Small Block & Big Block Chevy Specific - excitation wire alternator, alternator wont charge
03-28-2012, 10:35 PM
well, my buddys old wagoneer has a problem. I know, this is a chevy forum but I'm pretty sure the the alternators that use this method work the same. His old alternator burnt up and has not been able to get the new one to charge. The old one burnt up his excitation wire and he didnt know what it was. He just replaced it with a regular piece of wire. After I told him what it was, he went and got a 15 ohm resistor to put in it, but he just used like an 1/8 watt one so I know he just burnt it up. I told him that I did not know where we could buy the "resistor" wire at so we ordered a 15 ohm power resistor from mouser .com. Its a big wire wound unit rated at 50 watts. Anyways he hooked that sucker up to 12 volts to terminal one, termial two straight to the positive post on the fender mounted solenoid, and the output of the alternator to the same post. Why in the heck wont this damn thing charge? There is only two wires, the "sense" wire which is T2, and the "Excitation" wire which is T1. Hes only getting 12 volts and driving at night with the lights on drains the battery. I think his alternator is shot. Its a "new" rebuilt unit but hes tried a bunch of things trying to get it to work so he may have fried a diode or something. Any thoughts? Hes gonna pull it and take to get it checked tomorrow. IF it shows good, Im really at a loss of how to get it to charge.
03-28-2012, 11:55 PM
On older Chevy's the excitation wire has the ALT lamp in series. Goes from the IGN terminal on the ignition switch to the alternator.
Al 95 Z28
03-29-2012, 03:12 PM
post a pic of the back of the alternator, and model number if possible.
I don't understand why you need a resistor, or resistor wire. Only thing a resistor wire was used for that i can think of was the ignition coil when you have points in the distributor.
make sure the mounting of the alternator to block is clean so there is a good ground path.
03-29-2012, 05:42 PM
sorry I do not have a pic of the alternator. It just has a two conductor molex plug labeled T1 and T2, then the actual output hook up. Yes, some vehicles had the idiot light and some did not. The ones that didnt had the resistor wire. I even tried hooking up a light and diode isolated it from the switched 12 volts thinking maybe that would do the trick..nothing. He just called me and the alt checked out good. I dont know whats going on. The jeep has all kinds of grounds. He has addded several from the frame to the battery, to the alt directly to the frame, etc etc. Here is a diagram of the original setup. I cant figure out the problem. ITs probably one of those things that is really simple Im just overlooking.
03-29-2012, 08:41 PM
read the bottom how to wire GM delco remy alternator to make it work.
i would try this:
with battery disconnected,
-have the alternator installed and with belt on alternator ready to run, but don't hook up engine wiring to the T1 and T2 plugs on back of alternator.
-Hook up the big output wire to back of alternator.
-with digital multimeter, check for continuity between main output stud of alternator with wire connected to engine block or alternator shell, there should be none. If so there's a problem, either short on the main output wire or internally in voltage regulator.
-if no continuity then hook up battery. You should see battery voltage at the output stud on back of alternator with the big wire connected, if not then there's a break in the wire or bad ground between alternator shell and engine bracket.
-get 2 jumper wires rigged up with a spade connector on the end of them that you can easily plug onto the T1 and T2 terminals. T1 is the excitation. T2 is the voltage sense and is optional. Have the jumpers plugged onto the T1 and T2 terminals with the other ends hanging out not touching anything.
- touch T1 jumper to battery +, start vehicle, alternator should charge. if not charging, touch T2 jumper to battery +, will most likely not make a difference but if it does something else is going on maybe the T2 jumper is excitation. the T2 or whatever the voltage sensing terminal is optional so you could leave that disconnected. but the purpose is to run a good wire from it to as close to the battery as possible, usually the main bus that's in the fuse box near the battery, so the battery charges properly.
- normal wiring for T1 is from ignition switch when in run position through battery warning light to T1 terminal on back of alternator. When alternator not charging, T1 is ground and battery light illuminates. when charging, there's 12+ equal voltage on either side of the warning light so it does not illuminate. I don't think there should be any resistor or resistance wire in this circuit, unless the warning light bulb needs less than 12 volts.
-also with ignition key on, with meter measure to see if there is 12 volts on the end of the T1 excitation wire that goes to the alternator terminal. If there isn't, that's your problem. i'm guessing that's it since alternator checked out good, or what you think is the T1 wire is not.