GM Upgrade from an SI to a CS Alternator


Why go through the trouble? Well, the GM SI alternators are easier to get and have higher outputs. SI is short for Systems Integrated which means the regulator is inside the alternator case. The CS alternators are even better!

The GM 10SI Alternator

All 10SI have three terminals (including those with a 1 wire regulator).

  • The large BATT terminal which gets connected to your battery positive and a dual terminal
    connector. (available at any autoparts store)
  • The 1 or R – Relay Terminal. (Marked with a 1 and R on case) This terminal provides a pulsing DC signal which varies with engine rpm. The voltage is half system voltage as measured with a voltmeter. This terminal is used to connect to the dash warning light, or used as a tachometer connection For the warning light, a lamp is wired in series with a switched voltage source. During normal operation the lamp stays off. If the regulator is damaged, the 1 terminal provides ground, and the warning lamp will light. This terminal is also active on 1 wire regulator equipped 10SI alternators.
  • The 2 or F or Field/Sense Terminal. (Marked with a 2 and F on case) This terminal is used to excite the 10SI into operation. (3-wire 10SI) It is connected to the battery positive. For simplicity you can connect the 2 connector pigtail directly to the BAT terminal on the alternator. The terminal is present on 1 wire regulators. Used only for those that require the stock connector to fit snugly. If you are converting from a 3 wire 10SI to a 1 wire regulator you can hook up all your stock connectors, and run it as is. However, that’s wasted money unless you plan on cleaning out some wiring under your hood.

Typical Sources

  • Delco 10SI Series (61 amp/Single V groove pully) – Delco 1105360 / Lester 7127,7128 and 7282 used on 85-73 GM 4, 6, 8 Cyl
  • Delco 12SI Series (78 amp/Single V groove pully with M8x1.25 threaded ear) Delco 1100250,1105370,1105372 / Lester 7271, 7272,7272R and 7273 used on 83-89 GM 4, 6, 8 Cyl.
  • Delco 12SI Series (94 amp/Single V groove pully with M8x1.25 threaded ear) Delco 1101308 / Lester 7294 used on 84-87 GM 4, 6, 8 Cyl.
  • Delco CS130 Series (100-105 Amp /Single V groove pully) Delco 1101229, 1101275, 1101292 / Lester 7808, 7888 used on 88-90 5.0L Buick Estate Wagon, Cadillac Fleetwood Brougham (RWD), Chevrolet Caprice, Oldsmobile Custom Cruiser and Pontiac Safari.
  • Delco CS144 Series (140 Amp/6-Groove Pulley) Delco 10479891, 10480201 / Lester 8112 used on 93-96 5.7L Buick Roadmaster, Cadillac Fleetwood Brougham (RWD), Chevrolet Caprice and Chevrolet Impala 5.7L

If the one wire alternator is for cleaning out wires, you only need to retain the BAT wire. The 1 and 2 terminal wires can be eliminated. Don’t be surprised to find that the Field wire only goes a short way into the harness and spliced into the BAT wire. The 1 wire regulator comes with a dust plug for the 1 and 2 terminals.

AC Delco wiring package 1870921 (for those 6 to 12 volt conversions) for wiring up a 10SI. This contains the terminal connector AND an extra resistance wire pigtail to connect to the ignition system (don’t use a ballast resistor if you use a resistance wire). Also available is an ammeter package (1965400). Another piece (pn #8077 and 8078) is available from AC Delco for upgrading to a CS style from an SI. These SI-to-CS adapter plugs have a Molex connector to fit the existing wiring harness SI style connector, and the Delco Weather Pack connector to fit the CS-130 and CS-144. The AC Delco #8077 is used if you have a lamp on the dash and the #8078 has a load resistor for use with no lamp.

Alternate part numbers for the dash light version pigtails are Haywire P/N 2110 / Painless Wiring P/N 30707. General Motors P/N12102921 / Pico P/N 5331 are for no dash light installs.

