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Topic: Alternator question  (Read 551 times) previous topic - next topic

Alternator question

For some reason the alternator isn't charging the batteries. Alternator tests good, but still only 1.87 volts. When we put it back on we noticed that the lead from the negative terminal of the alternator goes to to the alternator terminal on the isolator. That isn't right, is it?

Re: Alternator question

Reply #1 –
No the negative lead goes to ground and the positive goes to the center post on the isolator.
Peter and Frieda Morin
1999 36ft. U320 Foretravel
Build # 5436
1998 Suzuki Sidekick Sport

Re: Alternator question

Reply #2 –
Quote from: kb0zke –
For some reason the alternator isn't charging the batteries. Alternator tests good, but still only 1.87 volts. When we put it back on we noticed that the lead from the negative terminal of the alternator goes to to the alternator terminal on the isolator. That isn't right, is it?
The good news is that the diodes in the isolator protected your alternator.
1992 Foretravel Grand Villa
U225 SBID Build No. 4134
1986 Rockwood Driftwood
1968 S.I.A.T.A. Spring
1962 Studebaker Lark
1986 Honda VF700C
1983 Honda VF750C
Charlie, the Dog was broken out of jail 24 Oct 2023
N1RPN
AA1OH (H)e who must be obeyed.

Re: Alternator question

Reply #3 –
More information: Last year, on our way to Nac, I noticed that the start battery voltage wasn't where it normally was. I asked MOT to check it, and they said the alternator was failing, so they replaced it. Things seemed to be working, but the problem showed up again on the way back to Wyoming. The "new normal" was voltage fluctuating from about 11.8 to 13.3. After the house batteries were replaced earlier this month I haven't seen anything higher than 11.3. No one has touched the alternator since MOT replaced it.

Questions: the red cable that is now on the positive terminal heads down towards the starter. Is that correct? If so, then should there be a second cable going to the isolator? Where should the negative cable attach? Or are the two cables simply reversed? Why did Foretravel use a RED cable for a ground and a BLACK one for positive?

Re: Alternator question

Reply #4 –
Quote from: kb0zke –
Why did Foretravel use a RED cable for a ground and a BLACK one for positive?
On our older coaches that have been "worked on" by many people over many years, you can't depend on wiring insulation color to tell you anything.  Sometimes so-called mechanics just grab whatever cable is handy and the right length.  If it matters, you must verify where the cable goes by checking at both ends.

Normal Foretravel alternator wiring is B+ output goes to center post on isolator.  On some alternators (which ground through the mounting bolts) that's all there is - one single wire.  Hence the name...

IF the alternator has a large negative terminal it should be connected to a good chassis ground - usually a nearby bolt on the engine block.  Even if your alternator only requires a single wire hookup, it is a good idea to add a separate ground cable.

The alternator B+ output cable should never be connected directly to the starter.  The starter should be powered by the start battery.  The alternator B+ output MAY be connected directly to the start battery.

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: Alternator question

Reply #5 –
Pierce, Andy, or anyone else with a similar coach, would someone verify the routing of the wires from the alternator? The isolator is a Victron (I think) with only the three main connections (alternator, house batteries, and start batteries) and a ground wire. I'm thinking that the positive cable should go to the alternator terminal of the isolator and the negative cable should go to a convenient ground point.

Re: Alternator question

Reply #6 –
You are correct the + cable from the alternator connects to the Alternator terminal of the isolator, usually the center terminal. The ground cable connects to a good ground frame or battery negative terminal. Withe the coach running you should have close to 15v at the battery terminal of the isolator/positive term of the alternator,  and 14.3 or so at the other 2 terminal or the isolator is bad. It takes at least 13.6-13.8 volts to charge 12v lead acid batteries.
Bruce, Linda, and Macy
Zoey RIP 1/20/19
1999 U295 40' build #5400
2017 silver Jeep Wrangler, 1260 watts of solar on top
Moving around the country

Re: Alternator question

Reply #7 –
You can check in the wiring diagrams for you coach also. Sounds like maybe someone got creative with the wiring for the alternator.
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 question

Reply #8 –
Well, we finally got the problems solved. Unfortunately, there are now some new ones.

The cables from the alternator WERE correct. You may remember that last year we started noticing that the chassis batteries were showing less than normal voltage, then would bounce back, only to drop again. Bumps on the road seemed to have more effect than anything else. That was a HUGE clue, but we didn't recognize it.  Long story short, the sense wire appeared to be firmly attached, but wasn't. It eventually wiggled completely loose, which told the alternator that no voltage was needed. Once that was taken care of, the alternator was once again working properly.

On to the next problem. Earlier this month I replaced the house batteries. I chose to go to the military style terminal rather than the old post. Naturally, some of the cables were too short, so we had to make do with using some long bolts. While we were changing those 150 pound batteries some of the cables took the opportunity to go hide, while another one joined some friends of a different persuasion. That also got fixed today.

What is now "broke" is the generator's starter and some wiring on the fuel solenoid. We'll get to Lincoln tomorrow and be there for several weeks. During that time I'll try to locate someone to work on the generator.

Lessons learned: I'm going to need some new cables for the house batteries anyway, so I'm going to replace more than just those. Any cable that connects to a + terminal will be red, and any cable that connects to a - terminal will be black. I'm also going to pay closer attention to simple things when an issue arises ( check ALL connections for tightness first).

Finally, a shout-out to Matt Gabrielson, owner of Gabrielson Truck Repair in Chillicothe, MO. He correctly tracked down all of the issues in only a few hours. He has a tow truck driver, Jacob, who brought is in (in the cold rain) Monday, and said that the coach would be fine there until we got back today. Why the delay? We were going to Lincoln for a funeral. Jo Ann's brother died unexpectedly last week, so we had to leave our campground hosting spot a bit early.

Gabrielsons doesn't normally work on motor homes, but we could see the challenge in Matt's eyes when we explained the situation. We were actually talking to the service manager, when Matt stopped by. He listened to the story, and said he wanted to take a look. He kept looking until he fixed everything he needed to fix to get us back on the road. While they were preparing the bill, we talked to the clerk and the parts manager. BOTH said that that's the way Matt works. He encourages his employees to do likewise.

I don't know if he would appreciate any more business from Foretravelers, but if you happen to be in the neighborhood when service is needed you might want to give them a call.