Wiring Alternator to Starter

[1968_K20_4x4]

Hello All, quick question about alternator wiring. Going through the wiring of my '68 K20 with a 307. The P.O. installed an SI alternator in the truck and ran a red wire from the "Bat" terminal of the alternator to the positive terminal of the starter motor (connected to where the positive battery cable connects to the starter). Here is my question: Should I install a fusable link to this wire to protect the circuit? Should the fusable link be 16guage?

They then connected the same red wire to the no. 2 post on the alternator and ran a tan wire from no.1 on the alt to the fuse block. The old voltage regulator unit has been completely bypassed.

Edit: I have seen other diagrams where this wire is tied in to the main red wire that comes off the battery and runs to the fuse block. However my alternator is on the passenger side and I would like to keep it wired up the way it is for cosmetic reasons. Just want to make sure it's wired in properly



Thanks for the help

[kwmech]

Sounds like it will work to me, although I would have run the wire directly to the battery instead of the starter

[VetteVet]

That's the way the later model trucks 74 and newer did it. You don't need a fusible link on the alternator to battery cable wire, but any wires from there should have one, and it will be four gauges smaller than the wire it protects. I.E. 12 gauge needs 16 gauge link.

The question is, do you want to retain the battery gauge function in the dash? If so then you will have to wire the truck like the stock 68 with the alternator charging wire going to a main junction on the left fender or on the radiator shroud near the horn mount. Then
you can run a battery charging wire back to the battery positive post on the right fender junction bolt and from there to the battery positive post using a fusible link.

Then you will be able to run a power wire to the cab to the fuse panel, the horn relay, the headlight switch, and the ignition switch.

Run the no. 2 alternator terminal wire to the same junction as the alternator charging wire for better downstream voltage sensing for the alternator regulator.

The no. 1 alternator terminal wire does not go to the fuse panel but it goes inside the cab and connects to a 10 ohm resistance wire from the ignition switch. It makes the connection on the inside of the cab in the firewall block behind the fuse panel. This wire was originally a 16 gauge brown wire which went from the engine side of the firewall block to the external voltage regulator.

Here is a diagram I made to illustrate the routing.





I would use a larger wire from the alternator charging terminal to the main junction such as a 10 gauge or an 8 gauge.

The 10 ohm resistance wire is needed to prevent backfeed from the alternator to the ignition switch after engine shutdown which would let the engine run on.

The black and gray wires in the diagram are the battery gauge wires, and they run from the passenger side fender terminal to the battery gauge, and from the alternator main junction to the battery gauge. Each one of them has a small four amp fuse in a fuse holder to protect the gauge. The idiot light dashes do not have these wires, but they do have the resistance wire.

The stock factory trucks used a 12 gauge red wire from the RH fender junction to the alternator main junction to act as the battery charging wire, and as an external shunt for the battery gauge. This allows the battery gauge wires to send voltage readings to the gauge so it can sense charge or discharge of the battery.

[1968_K20_4x4]

Wow thanks VetteVet for all the great info. That diagram is very helpful! I was wondering what the black wire with the little fuse in it was for. I definately want my battery gauge to work properly, so this is how I will wire it up. Thanks again for the excellent info and for sharing your knowledge! Much appreciated!

[LH Lead-Foot]

When saying battery gauge are you talking about the "Amp" meter? I would check to "F" terminal wire to the alternator after switching to a 12SI (With Fan) to make sure it has resistance using a test light. It should be dimmer than normal. If dis-connected, you can measure with a volt/ohm meter.
A 460 ohm resistor is installed between the ignition switch "ON" terminal to protect the voltage regulator, this protect the voltage regulator from current, while proving voltage to turn on with three wire alternators. Once started, the diode trio provides field voltage to keep the alternator turned on. If a diode trio problem occurs, one of three diodes will reduce the output voltage. This is why the dash light comes on dim and a little brighter when revved up.
The stator is a "Y" connect type and the field terminal is dead when not turning. When the engine is running, voltage is produced and matches system voltage. A this point, you have B+ on both sides of the idiot light, with no potential for current to flow, the bulb goes out.
The common practice in the 70's was an idiot light for charging using a 194 peanut bulb. Both the bulb and resistor acts as a current limiting device to the "field" terminal for turn-on signal. Typically used in parallel to the bulb & resistor, as this practice allows the alternator to stay on, if the bulb burns out. Also, since the "F" terminal is dead when off, this was typically used to control a choke relay coil. With voltage, engine running, the relay is energized and supplies the electric choke with voltage to heat up, usually from a fuse on the ignition switch on side.