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1978 F150 Alternator Wiring Question

Joined
May 24, 2020
Member Number
1298
Messages
492
Loc
OC, CA
I just installed a 130 amp JBG one wire alternator (I believe it's a 3g with a self-exciting internal voltage regulator) and have a couple of questions:

1. With the truck running, voltage at the battery terminals is 14.5v. The voltmeter gauge I have installed under the dash (taps into the fuse box) only reads 13.5. I think this is fine and good but just wanted to double check.

2. With the one-wire set-up I only needed to run a heavy gauge wire from the alternator to the positive post on the battery, and then I was able to get rid of the external voltage regulator and all of the associated wiring. After removing that wiring I found I needed to run the following wires as shown in the pictures below and am unsure about C)/the yellow wire:

A) Red wire going to a heavy gauge wire with an inline 20-amp fuse (replaced a less clean version of what was already there) to some junction on the fender well.
B) Thick Black wire with Yellow stripe spliced directly to the stock fusible link going to the starter solenoid. (I'd like to just replace this with a big enough inline fuse but not sure what amp fuse to use.)
C) Thin Yellow wire with a black or brown stripe that isn't hooked up to anything now that I've removed the voltage regulator/alternator wiring.​

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The truck runs, starts up, all of the accessories/lights/etc work so I'm not sure what that yellow wire with brown stripe is. My GUESS is the in-dash-cluster ammeter but was hoping someone knew the answer.​
 

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These old trucks are stone simple, but knowing what to cut out it not straight forward.

1. No, 1v is way off. does it change offset/error when you turn on lights and other accessories that craw current?
2a. what is the load?
2b. Don't bother with fuse. Selecting a fuse that will actually blow under any useful condition is non-trivial. A fuse protects the bat and that short wire from short in alternator. The alternator will never pop a fuse unless it is 1/2 the rating of output, or less.
2c. What does the schematic say the yellow wire is? On my 72 ammeter was yellow and red. Use that wire to feed your volt meter at dash so there is no voltage drop in it.
 
I was in a bit of a hurry to wrap up yesterday (had a chatty neighbor that wouldn't leave) and I should have checked the accuracy of the volt gauge against my multimeter (which I may not have set correctly for use)...

Here's the reading at the battery:
PXL_20210630_190422248.MP.jpg

Here's the reading from the gauge after I tapped it a few times:
PXL_20210630_190410105.MP.jpg

And my "fuse-tap" install (the top one that reads Eng. Sol.):
PXL_20210630_190730563.jpg


I then tested the voltage at the fuse panel where I'm tapped in using the multimeter and got a reading of 14.2-14.4v depending on lights/accessories being turned on.

Seems like all is good (except for the cheap Bosch gauge's accuracy)?

Also, a better terminal for the charging cable is in the mail.
 
How is voltage gauge grounded? That could also be issue. Measure voltage from bat- to volt gauge -, will need to extend one meter lead.. Should be zero. Condition of ground strap from cab to motor? motor to batt? Motor to chassis?
 
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