2g Alternator S terminals unused?

Curious about two things. My truck does not have charging issues or anything like that, but going down the rabbit hole on alternator-driven tachs is making me as these questions-

Section 1 - S terminals:
On most wiring diagrams for a 2g alternator, I see the S wire from the voltage regulator being connected to the S terminal on the connection of doom/B+ output plug. My truck has no provision for an s-wire on the fire plug, and has/had only wires going to the A and I terminals on the voltage regulator. I've never had a problem with charging, although running voltage is lower than I would expect at ~13.x volts. I replaced the ASI plug a few years ago, and just kind of have the S wire dangling/capped.

The questions I would have here is: Why does the S terminal jumper not matter on this truck? Or does it and someone in this truck's life didn't care?

Section 2 - Stator output:
Does the S terminal on the voltage regulator send or receive signal from the stator? I am curious if this would suffice to send signal to a tachometer?

We had a big discussion about the stator wire on the 2G alternator recently.
In fact Gary modified the EVTM diagram to show the correct pin out on the regulator plug.

The 'S' wire from the output plug goes to the choke heater on gas engines, but you have a diesel so it's not there.

I don't think the stator wire from the regulator plug is going to provide any meaningful pulse you can use for a tach. (but I'm not absolutely sure about that)
As i recall retrofitting a tach to an IDI was a challenge involving some hard to find parts.

Right, the S wire isn't used on a diesel.

But the EVTM says that the "Voltage at S is one half the voltage at B".  Given that I believe it is a half-wave rectified AC voltage so will look like the signal below and go from 0V to probably battery voltage.  Is that capable of running a diesel tach?

PossumTruck wrote

Does the S terminal on the voltage regulator send or receive signal from the stator? I am curious if this would suffice to send signal to a tachometer?

Have you installed a stock diesel tach cluster or are you considering an aftermarket tach?

IIRC the alternator pulley is 3:1 for diesels. How many pulses does the stator output for each alternator revolution?

As I recall I had to find a pump housing with the boss for the tach pickup (actually I swiped the housing and pickup+wiring from a wreck)

Our page at Documentation/Electrical/Alternators says the 420/IDI has a 2.18:1 ratio - although our 460's have a 3.0:1.  I don't understand that because the diesel doesn't rev as high as a 460, but whatever.  

I guess just another thing to consider if trying to use that wave as a signal, Gary

I'm very sure the Hall pickup in the stock configuration doesn't reach battery voltage.

(Clarity)

Here is a good article regarding using the stator output of 2G alternator to run a tachometer.

https://www.electricalrebuilders.org/eren/archive/ere_2008/ere_01_08_analyzing_the_alternator_tach_signal.pdf

The article... "Analyzing the Alternator Tach Signal" , is well written and very technical. It shows the stator output as a square wave.

Personally, I would have expected a wave form shown in Gary's post. Different DVM would try to interpret this differently, but it would be half the power output, giving the choke a slower opening time.

All 3 wires on the voltage regulator plug are inputs. Most likely the green wire was originally on the center position S. It is interesting that moving it to the I, and matching colors worked.

When thinking of a 2G alternator it is important to remember the are two separate components, the ALT and the VR which is screwed to the side of the ALT. Meaning the wiring on the VR needs to match the expectation of the VR manufacturer. What I am trying to say is if you install a newer VR you might need all 3 inputs to get it to charge.
1987 and on use 3 wires. The 2 wires on the VR is a 1986 Ford Mystery question.

Good luck, Jim #2

Interesting article Jim #2!  
I don't find it too technical, and think Nathan did a good job dumbing it down for the rest of us.
Surprised he suggests 'S' is sense in an article highlighting the 2G, when we know 'A' is sense 'I' is ignition on and 'S' is stator with these units.
Maybe he's pointing out that other (Delco?) alternators don't follow this convention?

https://www.delcoremy.com/alternators/alternator-features/remote-sense

It seems Sense or S can mean different things to different manufacturers.

Well, I had a free 3500rpm tach from a 90s HD truck. What I did today was replace the output plug on the alternator with a new one that had the stator output. I connected the stator output to what I believe to be the signal side of the hall effect sensor wiring on the truck (green/yellow stripe), and left the ground side attached to the stock tach sensor (Green/black stripe). Fired it up and I am now getting sufficient signal to move the tach, but its WAY too low - no amount of adjustment can get that thing to read anything above 150 RM at idle.

Live and learn, I was going to replace that plug anyway until I do a 3g conversion.

With the prices of factory tach clusters, I was hoping for a clever fix/miracle. Not so. Perhaps a gas tach could be made to read relatively close. I don't need laboratory accuracy, just a fun thing that seems accurate-ish.

Edit to add - All the wire splicing I did was on a factory tach sensor I pulled from a junk yard a few years ago. If it cost be $3 I would be surprised. I was not surprised that the hall effect sensor did not work with the free tach. I can wire it back together if I need to. I just wasn't going to cut into the truck's wiring harness any more than replacing that plug.