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**dmjlambert** · 08-13-2019, 11:11 PM

I don't know. I suggest work on one thing at a time. I don't think the alternator could be related to tachometer, unless you have a computerized truck.

The stock wiring harness has a resistor wire in the cab that goes between the ignition switch and the firewall connector, then from there it is a regular brown wire that connects to the alternator. I don't know if your replacement harness has the same resistance. It probably does.

The wiring harness instructions cannot say that you don't need a regulator, because whether or not you need a regulator depends on whether there is one built into the alternator. The original alternators on these model years of trucks we have did not have a built-in regulator, they depended on an external one. If you know your alternator is an SI series alternator, then that would have a built-in regulator. It may be the type that your new wiring harness is made for.

So what is the more complete picture of what you have going on? Was your truck up and running with the new wiring harness and an SI alternator and everything working for some period of time, and then the battery started getting discharged leading you to put another SI alternator in it and things started going bad from there? Or some other sequence of events? When you went to get the alternator, did you get it from a car parts store and tell them you have a certain make and year of vehicle to get them to hand you an SI alternator instead of giving you the original style alternator for your truck year? Or did you get the alternator from some other source?

I am wondering on a new wiring harness how was the alternator plug already gone bad and needing to be replaced. I am not judging or critical of your story, just trying to get the full picture of what's going on in order to possibly help.

I think maybe the pictures you will take over the weekend may help. If you still have the old alternator that was working before it started draining the battery would be good to see, front and back and connector plug. And the new alternator that is misbehaving, front, back and connector plug, and pictures of your new connector that you attached to replace the bad connector. And whatever wires you have that are now melted.

Sorry to hear about these troubles, I know electrical problems are a drag.

08-13-2019, 11:11 PM	#6
dmjlambert Senior Member Join Date: Apr 2016 Location: Cypress, TX Posts: 3,563	Re: 1970 C10 Alternator/wiring issue I don't know. I suggest work on one thing at a time. I don't think the alternator could be related to tachometer, unless you have a computerized truck. The stock wiring harness has a resistor wire in the cab that goes between the ignition switch and the firewall connector, then from there it is a regular brown wire that connects to the alternator. I don't know if your replacement harness has the same resistance. It probably does. The wiring harness instructions cannot say that you don't need a regulator, because whether or not you need a regulator depends on whether there is one built into the alternator. The original alternators on these model years of trucks we have did not have a built-in regulator, they depended on an external one. If you know your alternator is an SI series alternator, then that would have a built-in regulator. It may be the type that your new wiring harness is made for. So what is the more complete picture of what you have going on? Was your truck up and running with the new wiring harness and an SI alternator and everything working for some period of time, and then the battery started getting discharged leading you to put another SI alternator in it and things started going bad from there? Or some other sequence of events? When you went to get the alternator, did you get it from a car parts store and tell them you have a certain make and year of vehicle to get them to hand you an SI alternator instead of giving you the original style alternator for your truck year? Or did you get the alternator from some other source? I am wondering on a new wiring harness how was the alternator plug already gone bad and needing to be replaced. I am not judging or critical of your story, just trying to get the full picture of what's going on in order to possibly help. I think maybe the pictures you will take over the weekend may help. If you still have the old alternator that was working before it started draining the battery would be good to see, front and back and connector plug. And the new alternator that is misbehaving, front, back and connector plug, and pictures of your new connector that you attached to replace the bad connector. And whatever wires you have that are now melted. Sorry to hear about these troubles, I know electrical problems are a drag.