Positive Ground - Alternator Connection

[56LT]

I joined this forum because it seems that are a fair number of people that have knowledge with positive ground systems.

Truck is a 56 LTH and engine is a Big Cam 3 with a Leece-Neville 2500J alternator.

We have verified the truck is still wired for positive ground, with 24v starting through a parallel switch. In this case one of the battery positive posts goes to the starter solenoid post and then from that post to nearby ground on the frame.

For the alternator, it has a single positive post and a single negative post and the alternator case is electrically isolated from both posts, so it does not ground through the alternator case. From the wiring harness, one alternator lead shares the same ground point on the frame as the starter solenoid. The other lead goes to a "Main" toggle switch in the cab. I have verified that this lead has continuity with the battery negative cable and when the batteries are connected I have ~12.8v. As far as I know, the alternator has not been modified and is still wired for a negative ground system.

My question is, which alternator post should receive the voltage from the wiring harness? Being positive ground, my first thought is that alternator positive should still go to ground and alternator negative should still go to negative on the wiring harness. But then there's part of me that says the alternator should still be wired as if it were in a negative ground system in this case.

I would appreciate any insight from people more knowledgeable than I am on this.

[Geoff Weeks]

You are 100% correct the case is neutral on the JB series. so the positive goes to the frame on a + ground truck and the negative goes to the truck wiring. The internal voltage regulator doesn't care as  it is isolated from the alternator frame and gets its power from the + and - stud on the alternator itself. 

[56LT]

Thank you for your reply.

So in this case, 12v to the negative post is acceptable? That's the part that throws me for a loop.

Edited 3 hours ago by 56LT

[Geoff Weeks]

yes.

Both Delco and Leece Neville used to make duel output alternators that would allow you to eliminate the Series/Parallel and keep 24 volt cranking and 12 volt for everything else.

The Delco is still available (Delco 30SI-TR) but I only see it in negative ground. 40 years ago I had the part to convert that JB2500 to the same, but it is long gone. I haven't seen the JB kit since I had the one I had. 

Series parallel seams to "throw" people as bad or worse than positive ground.

 https://smithcoelectric.com/products/240-821-new-alternator-for-delco-30si-12v-90a-with-24v-transformer

[Geoff Weeks]

https://www.shopzorkos.com/Category/5116T

this is close to what I had. The one I had bolted directly to the back of the alternator, this one is remote.

[56LT]

Thank you very much. We're not looking to eliminate the parallel switch, we just want to make sure we don't damage the alternator by hooking it up backward.