cannot be removed because it is also turning the water pump, at least on the ISC. You probably have the ISL which is based on the ISC and so likely has the same belt arrangement. A picture would confirm or refute the similarity. The sense wire needs to be reading the starting battery voltage in order to allow the regulator to function correctly. It should be easy to check if the sense wire shows the same voltage as the chassis battery. All you need is a multimeter. Just make sure the boost switch is off so you can be sure the reading is affected by the house batterie's voltage. The center post is what the alternator is outputting, probably because the sense wire is not giving the alternator the needed info. There are several possible reasons, but if it is the isolator failing that has caused it, bypassing it should get you by until you can address it properly.
Good luck!
Don
Quote from: kathyg –
Thank you for the responses.
The isolator has 3 posts. The sensor wire is reading battery voltage at the alternator. I started the coach and checked voltage at the isolator. The center post was 17v.
It has a serpentine belt on it I don't think can be removed.
The selected media item is not currently available.
Don & Tys
1999 U270 3602 WTFE #5402
Xtreme Stage 1 w/Headlight, Step Conversion, etc.
2009 Honda Fit Sport with Navi
Freedom is NOT "just another word for nothing left to lose"... with apologies to Kris Kristofferson

Re: Alternator overcharging?

Reply #7 –
The sensor wire comes off the start battery side of the isolator, goes thru a 15A fuse and then to the alternator. Both sides of the fuse and the alternator read start battery voltage of 12.8v. Would this indicate the voltage regulator inside the alternator is bad?
Kathy
05 36' U270

Re: Alternator overcharging?

Reply #8 –
If you read the correct start battery voltage at the alternator sense wire connection on the alternator and there isn't a bad connection of the sense wire to the alternator's voltage regulator, then it sure sounds like the voltage regulator has failed. I would check and clean the connections on the alternator. Maybe someone else will chime in with other suggestions. If the regulator is bad, then the question becomes "are you in a place to address the issue". If not, then you need to take the alternator out of the loop electrically without potentially damaging it. Is the alternator self excited, or is there an external excitation wire that needs to get 12 volts to allow the alternator to create the magnetic field? If it is an externally excited alternator, then disconnecting that wire should allow the alternator to basically go into sleep mode, and then you could use the generator with the boost switch on to get you to where you can address the problem. Depending on the alternator, a rebuild job by a competent shop could be an option. However, finding someone who can understand these alternators may be a challenge. Typical truck repair facilities are not often familiar with them. There are many threads on the forum of people who had professionals install new or rebuilt alternators that were not suited for the two battery bank setups our coaches use. Yours is a 2005, so that may be different in your case, depending on the alternator originally installed. Do some research regarding your specific alternator model and maybe check with Foretravel to confirm what was OEM.
Don
Quote from: kathyg –
The sensor wire comes off the start battery side of the isolator, goes thru a 15A fuse and then to the alternator. Both sides of the fuse and the alternator read start battery voltage of 12.8v. Would this indicate the voltage regulator inside the alternator is bad?
The selected media item is not currently available.
Don & Tys
1999 U270 3602 WTFE #5402
Xtreme Stage 1 w/Headlight, Step Conversion, etc.
2009 Honda Fit Sport with Navi
Freedom is NOT "just another word for nothing left to lose"... with apologies to Kris Kristofferson

Re: Alternator overcharging?

Reply #9 –
You may just need to replace the regulator.
1994 U280, Build 4490
Deming, NM.

Re: Alternator overcharging?

Reply #10 –
Quote
The sensor wire comes off the start battery side of the isolator, goes thru a 15A fuse and then to the alternator.

I had a similar issue awhile back. The voltage at the isolator and battery were the same. Nevertheless, on advice from the forum, I removed the sense wire from the isolator and connected it directly to the start battery. No more problems. Hope yours is such an easy fix.
jor
93 225
95 300
97 270
99 320

Re: Alternator overcharging?

Reply #11 –
sense line or battery cable corrision  (?)
1993 U300, 6v92
build 4366
USAF retired, Flight Engineer, C124, C130
 ATP, A & E.  & ex AI

Re: Alternator overcharging?

Reply #12 –
Even with good voltage reading at alternator, running a new temporary wire from start battery to small voltage sense alternator terminal is highly recommended to eliminate the oem wire which came from isolator panel, and known to fail over time.

Alternator has two small terminals. How do you know which is Sense (battery voltage all the time) and which is Excite (voltage only with ignition key on).

'Old School' alternator/starter repair shops know about this type of RV alternator, most shops think they do and may cross the small wires. We only use these old school shops and always get our alternator rebuilt.  Never exchange or install 'factory' rebuilt, where they usually only replace the bad part leaving all the other worn parts to fail on your watch.

Re: Alternator overcharging?

Reply #13 –
If you have 17 volts to the center post of the isolator and only 12.8 on the engine side of the isolator it sounds like a bad isolator.
previous 1984 35 ft ORED 250 HP 3208 Cat       
previous 1998 40 ft U295 CAI 325 hp Cummins
previous 2003 40 Ft u320 build #6140 450 Cummins M11.                                                         
1999 Mazda Miata
Ron, Nancy, Tipper the cat, Max The dog
1997 U 270 36 ft build number 5174 8.3 Cummins

Re: Alternator overcharging?

Reply #14 –
Quote from: red tractor –
If you have 17 volts to the center post of the isolator and only 12.8 on the engine side of the isolator it sounds like a bad isolator.

That was it.
I drove the coach to Tennessee RV today. They worked it into their schedule and had Dave, their foretravel tech diagnose and repair it.
1.5 hours later- new isolator and trickle charger- problem solved.

The isolator had failed and was not passing current to the start batteries so the alternator kept increasing output. The trickle charger had not survived and he replaced it as well.
I can't recommend Tennessee RV enough. All their service bays are full and coaches waiting for work, yet they took the time to look at my problem, repair it and get me back on the road. Great customer service.
Kathy
05 36' U270

Re: Alternator overcharging?

Reply #15 –
Quote from: red tractor –
If you have 17 volts to the center post of the isolator and only 12.8 on the engine side of the isolator it sounds like a bad isolator.

Well, yes and no-- depends on which dash gauge (chassis battery or audit which reads house battery) read 17 (see original post).

If 17 on audit, makes perfect sense-- diode to chassis battery out, so alternator kept raising voltage which only went to the house bank.

But, if 17 was on the dash gauge itself (which measures chassis battery voltage) then it does not make sense (reading of 17 at dash and yet 12.8 at chassis battery??).

BTW, for clarity, let's continue to call the two battery banks:

CHASSIS
HOUSE

Thanks.
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