So I finished up the wiring on my 36 gmc resto mod. 4.3 vortec, 12v system. I have a one wire alt. I got all done closed up the shop for the night and the next day the battery is dead So I pull the neg cable off and connect my test light between the neg post and the cable, The light glows. So I start disconnecting wires and the light will not go off. I go out to the alt and disconnect the wires there and the light goes off There are 3 wires there, one from the starter hot post, the alt post to the junction block, which then powers up the fuse box and the 3rd wire is power for the electric fan relay. If I touch the wire from the starter to the alt wire the light comes on. So I must not have this wired incorrectly. should that alt wire go to the ign switch or headlight switch first and then to to junction block? Not using the amp gauge, it's a volt gauge now. once I get this fixed I can start hanging sheet metal. Thanks for any help with this problem.
Last edited by FAST55; 02/27/20253:21 PM. Reason: spelling
'57 GMC 102, Original 347 V8, HydraMatic, 3.08 rear gear, added A/C, disk front brakes, HEI, AFB carb, '98 Honda Black Currant paint. T-boned and totaled 10/12 '52 GMC 152 Stake Bed, Original 228, SM420, added A/C, HEI, disk front brakes, '67 Chev 3.55 rear gear. Gets used as a real truck.
Disconnect the big wire at the alternator output terminal and see if the light goes out. If it does, you've got a shorted diode bridge. Even a "new" alternator can have bad circuits. A parasitic drain in the regulator (the two small wires) usually takes several days to kill a fully charged battery. One faulty diode will still let the alternator charge, but the max output will be less than 1/3 of its rated capacity. Jerry
"It is better to be silent and be thought a fool than to speak and eliminate all doubt!" - Abraham Lincoln Cringe and wail in fear, Eloi- - - - -we Morlocks are on the hunt! There is nothing noble in being superior to your fellow man; true nobility is being superior to your former self. - Ernest Hemingway Love your enemies and drive 'em nuts!
The only time the light comes on is when the wire from the starter and the alt are connected together. I can hold them in my hand and connect and disconnect and watch the light go off and on.
If I take the wire from the starter and run it to the run side of the ign switch would that fix this? Or the alt wire to the ign switch and then to the junction block, with a fuse by the junction block
I'm helping a buddy change the engine in his Dakota so the alt problem will have to wait a week or so before I get back to it. I'll update when I get back to it. Thanks for everybody's help so far.
So I pull the neg cable off and connect my test light between the neg post and the cable, The light glows. So I start disconnecting wires and the light will not go off. I go out to the alt and disconnect the wires there and the light goes off 
There are 3 wires there, one from the starter hot post, the alt post to the junction block, which then powers up the fuse box and the 3rd wire is power for the electric fan relay. If I touch the wire from the starter to the alt wire the light comes on. So I must not have this wired incorrectly. should that alt wire go to the ign switch or headlight switch first and then to to junction block? Not using the amp gauge, it's a volt gauge now. once I get this fixed I can start hanging sheet metal. Thanks for any help with this problem.
