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Still confused on my sense wire

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Old 12-07-2017, 08:23 AM   #1
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Still confused on my sense wire

So I have been searching and thinking but I still haven't found an answer. Basically, intermittently my alternator way over charges. It reads 18vdc at the battery. To solve the problem I replaced the stock alternator with a CS130 and did the big 3 while I was at it. But it didn't fix the problem. So I looked at the schematics and the only guess I could came up with is that the sense wire is reading the wrong voltage. So while it was running at 18vdc I reached over with my wire cutters and clipped the sense wire and the voltage dropped to 14.4vdc and it has been fine ever since. However my OCD is driving me crazy and I want to figure out whats wrong with the sense wire so I can hook it up right. Does anyone have any ideas on what could be the problem?
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Old 12-07-2017, 08:38 AM   #2
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... while it was running at 18vdc I reached over with my wire cutters and clipped the sense wire ...
.... However my OCD is driving me crazy and I want to figure out whats wrong with the sense wire so I can hook it up right. Does anyone have any ideas on what could be the problem?
If you have OCD you should really figure out exactly what is wrong before doing anything.
Exactly where did you clip the S wire?
How about:
1) Clip it close to alternator connector,
2) Splice or solder a piece of white wire (16 or 12 AWG) to the connector side,
3) Terminate other end with 5/16" ring terminal,
4) Connect that ring terminal to positive battery terminal
That's where it should be so it reads battery voltage accurately.
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Old 12-07-2017, 09:06 AM   #3
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My guess (read RAD4Runner's post for the steps to reduce that from a guess to an estimate) is that your sense wire was shorted to ground, not battery, somewhere. With the sense wire pulled to 0v, your alternator was trying desperately to bring it up to 14.4v (and 18v was as far as it could reach). With the sense connection now "gone," the alternator is outputting "open loop" at 14.4v. But the voltage that ends up on the battery terminal will wander up and down with varying load, because your alternator can no longer "sense" the voltage at the battery to compensate.

I would start by putting a voltmeter on the clipped end of the battery-side sense wire. It should read (duh!) the same voltage as the battery, and should track up and down with load on the battery.

If you don't fix the sense wire, you might have dim lights, intermittent radio reception, and possibly an under-charge situation. Or not.
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Old 12-07-2017, 09:50 AM   #4
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... your sense wire was shorted to ground... your alternator was trying desperately to bring it up to 14.4v (and 18v was as far as it could reach)...
Yes (no voltage regulation) ^^^
Bill Edgar,
Although you can fix the regulation problem by what I suggest above, I highly-recommend to physically trace the S wire to see what's really going on and make sure it will not cause a short again in the future. The sense wire is spliced to the "B" wire somewhere near the fuse block.
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Old 12-07-2017, 10:12 AM   #5
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I did most of these troubleshooting ideas. I checked the voltage at the sense wire with the plug unplugged at the alternator and it read 12 to 14 vdc somewhere in that neighborhood but as I said the problem is intermittent I don't have 2 multi meters to check voltage at the battery and the sense wire at the same time. So then I traced the wire and it is white and runs from the alternator through the wire loom in front of the radiator along side the alt B+ wire to the fuse box 80a alternator fuse. I pulled the fusebox and looked underneath and both wires literally join each other at the fuse box. So I thought well electrically they must be the same so why not connect them closer to the alternator. So there is a terminal on the drivers side inner fender that the Alt B+ wire connects to, for what I don't know but it has a little cover on it that says B+. So I cut the sense wire put a ring terminal on it and attached it there. So that eliminates all but a few feet of the sense wire. It still does it, that is when I just cut the wire to see what happens. The only thing I can figure is that the short is in the plug in the wire loom underneath the air box or somewhere along the wire between there and the alternator which is only like a foot of wire. Or its something else all together but seems to me its gotta have something to do with the sense wire because it hasn't overcharged since I cut the wire. Remember this is an intermittent problem though it happens more often than not.
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Old 12-07-2017, 10:18 AM   #6
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Originally Posted by RAD4Runner View Post
If you have OCD you should really figure out exactly what is wrong before doing anything.
Exactly where did you clip the S wire?
How about:
1) Clip it close to alternator connector,
2) Splice or solder a piece of white wire (16 or 12 AWG) to the connector side,
3) Terminate other end with 5/16" ring terminal,
4) Connect that ring terminal to positive battery terminal
That's where it should be so it reads battery voltage accurately.
That is probably what I will do, completely run a new sense wire to the battery. I just would rather stay in the factory harness and fix wherever the short is, instead of laying wire on top of wire. Or worse yet, run all new sense wire for nothing because I am barking up the wrong tree and its the 12v ignition wire causing the problem or the lamp wire or something.
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Old 12-07-2017, 10:58 AM   #7
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... Or worse yet, run all new sense wire for nothing...
Well, I'm a big fan of fixing the problem rather than trying a work-around, but RAD4Runners suggestion should set you back about $.50 if you buy all new parts. You can even just zip-tie it in place until you determine that was the problem (or not).

I recall that you had some difficulty trying to figure out how to convert the wiring for that CS130 to Toyota 4-wire. (Perhaps something to do with running the excitation current through the alt lamp?) If you used an all-stock adapter harness, you probably don't have a short IN the adapter, but if you had to do any splicing or re-pinning, I would sure look for a short where you did that work.
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Old 12-07-2017, 11:16 AM   #8
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Depending on vehicle, I believe the sense wire gets its signal from somewhere within the fuse box after a fuse. If you have a loose or corroded connection in there it will cause voltage drop and the alt will try to compensate. Compare Voltage at the battery to voltage in the cabin. If it drops significantly I'd start looking for loose and corroded connections. If it checks good its likely the sense wire
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