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Alternator question

Last year I noticed that the dash voltmeter was not reading as high as it used to. Occasionally it would drop down into the yellow range, so I mentioned that to MOT. They diagnosed a failing alternator and replaced the alternator. On our way north the voltmeter dropped into the red a few miles out of town, so we went back. They poked and prodded and pronounced us good to go. Same thing happened in the same place. Keith came out and asked to drive the coach with a tech stuck in the back with a device connected to something in the engine. We headed out and the same thing happened. This time they had an idea of what the problem was. Back at MOT they discovered that the +12vdc for the alternator was being taken from the same place that provided the power for the radiator fans. When the coolant warmed up enough, the fans took the 12v away from the alternator. Keith did a bit of rewiring, which also gave us both low and high speeds on the fans, and pronounced us good to go for the third time.

Since then we've had the same problem. The alternator will sometimes drop out and then come back by itself. I suspect that the wire is the problem (after 30 years, you think that might be so?), so yesterday I bought some wire and thought I'd get that +12vdc directly from the battery bank, with a fuse at the battery connection.

Questions: Other than possibly running the batteries down, is there any problem with getting the 12 volts for the alternator directly from the batteries? If not, what size fuse should I put in? The wire is 10 gauge.

Re: Alternator question

Reply #1 –
I can't imagine fans wired directly to alternator output.  Yes, the chassis battery would be where the power should come from.

But, you need to verify that amp draw of the fans-- are they really exceeding alternator output???

What fans are these-- I thought your cooling fan is hydraulic.

Yes, puzzling.
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: Alternator question

Reply #2 –
Thanks, Brett. Yes, the fans are hydraulic, but they have an electrical device that switches from low to high. That's where the +12vdc was taken from. I suspect that it was on the low side of the switch, so when the fans went to high the alternator didn't get the 12 volts it wanted.

What size fuse should I use for the alternator? I suspect that the 10 gauge wire is probably heavier than necessary, but that's what I found, and the rest will be used for another project.

Re: Alternator question

Reply #3 –
David,

Is the wire to supply the fan controller connected to the B+ terminal? Sense terminal? The isolator-- if so which battery's lug?

The fan controller should require very little power/few amps.

And, when you ask "what size fuse for the alternator", which wire are you considering fusing (from where to where)?
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: Alternator question

Reply #4 –
From battery to the IGN terminal on the alternator. That's the wire that was connected to the fan controller, and I suspect that the wire has a problem.

Re: Alternator question

Reply #5 –
Quote from: kb0zke –
From battery to the IGN terminal on the alternator. That's the wire that was connected to the fan controller, and I suspect that the wire has a problem.

Who in the world would have wired it that way????? That is a low amp wire that just tells the alternator that the ignition is on and should NOT have power except with the ignition on.  A very poor choice for supplying power to something else!

Plenty of "ignition hot" sources in your engine room that can be utilized for that fan controller function and excite function. If low amp draw, it can be wired directly from an ignition hot source.  If higher amp draw, use that ignition hot source to close a RELAY.
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: Alternator question

Reply #6 –
Quote from: kb0zke –
From battery to the IGN terminal on the alternator.
It sounds like you are talking about the "EXCITE" wire.  Does your alternator have both a "EXCITE" wire and a "SENSE" wire?

The "EXCITE" wire should only show battery voltage when the ignition switch is "ON".

The "SENSE" wire should be always HOT and connected either directly or indirectly to read START battery bank voltage.  It should be fused at the end close to the battery connection.

If you are talking about the "EXCITE" wire, then it should not matter where it is connected, as long as it sees 12 volts ONLY when key is "ON".

DO NOT connect the "EXCITE" wire directly to the battery.

All the excite wire does is "turn on" the alternator.  It does not affect the output of the alternator.  If your alternator is not putting out correct voltage I would look elsewhere for the problem.

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 #7 –
Okay, I'll connect it somewhere else that is on only when the key is on. I'm assuming that a 5A fuse is sufficient for this.

Re: Alternator question

Reply #8 –
When you say "it", are you referring to the alternator excite terminal of the alternator? Fan controller? Or a separate wire and fuse to each.

And, as Chuck asked, make sure you are referring to excite terminal, not sense terminal of the alternator, sense terminal needs to be connected to chassis battery either directly or indirectly (often at chassis battery lug of battery isolator).
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: Alternator question

Reply #9 –
5 amp fuse should be fine.  You just want a fuse that will blow before the wire melts in the case of a dead short.

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 #10 –
Yes, excite wire. The alternator has four terminals: Positive (big red wire), Negative (big black wire), Sense (smaller red wire), and IGN (which is the one I'm working on). In a few minutes I'll go out to the coach and connect the engine batteries Then I'll find a place that provides 12v only when the key is on.  I'll connect the new wire to that point through a 5A fuse.

Depending on what time it is and how sore my back is, I may go for a test drive today. More than likely, though, I'll move on to the next project and test both at once, maybe next weekend.

Re: Alternator question

Reply #11 –
And, how will you wire the fan controller? Again, depends on amp draw to determine if it can be wired directly from an ignition hot source or if you need to use a relay with ignition hot closing the relay.
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: Alternator question

Reply #12 –
Quote from: kb0zke –
In a few minutes I'll go out to the coach and connect the engine batteries Then I'll find a place that provides 12v only when the key is on.  I'll connect the new wire to that point through a 5A fuse.
A common place to connect the "EXCITE" wire is at the remote start panel in the engine room.  Turn your ignition key ON and check the terminals on the back side of the panel.  One will be HOT with ignition.

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"