Diode or No?

[The Rocknrod]

If my wire harness has a switched line (white wire) going to the Alternator (tickle) do I still need a diode?
What do you think?

[VetteVet]

I'm assuming you are talking about the 64 work in progress truck.
If so you should have a resistance wire on the key switch that will serve as the tickle wire to the alternator via the external regulator. If you have a generator light in the dash it will also provide resistance to the alternator white wire when it joins the brown wire at the bulkhead connector.

This resistance is needed to excite the fields in the alternator and also to prevent current feedback to the ignition switch once the alternator is charging. This feedback current will continue to power the ignition switch and the coil, even if the key is shut off, resulting in the engine continuing to run after the key switch is turned off.

A diode would also prevent this run on, but if you have the resistances I mentioned you should not need a diode.


This diagram shows the key switch in the top middle of the diagram with the 24 gauge brown/white wire running to the forward bulkhead connector where it joins up with the brown wire from the Gen Tell tale light (if you have a light dash instead of a gauge dash).




From there the two wires become one brown wire which runs to the regulator to terminal 4, and sends power to the alternator when the key switch is turned on.

[kipps]

Agree with VettVett. I ran into that with a skidloader I rewired. It was a diesel, but operated with a key switch and a fuel solenoid. Thus, the alternator could power the fuel solenoid if the diode wasn't installed in the wire like it was supposed to be.

I remember the struggle of finding that diode in the old harness. It was hardly any fatter than the wire itself, and was buried in a non-serviceable wire loom. I just went along chopping the loom, and back testing with an ohm meter till I found it.

[The Rocknrod]

Thanks for the answer. My wiring harness is aftermarket. It has a white wire (from the fuse panel) that is supposed to attach to the generator post 1 to tickle the unit. I will not be using a light in the dash as it's fully gauged unit and already built out and wired in. I have the diode in hand I just thought "maybe" I wouldn't need it, but installing it wont kill me either way.
Thanks again VettVet.

[jbarron]

Just went through this.
Single white wire for excite to alternator.
First run of engine it would not shutoff.
Installed the diode and everything works good.
It will not hurt to have it, but you can try without and see if your engine shuts off with the key.