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Fffrank · 10-15-2021, 01:15 PM

I'll do another thread about what went wrong..... This one is just for the current issue.

Here's how my old alt was wired.

I connected B+/B- to the new alternator in the specified places, connected the
KEY SWITCH wire to I and the DUVAC wire to S. The 4th wire I assume is the tach and I connected it to the R terminal.

Now I don't have a tach. I seem to be getting good voltage (14.4) even at idle and there are no ALT related dash lights. My BIRD is working and my battery monitor shows the battery is charging.... So I think things are wired correctly except for the tach. Any thoughts?

jacwjames · 10-15-2021, 02:35 PM

I assume the old alternator was a Leece Neville.

The 4th wire is for the Alt Relay which triggers a Alf Fail light on the dash.

On my chassis wiring diagram my tach gets it's signal from the ECM. You can go to the Cummins Quickserve site to see if yours get a signal from the ECM

153stars · 10-15-2021, 07:57 PM

See if this helps. There are a lot of 28si posts with whatever mistakes were made and fixed. https://www.irv2.com/forums/f115/help-delco-alternator-wiring-444372.html

bjohnsonmn · 01-24-2023, 08:14 PM

Quote:

Originally Posted by 153stars

See if this helps. There are a lot of 28si posts with whatever mistakes were made and fixed. https://www.irv2.com/forums/f115/help-delco-alternator-wiring-444372.html

I read that post, but he lost me at adding a relay.

What would the consequences of not connecting the ignition wire on the alternator?

153stars · 01-25-2023, 09:13 AM

If your asking me I'm not sure. There are many posts with diagrams. It may have been to retain the alternator fail light. Is back feed to it that keeps motor running IDK sorry.

Quote:

Originally Posted by bjohnsonmn

I read that post, but he lost me at adding a relay.

What would the consequences of not connecting the ignition wire on the alternator?

Ted DuPuis · 01-26-2023, 06:31 AM

The two items may not be related, just coincidental. If you've got a 2000 Diplomat I believe that should be a Freightliner XC chassis. On mine (2000 Endeavor, Cat 3126) the instrument cluster as I understand it is fed from the computer's ECM, which gets its engine speed signal from the crank sensor.

I've seen some weird things with the tach. For example sometimes it will start reading low for no apparent reason (if there's a common thread I haven't found it) and will stay reading low until I turn the engine off and restart it. I have a BlueFire which continues to read the correct RPM (if the ECM didn't know the correct RPM, it couldn't run).

Incidentally, I converted my engine side to a 1-wire 28si (from a Leece-Neville) and have had no changes in the tach behavior. This occasional oddity happened sometimes with the original alternator and still happens sometimes with the new alternator. In my case, I put the 28si where the AC compressor was originally, and then put another Leece-Neville (270A) where the original alternator was, and that feeds the house batteries. The setup has worked really well.

So, my guess is something else somewhere. Maybe try turning your batteries off, leaving it that way for a day or so, and restart.

Fffrank · 01-29-2023, 11:55 PM

This is an old thread that somehow got bumped! I'm not sure exactly how I fixed it but I did swap some wires around and the tach started magically working again.

I do have an issue occasionally where the tach bounces around. I suspect it's related to a bad/weak ground in the dashboard but I've been unsuccessful tracking it down.

28si works brilliantly and seems to charge batteries at around 80-90a at idle. I'm very happy with that performance!