[PurdueSD]

Originally Posted by VetteVet

If you look at the first sentence of Of what I posted you will see that there were two types of plugs PLIS and SFLP. You have a SFLP plug that was the donor plug in the harness that you cut which was unmolested.
You also stated that the P and the S slots in the plug were uninhabited. If you read the quoted again you will see that the F slots were not used in some vehicles and the P slot was used for accessories like a tach so it is entirely possible that the vehicle did not have those features

The purpose of the S terminal is for voltage sensing on the vehicle circuits which tell the alternator's internal regulator what the circuit voltag is and to increase or decrease it accordingly. Some vehicles read that data off the vehicle main junction via the ECM so the S terminal was not used. Some of the later year CS alternators had features that red the circuit voltage off the large output wire on the alternator. Your alternator might be one of those or it might not.

In any case, look at the terminals inside the alternator where the plug goes and see if you have the four terminals and if so you only need the L and the S terminals and in fact you may only need the L terminal if the alternator is post 1994. It would not hurt to run a sensing wire to the S terminal anyway. It would be constant hot from some hot source preferably the main junction of the battery and alternator.

You will definitely need to connect an ignition on source with a resistor inline to the L terminal to be the alternator exciter circuit.
I don't see any way that connecting it like that will hurt the alternator or the harness.

If it were me I would locate an alternator plug from a vehicle that matches the alternator and splice in these two wires to the alternator. The S and L wires. Here is the diagram I use to describe the conversion. It has been used by countless people and it works.

The brown wire from the firewall is the exciter wire from the ignition switch, and it shows the resistor wired inline to the L terminal and the other S wire comes from the man junction.

Attachment 1866108

Originally Posted by dmjlambert

Since the donor vehicle didn't use the S lead, it is possible the alternator doesn't need it to work, especially in the 1987 truck. In your simpler wiring in the older truck where you are moving it to, it is possible connecting S anyway will better control the alternator output. To do that you would need to buy another plug that has the connector in the S position, or buy one of those adapters like VetteVet gives the part number for.

The American Autowire pdf I gave the link to they say you just need the L wire connected with a resistor as shown in their adapter diagram. That you can do with the donor plug, you just don't need to connect the red/black-stripe F wire to anything.

I'll agree this stuff is confusing. I bought a CS144 alternator for a 1993 Chevy truck, but I bought it new from the parts store instead of getting it with the plug out of a donor truck. So I don't have the benefit (or added confusion) of knowing what terminals were used in the 1993 application, but I just wired it using VetteVet's guidance in the many forum posts on this subject. I used the L and S terminals as shown in VetteVet's diagram above. Works great!

Thanks! You guys are the best! @vettevet thanks for the lesson! And @DMJLAMBERT