The 1947 - Present Chevrolet & GMC Truck Message Board Network

Register or Log In To remove these advertisements.

Go Back   The 1947 - Present Chevrolet & GMC Truck Message Board Network > General Truck Forums > Electrical


Thread Tools Display Modes
Old 03-08-2019, 05:13 PM   #1
The Rocknrod
The Rocknrod's Avatar
Join Date: Aug 2011
Location: China Spring, TX
Posts: 4,217
Diode or No?

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?
The Rocknrod is offline   Reply With Quote
Old 03-08-2019, 09:13 PM   #2
Msgt USAF Ret
VetteVet's Avatar
Join Date: Jan 2005
Location: Kalamazoo, Michigan
Posts: 8,315
Re: Diode or No?

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).

Name:  64_wiring_page2.jpg
Views: 107
Size:  96.5 KB

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.

Name:  64_wiring_page1.jpg
Views: 101
Size:  38.1 KB

metallic green 67 stepside
74 corvette convertible
1965 Harley sportster
1995 Harley wide glide

Growing old is hell, but it beats the alternative.
VetteVet is offline   Reply With Quote
Old 03-08-2019, 09:40 PM   #3
Registered User
Join Date: Jul 2014
Location: North-central Virginia
Posts: 510
Re: Diode or No?

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.
kipps is offline   Reply With Quote
Old 03-08-2019, 10:51 PM   #4
The Rocknrod
The Rocknrod's Avatar
Join Date: Aug 2011
Location: China Spring, TX
Posts: 4,217
Re: Diode or No?

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.
The Rocknrod is offline   Reply With Quote
Old 03-11-2019, 03:15 PM   #5
Registered User
Join Date: Apr 2006
Location: Spring, Texas
Posts: 145
Re: Diode or No?

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.
'72 3/4 Cheyenne Super 402/TH400 Gear Vendors
'72 3/4 Cheyenne Super 350/200R4
'50 1/2 Chevy 3 Window
jbarron is offline   Reply With Quote


Thread Tools
Display Modes

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off

Forum Jump

All times are GMT -4. The time now is 08:31 AM.

Powered by vBulletin® Version 3.8.11
Copyright ©2000 - 2020, vBulletin Solutions Inc.
Copyright 1997-2013