ammeter bypass wiring Q

[rirealtor]

reading up on this bypass from Mad Electrical and posts here, want to make sure this sounds correct. my plan is to eliminate the yellow wire, and run new wire into cab as it looks as if red wire might have been exposed to heat. old Motorola alt will be replaced, eliminating external regulator, replacing with PowerMaster 150. My questions are as follows:
If replacing the red wire back to cab, should the wire gauge be changed for any reason? do I need to drill out connector or can I utilize another grommet through firewall? I wont be using connectors, so does it make sense to remove altogether in engine bay? If running through grommet, what's best way to connect to big splice? Just in front with tin butt connector, followed by heatshrink? In doing so, the old red wire can be left on the ammeter post or remove?
what breaker should be used from solenoid to big splice if keeping with 10ga? 40?
2 gauge wire from 'bat' on alternator to breaker and then 2 gauge to solenoid. What size breaker here with the alternator? 100? This is a stock vehicle that will see headlight upgrades in future, but not much else.
Anyone run the 1 wire alternator with a similar year(75)? seems like I have the funky bracket which creates issues with other alternator options.

[tgreese]

What I did for my J20 is to remove the return wire, and leave the wire to the dash in place. The 10ga wire to the dash powers the cabin and all the lighting, so you don't need to remove it, and you can abandon-in-place the 10ga wire from the ammeter back to the bulkhead connector. Instead of cutting the wire, you can take the bulkhead connector apart and remove the connector and wire complete from the engine side of the connector, and then wire directly from the alternator charging line to the battery. I replaced the ammeter with the SunPro voltmeter, which has been covered many times here and on IFSJA. I took power for the voltmeter from the switched power on the cluster.

If the red 10ga wire to the dash is damaged, I would clip out the damaged part and wire in new. You can remove the wire intact from the loom and bulkhead connector, and reuse the ends, or buy new ends and replace them, or just snip out the bad part. If you choose to, you can drill through the bulkhead connector and run continuous wire to the splice under the dash. I expect you'll have to take down the fuse panel on the inside of the firewall to do this. If the 10ga wire is heat-damaged, I would expect heat damage from the splice to the ammeter too. it's the ammeter shunt internal to the ammeter that is typically the focus of this wire overheating.

Take a look at your wiring diagram, and trace it through. You will see that the 10ga wire into the dash goes to a splice under the dash that powers everything other than the alternator. What I'm telling you is from memory, so be sure to verify by tracing through the changes you will make. Get the '75 TSM with the proper wiring diagram, if you don't already have it. Cheap on CD at RockAuto.

In the same session, I also bypassed the "regulator" in the fuel gauge and replaced it with a 7805 chip mounted on the cluster.

My '82 comes with the 10SI alternator, so I can't advise on the mechanical issues of installing one in a '75. For the wiring, refer to the later model factory wiring diagram. IMO paying extra for the 1-wire version of a 10SI is a waste - plenty of discussion on the net already.

[rirealtor]

Hi Tim,
Thanks for the follow up. Initially, my thought was to keep the factory wiring, and just clean up some of the black gunk off the connectors on the bulkhead ( contact cleaner with brush?), but as I peeked under the dash to trace the red, there were signs of red wire overheat at the connector. This lead me to possible re-wire from the solenoid all the way back. My thought was to follow Serehill's method with the small 40a breaker and then into cab.
Apparently mid year 75 there were changes to the alt bracket design and the Delco series was not out yet. My understanding is that the modification of the existing bracket is difficult. My preference would be a more drop in version, and all that I can find is the one wire PM. The CS series looks great also, but again, I am concerned with mods needed.

[tgreese]

Hi - It's just dielectric grease. Clean it if you want - IMO it won't make a difference.

Now that I think about it while looking at the diagram, I must have abandoned the red wire and used the yellow wire to feed the dash. You can remove all the red wire if you want, as long as you connect yellow to red at the ammeter. Then you'll need a connection from the alternator direct to the battery. No need to repair the red 10 ga wire if it's heat damaged.

IIRC I added another fusible link to whichever part of it was not now protected - the lead to the alternator, I think. I would be inclined to use the existing wiring rather than run another wire - the bulkhead connector for that wire is larger gauge than the rest and fine to power the dash and lights. It could probably be smaller gauge if it did not have the charging current running through it.

[rirealtor]

In poking around, the red wire is in good shape and does not need to be replaced. it was the ac wire that had a little Melted portion at the connector. Easy fix.
So with that said, if I am leaving red wire intact and moving from alt to solenoid, and removing yellow wire altogether, what is to be done with red wire on ammeter post? The yet to be installed voltmeter will come from a separate keyed source.

[tgreese]

Rather than a lot more text, I made a drawing for you. Here's your existing circuit and how I changed it:



Note that the 14ga wire from the solenoid to splice K is a fusible link. My mod takes the existing red wire from the alternator and sends it to the battery via the solenoid post. I added a second fusible link to that wire.

If you move the red wire directly to the solenoid/battery, you can abandon the rest of the red wire in place. Tape off the dangling wire under the hood. I removed all the excess wire because I didn't want an extra wire under the dash or under the hood that could someday short out or accidentally be connected.

[will e]

Has anyone attempted to leave the Amp meter circuit in place and add a shunt ciruit?

https://www.youtube.com/watch?v=Q-CE-ZbBuTQ

You could decrease the sensitivity of the amp guage. It would provide 'false' values but you could still see if you are charging or not charging. This would be handy if you went with a higher output alternator and should be fairly easy to do.

Does anyone know the internal resistance of our amp meters?

[tgreese]

Likely quite low. There is a shunt in the meter already. It's the voltage drop across the shunt that deflects the meter needle.

You could take a meter apart and measure the resistance across the shunt. I expect it's really low, and it's difficult to accurately measure low resistances without special equipment.

Possible you could put an additional resistor across the ammeter leads though I think the 10ga wiring will be insufficient with a larger alternator.