3.8 volts brown wire at alternator ???

[VetteVet]

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Originally Posted by pilotXC

Thank you as always, this is great. Do I need the busbar to starter 8 gauge wire if I went that route? The only reason I had it on the diagram I did because that was the only route to the battery from the alternator without jumping to the terminal block in your second diagram. If I used your diagram, the busbar would route to the battery from the alternator correct?
The factory only used the 12 gauge shunt wire to power the junction from the battery. It was the charging path to the battery from the alternator and it was also the power path to the junction in case the alternator failed. So the answer for a stock system would be no.

If it didn't affect the working of the ammeter, which needs the shunt wire, then I would go ahead and use it if you have upgraded to electric cooling fans or AC or any higher amp draws that are so necessary with the modern engines. Yes the busbar would route to the battery instead of going directly to battery positive and bypassing the shunt. In this diagram, the ammeter is not used and a voltmeter replaces it so the shunt wire is not necessary. I would plan to use it if you decide to replace the ammeter.

Would I need to fuse the 8R wire to the terminal block on the fender well? I assume so, just want your input on what amp fuse I should use. Also, you think it's alright to splice the brown ignition wire into the L wire going to the PCM? I'm going to call the person that altered my harness and ask him on this as well...

I would use a 30 or 40 amp maxi-fuse depending on the amp draw of the system. The fender well junction on the stock 67-72 trucks used a 40 amp fusible link which protected the battery powered source to the truck harness. You can never have too many fuses just like you can't have too many grounds in any electrical system.

I would definitely not use both inputs to the alternator. You risk damaging the PCM and the alternator regulator with over voltage. The videos tell us not to ever connect an unrestricted 12 volt source to the L terminal. IF wired right, with the proper resistor, the brown wire from the key to the L terminal is the way I would go. Bypassing the PCM altogether. Use the S for voltage sensing instead of the F wire. If you had a complete harness with the Nineties gauges and dash and all the sensors then I would use the PCM, but you don't so my advice would be to use the tried and true wiring that the video describes.

[pilotXC]

Thanks. The person who did my harness said do not mess with the wires from the alternator to the PCM and strongly recommended to not use the old wiring system. He said he has done a few C10 trucks with the LS terminal block at the alternator and had no problems at all, except for the "ammeter" not working and said to get rid of it as it's a fire hazard and replace it with a volt meter. I might go this route to get it running as I don't want to mess with the PCM on my own and keep researching...

[VetteVet]

Well you are certainly within your right to use whatever choice you want to make to wire up the charging system. I did not say that using the PCM was wrong in fact if the alternator is a newer model it relies on the PCM to regulate the output voltage. If the alternator is older then it might not work because the regulator is inside the alternator and it would need to have the S pin wired so that the regulator could sense the system voltage and adjust the field strength as needed.

I still think the ammeter would work if the system is wired properly. I strongly disagree with the statement that it is a fire hazard. The older model trucks Pre 1963 used an amp meter which had all the system amperage ran through it and I believe your wiring guy is referring to that style. The 63 and later models were changed by the factory to avoid the fire hazard, and that style was replaced by the one you have. It only uses a small amperage and voltage to work and it is fused.

It is sometimes confusing when it comes to mix and matching new and old, and different systems some things work great and others may not.
Good luck to you and let us know how it works out.

[pilotXC]

I know, and I appreciate that... I'm just nervous about messing up the new system since it's more automated than the old. It is a 2001 LQ4, so from your previous post would not need the S wire as the PCM would monitor that I think anyway. Just need to ensure I have power to the firewall and figure out how to make the ammeter work with the new system.

[pilotXC]

Here's how I ended up wiring everything and all gauges work, including ammeter... Thank you VetteVet for going back and forth with me to figure this out.