Digijet 8385 Shoebox hall sender light question

[sasmith]

Recently purchased the Shoebox 8385 diagnostic tool mainly for fun, but also because I have 3 83-85 digijet vans that I work on and knew it would be beneficial as time goes on. Was hoping I could find some readings that would help better fine tune my van's system, but wasn't expecting anything out of the normal since my van starts, idles, and runs well.
But when I connected unit, the initial diagnostic with key on, engine off, had the hall sender light on, when the light should be off. With engine running, the light pulses as it should. I connected to other 2 vans and light was off as should be normal. I have spare distributor, ignition module, ECU with shoebox pigtail, and idle control stabilizer. Switched those in as many combinations as I could do to rule out something but nothing changed the light being on. Tested grounds and all tested as they should be. Have new ground straps and new wiring harness (2 years new). With engine running, all lights on shoebox unit indicate as they should and the readings, although slightly off, are linear and with no big concerns.
Disconnecting plug at distributor has no effect on the light, with key on, engine off, with either distributors.
I know this is a very specific question confounded with the fact that the van runs fine, but something is not right for this light to be on. I spoke with Jim at shoebox, and he also thought it strange and suggested maybe a polarity issue, but having switched out components in various ways, it would seem to rule out a polarity issue.
Anyone with special knowledge of the hall sender and ignition module wiring or function that could toss any ideas out?

[MarkWard]

Try bypassing the digitial idle stabilizer next to the ignition module. The harness is meant to be unplugged and connected to itself for setting the timing. Retest.
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[sasmith]

Thanks for the suggestion Mark. I was in the shop so thought I would test to give some feedback. No change when disconnecting idle stabilizer and connecting plugs for bypass, light stays on. I hadn't tried that yet, although I had switched idle stabilizer units prior to this. Only time light goes off is when I disconnect the ignition control module.

[MarkWard]

Not familiar with the shoebox. The difference between the DigiJet and the Digifant is that the digijet has a completely standalone ignition system. The ECU gets only an rpm pulse directly from the coil. Digifant has the coil drivers and fires the coil directly. I assumed the shoebox plugged into the ECU harness, but really don't know. Can't help more than that.
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[sasmith]

Mark,
The Shoebox tool does have a pigtail that is soldered into the ECU to connect the unit. Both of my ECUs have the pigtail soldered and I have tested with both. I read a post from a few years back that mentions, however, that the Hall pulse is not directly "picked up" by this unit, but is scavenged from other signals, but I am vague on the details and not knowledgable enough in the particular design to offer more than that. There are some very valuable readings that can be obtained from this unit for troubleshooting, but I was not expecting this variance and am not sure what it is telling me with key on, engine off.
And I know I am in trouble when Mark Ward says he has nothing else to offer when it comes to diagnostic work. Ha. Thank you again for your suggestion. As always it is greatly appreciated!

[MarkWard]

If you are getting a Hall signal, it's likely the rpm pulse from the coil, not the actual hall itself, but if it is working, you could call it a hall pulse, because no pulse, no run.

I've read there are early and late Digijet systems. The difference was the later digijet had a digifant style TPS switch. So compare manufacturing dates.

I guess it depends how bad you want to sort this out. You have 2 vans giving you the result you are looking for. So, you could get carried away and start swapping parts, but I'd have to be loosing a lot of sleep over it.

One last thought. Would the position of the distributor when the engine is off cause this? Oddly enough most engines stop in the same position time after time. So, if the shutter wheel stopped in an open notch vs closed, could that account for it? Good luck.
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[sasmith]

Mine is the late digijet with the single, undermount TPS on the TB. The switch is operating correctly, but is there something in the circuit that could cause the stock single switch wiring to affect this? The other 2 vans are early digijet, and although they have been converted to single switch, it involves a harness adapter to convert from the 2 switch system.
The distributor position is an interesting thought and something to look into. But I have to admit that line of thinking is currently above my pay grade as far as knowledge is concerned.

[MarkWard]

Under the distributor rotor is a metal disc with 4 cutouts. The disc rotates through the hall pickup. Each time a cutout passes, the hall turns that into signal the ignition module uses to fire the coil. So, the cutout can stop directly in the hall section or outside of it.

I was thinking the switch so much as the system version. So, the vans that work as expected are earlier vans forgetting the switch, and the one not behaving is the later version.

So, that might account for the differences.
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[sasmith]

I may have to follow up on the shoebox design a bit more to see if there are some design differences between the early and late digijet to account for the light.
And after I thought about the distributor position in relation to the signal that would be seen with key on, I can rule that out because the light stays on whether the plug is connected to the distributor or not.
Because of that, and earlier in my process, I had thought I could find the issue through a process of elimination within the ignition system "down the line" because the distributor and hall sender did not seem to be a part of the issue since the connector did not make a difference with either of my distributors when testing.
Just something to think about and fortunate it does not appear to be affecting the engine running, although it does make me wonder if it is affecting my tuning capabilities. But not enough to lose sleep over for sure.

[MarkWard]

Unplug the two wires on the negative side of the coil. That should isolate it from the ECU.
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