Skip to main content ↑ ↓
Topic: Chasing down electrical issues (Read 614 times) previous topic - next topic

Chasing down electrical issues

On the way to Nac this summer I noticed that the start battery voltage was fluctuating, so when we got to MOT I had them check it out. They determined that I needed a new alternator and put one on. When we left we didn't get very far before the start battery voltage dropped drastically, so we returned to MOT. They poked around some, didn't find anything obvious, so they sent us on our way. Same thing happened in the same place. Turned around again. This time Keith got involved. We took it for a test drive, and got the same result a third time. This time, though, there was a tech way in back with something connected to the wiring. He sang out about the same time I did (I was watching the computer) and said it was the ignition circuit.

This alternator requires an exciter wire, and the point that was used to get that +12vdc was a solenoid that kicks the radiator fans from low to high. As soon as the radiator warmed up the fans took the +12vdc away from the alternator and gave it to the fans. Alternator wasn't happy about that and started pouting.

Keith changed the wiring a bit and pronounced it good. Since then, though, the alternator seems to be dropping out and coming back. I'm suspicious of that excite wire. I wonder if there is another place that I can get that +12vdc from.

Also on the way back from Nac the turbo would drop out and come back. Andy put his Pro-Link on it and the machine said the boost sensor was bad, so I replaced it. Naturally, the new one isn't exactly like the old one. Close, but the three wire plug wouldn't seat properly in the new one. There was a spacer on the plug, so I took it off and now I think it seats better, but maybe not making a great connection yet.

What's interesting about both of these is that they seem to be connected, in that both alternator and turbo are working when I start out, then the alternator starts changing voltages and then the boost goes away. Turning cruise off seems to help sometimes, and sometimes when the Jake comes on the boost will return.

In talking to others, one thought was that maybe there is a ground problem. There is a ground point by the electric boxes at the foot of the bed, so I ran a ground cable from that to the negative terminal of the start battery bank. Didn't seen to make any difference.

Tomorrow I think I'll go out and raise the coach up as far as I can, put the safety stands in, then disconnect all of the batteries and crawl under the coach to see if I can see where the alternator's negative wire goes. Maybe the problem is down there somewhere.

Other suggestions?

Re: Chasing down electrical issues

Reply #1 –
Plenty of places that are "ignition hot" in the engine room to connect to the excite terminal. 

Also, any location on the chassis battery side of the diode-based isolator is fine for the sense (on edit-- corrected. thanks Peter) wire.

If in any doubt, just run a new one.
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: Chasing down electrical issues

Reply #2 –
Quote from: wolfe10 –
Plenty of places that are "ignition hot" in the engine room to connect to the excite terminal. 

Also, any location on the chassis battery side of the diode-based isolator is fine for the excite sense wire.

If in any doubt, just run a new one.

I think Brett meant "sense" wire here...
Peter and Tammy Fleming
1991 U300 GV 40 - Sold, owned for 4 years
Downsized to Roadtrek Popular 210 class B

Re: Chasing down electrical issues

Reply #3 –
Yes, MOT ran a new, bright red red sense wire already. The excite wire is is what is attached to the solenoid that runs the fans. I was going to go out tonight and work on it, but we have grandsons tonight, so I'll go out there another evening.

Re: Chasing down electrical issues

Reply #4 –
Quote from: kb0zke –
I wonder if there is another place that I can get that +12vdc (excite wire) from.
One place to tap into "hot with ignition" power is the remote engine start panel.

Another place would be the power wire going to the fuel shutoff solenoid.

Another place would be the power wire to heater element on the air dryer.

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: Chasing down electrical issues

Reply #5 –
A common Foretravel problem is caused by where they connected the alternator voltage sense wire. This wire wants to know true start battery voltage, so we ran the alternator sense wire directly to start battery positive post.

Sense wire is ignition source.

With alternator problems, many have driven for weeks until it can be fixed, by running generator to power house battery charger, than turn boost solenoid on to then power start battery.

Re: Chasing down electrical issues

Reply #6 –
Reply in SOLID LETTER CAPS.

Quote from: Barry & Cindy –
A common Foretravel problem is caused by where they connected the alternator voltage sense wire. This wire wants to know true start battery voltage, so we ran the alternator sense wire directly to start battery positive post. ACTUALLY, ANYWHERE ON THE CHASSIS BATTERY SIDE OF THE DIODE-BASED BATTERY ISOLATOR SHOULD/BETTER READ EXACTLY THE SAME VOLTAGE AS AT THE CHASSIS BATTERIES.  BETTER NOT BE ANY VOLTAGE LOSS IN THOSE VERY LARGE GAUGE CABLES.  IF SO, THE CABLES ARE BAD.

Sense wire is ignition source. SUSPECT YOU MEAN EXCITE WIRE IS AN IGNITION-ON SOURCE.

With alternator problems, many have driven for weeks until it can be fixed, by running generator to power house battery charger, than turn boost solenoid on to then power start battery.
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: Chasing down electrical issues

Reply #7 –
I Agree with comments and corrections, thanks for the post.

Our alternator voltage sense wire was factory-originally connected to a small size wire on the inverter panel.  I also fused the wire at new point of connection.

Re: Chasing down electrical issues

Reply #8 –
Quote from: Barry & Cindy –
  I also fused the wire at new point of connection.

Yes, a very good idea.  Again, this wire carries almost no current, so you are not looking at high amp fuse or large-gauge wire.  It is just a sense wire to tell the alternator what the post-isolator voltage is.
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: Chasing down electrical issues

Reply #9 –
Yep, alternator voltage sense wire has very low amp fuse at start battery mainly to handle accidental grounding along wire run.

Speaking of start battery: We are now parking coach under cover so solar is no help. Put $40 BlueSea high-amp switch on start battery bank positive but have been reluctant to trust any switch for starter current. Found our coach has a separate OEM battery cable going to starter solenoid that I did not switch. To eliminate all start battery drain, switch is on another large diameter cable going to inverter panel.