Can you use a CS series GM Alt. in place of an SI?

[Stationwagonguy]

Alright, I needed another alternator for my '78 Impala, and I picked up this 120 amp brand new CS series alternator, meant for a '90 Caprice cop car. Didn't learn about the different series alternators until after the fact, but oh well. Since the '78 came with a 10SI alternator, they mount the same, but have a different plug connector- the original has a two prong one, and the new one has 4 small prongs. Is there a way to use this alternator easily, or should I just go buy another SI?

 

[glh_turbo22]

Yes, here is what you do.

On the CS alt. the 4 treminals shold be labled

S
F
L
P

F and P are not used

S= battery supply. Test to find which wire on your old connector is always supplying B+ when the key is off and connect it to the S terminal.

The other wire that is left over is the field energizer wire that comes from your BATT light on the dash and that will connect to the L terminal on the CS alt.

If you need a connector to use on the new alt. you might be able to get a pigtail from the auto parts store or just cut one off a car at the junk yard.

Good Luck

 

[glh_turbo22]

One other thing, you need to check to make sure that you have 35 ohms or more at the wire that connects to the L terminal. Anything less than that and you will fail the alt. Usually the light bulb is the resistor in the circuit that will give you that ohm reading. To check this you need to dissconnect the battery, turn the key to the run position and connect one lead of the ohm meter B+ and the other lead to the wire that will connect to the L terminal. If it's greater than 35 ohm then your fine, if not then you will have to connect that wire to the F terminal.