Pulleys can be interchanged between the old externally regulated and the 10SI/12SI/CS-130, and even with the old externally regulated FORD alternators.

Always use a voltage gauge to monitor your charging system.

Don’t expect your alternator to do anything for you at idle speeds. Alternator output increases with rpm, even a 100 amp unit won’t put out much more than a 63 amp unit at 1000rpm.

The CS130 Alternator

In 1986 GM introduced the completely new, 105-amp, CS130 Delcotron alternator (CS130 = Charging System with 130 mm diameter stator) because the SI series alternators could not keep up with the increased electrical demand and because overdrive transmissions were lowering engine and alternator rpms. The CS130 weighs less, is smaller in diameter but uses the same 6.6" mounting-hole, center-to-center distance, uses less internal parts, has a better voltage regulator system, has increased durability and is less noisy (audibly and electrically) than the SI alternator it replaced. There is also a 120-amp, CS144 version if you need more output because you’re running large amounts of electrical equipment in your vehicle such as high-powered stereos.

The 105 amp version size is close to typical SI alternator. So you can use the original brackets. Typical SI alternator is only 63 amps and at higher RPM’s than what the CS’s need. In other words, at idle, the CS is putting out, where as the SI is just spinning.

The 105 amp CS130 alternator is on late-80’s GM truck or full size car with mounting ears that are 180-deg apart. Most will come with a serpentine belt pulley so you will have to change over your pulley if you are still using a V-belt.

The flange for the tensioning bolt and the alternator bracket are tapped for metric bolts so you will need to get 8 x 1.25mm bolts. The alternator bracket may need to be modified and you will probably need a couple of washers to take up the space between the alternator bracket and the new alternator.

Most of the connectors for the CS alternators are four wire but will only use two of them and the wiring is the same as the SI.


5 thoughts on “GM Upgrade from an SI to a CS Alternator

  1. Stuart

    Bob, help! I put a 502ci GM crate ingine in my 1973 Corvette and because of the taller heads and valve covers I could not use the factory original brackets and accessories with SI style alternator, so I went with an aftermarket serpentine system which uses a CS 130 alternator. It’s 140 amp 3 wire style.

  2. Stuart

    Bob, help! I put a 502ci GM crate ingine in my 1973 Corvette and because of the taller heads and valve covers I could not use the factory original brackets and accessories with SI style alternator, so I went with an aftermarket serpentine system which uses a CS 130 alternator. It’s 140 amp 3 wire style. The car has an ammeter instead of voltmeter or warning light. Also the horn relay appears to be in the middle of the terminals 1 and 2 and battery and ammeter/solenoid.
    Please help, I don’t want to burn anything up.
    Thanks, Stuart

  3. Travis

    Hi bob,
    Just wanted to let you know that your description of the 10si wiring scheme is completely wrong. I work for a shop that re-builds a lot of these units, and the correct scheme is as follows:

    1 Field exciter lead
    2 Voltage sensing lead
    R Relay terminal post (if equipped, most non-agricultural units are not)
    BAT Battery terminal post

    You have indicated in your article that the number 1 terminal and R terminal are the same. They absolutely are NOT, however the proximity of the two markings has led many people to believe as you do. The R terminal is a separate stud on the back of the case, and most automotive versions of the 10si don’t have one. The number 2 terminal is for remote voltage sensing only, and does NOT excite the field.

    Typically the number 1 terminal is wired to the ignition switch through a warning lamp. This powers the field windings during start up, and once the rotor begins spinning, outputs full system voltage. The number 2 terminal is typically wired to the main power distribution junction, and carries full system voltage. The R terminal is usually connected to a tach/hour meter on off-road equipment, and in some cases used to power 6 volt relays. It carries half system voltage, pulsed at a frequency equaling input rpm multiplied by generating factor.

    • Thanks for the updated info. While I am a self-proclimed “know-it-all” I stand corrected. I will fix the article ASAP. I guess someone actually does read some of my stuff. Thanks again for the correct information from a great direct source.

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