Quote:
Originally Posted by grif
Thanks, VetteVet. You've made this sound pretty simple. My only question has to do with running the separate wire from a junction of the battery and alternator feeds: SEE NO. 2 BELOWWhat does this existing junction look like and where can it be found? I'm looking for a 12 gauge red wire coming from the firewall connector, right? Can I run this wire directly from the positive battery terminal instead?
Thanks for all your excellent help!
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This will lead to a tutorial on how the battery gauge works and how it's important to wire the battery and alternator feeds a certain way.
The battery alternator junction is buried in the wiring harness on the left side of the truck in the main harness and it looks like this.
It is simply a four wire junction soldered together. if you have the gauge dash with the battery gauge you will have a small black wire eith a white stripe soldered there also. On the 1966 and older year trucks they used the orn relay for this junction
These wires consist of
1. The large wire from the back of the alternator
2. The red sensing wire for the external voltage regulator to the no.3 terminal or it can be run to the no.2 terminal of the internally regulated SI alternator. or if you want more amps then it can be run to the S terminal of a CS alternator. This is the wire that does the remote voltage sensing for the alternator and allows it to compensate for voltage drops in the downstream loads of the circuit such as the electric cooling fan, the heater fan, and the wiper motor etc.
3. The cab feed wire which is the 12 gauge red wire that runs to the firewall connector block and feeds the circuits inside the cab.The headlight switch, The ignition switch, the fuse panel, and the horn relay.
4. The fourth wire is a 12 gauge wire that runs from the terminal on the passenger fender across the radiator and down into the harness to the junction. This wire is called the SHUNT because it connects the battery positive terminal to the alternator output feed wire in order to charge the battery. This is the wire which feeds all the circuits when the alternator is not working.
There is a common misconception that the battery is what supplies all the power for the vehicle and the alternator just recharges the battery. In fact the alternator supplies the power and the battery is used for starting and backup and it supplies power during load switching to reduce surges from the alternator.
This soldered junction can be replaced by most any type of aftermarket junction or fuse panel and I recommend it when upgrading the electrics on the truck.
This brings us back to the SHUNT wire and the battery gauge wires. There are two small 20 gauge wires that feed the battery gauge that us old timers use to call the ammeter. One of them is connected to the soldered junction and it goes into the cab to the dash cluster plug to terminal 1. The other one is connected to the junction on the passenger fender with the fusible link wire from the battery positive terminal. It runs through the firewall connector block to the cluster plug to terminal 12. It is usually a solid black wire.
Notice in the diagrams above these wires are shown connected as stated with two small fuses inline to protect the gauge in case of a short or break in the main feed wire which would alll all the alternator or battery feed voltage to go through the gauge and burn it up. One or the other of these two fuses is the main reason that the gauge doesn't work when someone buys a truck with the ammeter in it. They are only 4 amp fuses and they are located in the harness by the left headlight and by the passenger fender junction. They are easy to spot because the holders look like a small black football.
The next item is the shunt wire that is just a normal 12 awg wire running from the passenger fender junction across the radiator to the soldered junction. It's purpose is to shunt power from the alternator to the battery and back from the battery to the system. It's length and resistance is calibrated by the factory for the truck's demand and the battery gauge is matched to those factors.
Here's a good diagram I've posted many times that shows the wiring I've been discussing. The shunt wire is labeled and the fuses are circled.
Notice that the ammeter wires are connected on each end of the shunt
wire. This is so the gauge can sense which voltage is higher or lower the battery voltage or the alternator voltage. Normally the alternator voltage will be higher so the gauge will read charging or to the right of center. If the battery voltage is higher it means that the alternator is not charging and the gauge will read to the right or discharge.
If the alternator output wire is run directly to the battery post or to the passenger fender junction or even to the starter cable connection it will still work the system but that bypasses the shunt wire and the gauge won't be able to read accurately.
Here are the in cab wiring diagrams and you can follow the large red wires and see how they feed the cab circuits I mentioned above. This one shows the red feed wire from the soldered junction coming through the firewall block and feeding the Key switch and the horn relay and the fuse panel.
This one shows the red wire feeding the fuse panel.
This one shows the red wire feeding the headlight switch
