My car originally had the 75 amp SI type alternator in it. My friend gave me a 140 amp CS style alternator out of his 96 trans am, and told me it needed a rebuild since it was only putting out 100 amps. I completley went through it and put in a new regulator good to 160 amps. I bought the SI to CS adapter harness from GM, and installed the Alt. Its not charging at all, I get like 9 volts on the guage and the car drains the battery after running for 20 minutes. I took the alternator itself to two separate shops and it bench tested fine. I can't find any problems, the factory brown wire is connected to the center pin in the 4 prong connector, and the big red wire is connected to the large pin in the CS connector then loops up to the post on the alt (this is how the adapter connects the wires). I have a perfect ground (4 gauge stereo wire with gold terminals bolted right to the case) and I'm feeding the alt with 10 gauge stereo wire. Do I have it wired correctly? Any suggestions on what it could be? Its driving me nuts, I've spent 3 nights trying to sort out the problem to no avail. When the alt was in the trans am it only used the brown wire, so I disconnected the red wire from the post and I still get nothing. Thanks for any help.

 

ok, disconnect your battery, then, with key ON(batt still disconnected), check the resistance from your positive batt cable to the brown wire at the alt(should be the "L" terminal (labeled on the plastic of the alt.) , you have to have at LEAST 35 ohms resistance, and I belive, no more than 300 ( don't quote me on the 300), you will have this with key on only, if it is there wih key off, wrong wire.
If you search the web for CS alternator wiring you will get more

 

Ok I did that test, and I get nothing on my digital voltmeter which tells me I have an open circuit. Any ideas what could cause that? The car worked fine before I swapped the alternator. I'm confused. When I did the test you descirbed I still had the ground hooked up to the battery, and I disconnected the postive lead and unpluged the harness on the alternator, then turned on the key and put one probe on the disconnected battery lead, and the other in the L terminal on the connector (brown wire). I assume I did the test right, so now I'm out of ideas, I'd try and trace the wire but after the bellhousing I can't see it/reach it. Any further input would be appreciated, thanks.

 

does your car have a charge/batt light ?
if yes, is the bulb ok?
if no, the alt. can use any key on power as long as you put a 35 ohm resistor inline to the "L" terminal

 

The brown wire is the "sense" wire. With the plug off the alt you should read approx 12 volts on both wires. They use a resistor to provide a slight voltage drop for the sense so that the alt always provides a small charge to the battery. If the sense lead did not have a resistor the alt would only charge at a trickle at best. Some models use a resistance wire in the harness for sense. You can probably buy the resistance wire from the dealer and simply tie the wire to the L terminal and then jump it to the other wire on the plug. Go to www.alternatorparts.com, they have a free online manual for the CS alt complete with wiring diagrams.

 

My car has a factory volt gauge and no charge light in the circuit. I assume it has a diode somewhere to give the required resistance. I can't see anything being wrong with the diode since the car worked perfect before I swapped alternators, and I didn't change anything else besides the alternator with the exception of removing the cruise control harness (it was already unplugged and deleted before I just wanted the harness out of the way to neaten up the engine bay. I'll play with the voltmeter some more and maybe rig a resistor into the field wire to see if that helps. Thanks for your replys.

 

Just dug my voltage notes up. Typical TG(89 used for tests-CS alt) had at heavy red on plug wire 12.5 volts, sense wire measured 12.19 using Fluke 77 DVM. A diode will drop the voltage anywhere from .3 to .6 volts. The sense wire is fed on most models from the fuse panel, either the guage cluster fuse or the blower motor fuse. They do this again because they want a circuit that has the largest voltage drop so they can keep the voltage in the car around 13.8. Remember the vehicle wiring also has resistance. Check for a blown fuse in the fuse panel. Unplug the alt connector, you can leave on the big wire on the rear and turn on key then measure both wires. You should read batt voltage on the heavy one, slightly less on the sense. Assuming the case of the alt has a good ground and both voltages are there then suspect a wiring error in the adaptor or regardless of testing the alt. keep us posted, dan

 

Ok, I've got 12.8 volts in the red wire at the connector, and hardly anything in the brown wire with the key on. Checked all the fuses and all looks good. Any ideas? At least I narrowed it down to the field wire now, heh.

 

My concern is that the old alt worked. When you measured the sense wire did you do it with the CS adaptor or directly to the old plug harness. Is it possible where the adaptor and the old plug mate it is off by a pin? Is there a pin bent over in the connectors? Unless you shorted something inadvertently or saw a spark I can't imagine anything being blown. You may have to try to trace the sense wire. I'll look some more in my books to see if I can narrow down where the resistor and/or diode is. I suspect it is in fact a resistor wire. The next question, if it is blown why did it go. Double check those plugs, and do the same tests with the adaptor unplugged if you have not already and I'll repost when I get more info.

 

Actually when I did that test I went as far as too remove the brown wire right out of the facotry connector and test it directly, so its definatley screwed. I checked the adapter harness and all the pins are making contact, if I do a resitance test from one side to the other its perfect. All the pins are perfect and everythings making contact. I can't figure it out, I'm pretty sure nothing shorted, I touched my wrench to the valve cover while tightenening the nut on the alternator post, and that shot some sparks, but this was after I startered having problems with it (this was after installing the CS alt for like the 5th time). Argh. Thanks very much for checking that stuff for me.