Internally Regulated Alternator Sense Wire

[Captain Obvious]

The internally regulated alternators (used on the 78 and beyond) use a sense wire to control the alternator output. If the voltage on the sense wire is too low, it bumps the voltage up, and if it's too high, it cuts the alternator output down.

Question is... On the cars that came native with the internally regulated alternator, where did they connect the far end of that sense wire?

My engineering sense tells me that it would be best to connect the sense feedback wire right to the "+" terminal on the battery, but that's not what they did. And unfortunately this isn't the kind of info you can glean from a wiring diagram. According to the wiring diagram, it's connected right to the alternator output wire, but in reality, it's not. It should be on the other side of whatever voltage drops are expected along the wires under load.

Is any of this making sense? Does anyone have any idea what I'm talking about?? :paranoid:

[Mikes Z car]

You are making perfect sense to me. I just wish I had an answer for you and hope someone else does as I am curious about this. GM has a one wire alternator, not sure how they wire that to get around resistances.

[Zed Head]

Weird that I saw that same exact S wire connection in a wring diagram the other day and wondered myself, again. There are discussions and recommendations around the internet about where to put it. The one I like, that seems to make "sense" is to attach it in some central location to all of the various loads. Consider the battery to be a load, not a supply, since it is a load on the alternator once the engine starts. I ended up attaching mine to the starter lug for while, which is where the charge wire ends up after feeding the fusible links, and recently moved all of my loads, from the starter lug and the positive battery post, to a central one post attachment point under the relay bracket, and attached the S wire there too.

I did have the S wire attached to the battery post for a while but it seemed to make my voltmeter jumpy for some reason.

This on a 76 with the alt swap so I had the dilemma you're describing.

[djwarner]

See this site:

http://www.chargerr.com/Alternators/ALT.HTM

It says to connect the sense wire to the farthest point from the alternator, or rather where the lowest supply voltage will be found. This would be a function of wire size, wire length, and load current.

[Captain Obvious]

Thanks guys. Well at least it seems I'm asking the question in a reasonable way. You guys are talking the same thing I am.

My timing on this is that I'm planning some electrical changes on my car and as long as I was in there, I was gonna take the opportunity to switch over to the internally regulated alternator at the same time. And I'm planning to go in pretty deep and have the opportunity to do the swap a little more elegantly than the traditional commonly used method of just yanking the voltage regulator and jumpering a few wires.

That method, by the way, effectively just DOES connect the sense wire right to the alternator output right there at the alternator. It'll regulate the alternator output right there at the alternator, but doesn't account for load drops in the wires beyond the alternator.

So nobody has actually found the other end of that sense wire in an original system? I'm guessing they buried a splice in the white/red wire just before that white/red gets to the fusible link block. You can tell from the wiring diagrams that the two are joined on the same side of the fusible link, so it's not originally connected to the starter or directly to the battery.

Considering the above, I'm thinking the two locations that would make the most sense are right before the fusible link or at the fuse box. Would be nice to know what they originally did.

[240260280z]

Here is a block diagram for an internally regulated Z and it shows the sense tapped from a downstream (of Fusible link)

For convenience, when converting from external to internal regulation, I connected the yellow sense to the 12V bus wire that is downstream from the battery through the fusible link. I guess if the fusible link burned up the sense circuit would fail however I am guessing the sense wire should be fused for safety.

Here is a blurb on the sense wire:

2. An "S" connection which goes as close to the positive terminal of the battery as physically possible. The "S" connection "senses" the battery voltage and this is the voltage that the regulator is tying to control. This connection has a high impedance, so it only draws only micro amps from the battery, so it can be left connected without fear of battery discharging.

[Zed Head]

Considering the speed of electricity the only thing that should really matter is that there is no voltage drop (resistance) between the S wire and the feed to the loads. The main problem with the positive post connection, opinion, is that it is typically somewhat dirty and corroded, and has an extra interface between the wire end and the other loads. That's why I moved all of my stuff off of the battery, and just a ran smaller wire to the battery to charge it. I also ran my negative wire to a modified connection (a metal strap/plate from the starter bolt) though so that I could disconnect the battery without touching the posts. Battery posts are a pain in general.

Edit - as of 1995, Nissan was running the S wire almost to the battery post, or at least one connection point away from the alternator. Notice which side of the fusible link the junction is on, it's not actually at the battery, but it's still almost connected directly to the charge wire at the alternator.

Edit 2 - Forgot to say that one thing that I always consider when looking at how the manufacturers put their cars together is that the cars are designed for efficient assembly. So what you see may not be the best way to do things, just the best compromise.

Edited May 12, 2014 by Zed Head

[Captain Obvious]

Thanks again for the input guys. I've been thinking about it some more and I've come up with a problem with using the positive battery terminal... You don't ever want the sense wire connected on the far side of any fuse or link. In the event that the safety device opens up, the alternator will go full bore on trying to bring the sense wire voltage up. Problem is that the sense wire has been disconnected from the alternator output and the feedback loop is broken.

I think I'll connect it close to the fusible link but on the same side as the alternator output. Like the 1995 diagram Zed posted. That'll account for the drop on the wires between the alternator and the link. Essentially it will regulate the alternator side of that fusible link.

[Captain Obvious]

And a little off topic, but about battery cables... Last year, I switched to VW battery cables. They're fantastic. Best design I've ever seen.

They've got great top hardware clamps and four taps for auxiliary wires. Room for zero to four additional wires on each post. I've got my FI power connected to one each +/- and I've got plans for some others.

If you don't know what I'm talking about, let me know and I'll snap some pics.