e4od, VSS and US Shift intermittent issue

[nelstomlinson]

I recently bought a US Shift controller and installed it in my '94 ambulance. I like the way it shifts, I like the visibility it gives me into the transmission. It hasn't solved the problem that periodically the trans downshifts to 1st and won't upshift. It was doing this occasionally with the factory controller, same symptoms, except the OFF light would flash and of course no insight into what speed the factor controller was reading.

Now that I have the US Shift controller installed, I can see that the controller is seeing zero mph when this happens. The speedometer continues to read correctly when this happens. The speedo never wavers, never bounces, never gives an unlikely reading, so I have assumed the VSS and tone ring are OK, and the wiring from rear axle to dash must be OK. I replaced the VSS anyway, just because. That hasn't fixed it. Really not sure what to try next. After a few minutes, it starts working again. Shutting off the engine doesn't make it work, stopping and putting it in park doesn't work, but wait a few minutes and it again can see vehicle speed and starts working ... so far.

I tapped into the gray and black wire with the VSS tone which runs into the back of the instrument cluster. The green US Shift wire with the VSS tone runs straight across the dash to the controller.

Does anyone know anything about the speedo and how it gets the tone from the VSS? Per the EVTM there is a PSOM which takes the VSS tone and converts it for the speedometer, but "PSOM repair services" all talk about fixing the speedometer.

 

[Nero]

If I remember right, the vss goes to the speedo as an analog signal, and it converts it to a digital one for the TCM. could be a wiring between speedo to TCM or bad microprocessor issue on the speedo if you're losing signal.

 

[nelstomlinson]

This evening I'll post a page from the EVTM that covers this. From memory it says the PSOM is between VSS and speedo, which doesn't match a lot of the info out there. That mismatch is a big part of my confusion.

 

[Nero]

I was struggling with it as well working on my in laws truck. There's a split year where it does and doesnt. If you have a cable driven speedo it doesn't. If you have a fly by wire speedo it does. I think the cutoff is when it went to the obs aero cluster in 1992

 

[snicklas]

I was struggling with it as well working on my in laws truck. There's a split year where it does and doesnt. If you have a cable driven speedo it doesn't. If you have a fly by wire speedo it does. I think the cutoff is when it went to the obs aero cluster in 1992

Correct,

The Brick speedometer (87-91) is still cable driven. There is a VSS in the rear differential , but it is only used for the RABS. The E4OD Bricks have a VSS that plugs into the speedometer port in the transmission/transfer case and the speedometer cable attaches to that VSS. This is what the TECA uses for the speedometer signal on a brick.

On the OBS (92-97), there is only the VSS in the rear differential. This feeds the PSOM in the speedometer (OBS Speedometers are electronic) then the PSOM passes it to the TECA. There have been members on here that have a perfectly working speedometer, but the transmission is in limp mode, O/D Off light flashing, because something happened in the PSOM and it is not relaying the signal.

 

[nelstomlinson]

So, the gray and black wire running into the back of the instrument cluster is carrying a signal _from_ the speedometer/PSOM to the PCM? The palm isn't separate from the speedometer?

 

[Nero]

That is correct, the psom receives the signal, changes it a little, then spits it back out on another wire straight to the transmission controller.

 

[david85]

Have you tried reaching out to US Shift? It's been many years since I installed mine, but they were very helpful when I was first getting mine set up. Karl Baumann himself ended up helping me through Emails and a couple phone calls.

 

[nelstomlinson]

Yes, US Shift has been helpful, but I didn't understand what the PSOM was. I had completely misinterpreted the info in the EVTM.
This is the e4od page of the EVTM which shows the PSOM:

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What it's actually telling us is what Snicklas and Nero told us: PSOM is the Programmable Speedometer Odometer Module, and the signal from the tone ring goes to it, and apparently the PSOM monitors both sides of it and probably affects the signal somehow. We can see that more clearly on the RABS page, which shows the PSOM sitting between the two wires hooked to the VSS.

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The dash RABS light is on. Since it's green, I had thought it meant all was well, but looking at the manual it probably means the opposite. Perhaps getting the PSOM repaired will fix the intermittent shifting problem _and_ the RABS light?

 

[nelstomlinson]

The cruise control page tells us that the PSOM converts the signal from the VSS into a standard 8,000 pulse/mile signal.

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[nelstomlinson]

On FTE someone suggested that the RABS module might have a problem and be muting the signal. I unplugged this connector and drove around for half an hour without it recurring. This looks like it might be the RABS connector, but the ABS light on the dash didn't go out.

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It makes sense that the RABS module might somehow mute the signal, I suppose.

Unfortunately I started getting a loud chirp which turned into a squeal from somewhere in the back near the end of that drive, so road testing may be delayed, depending on what's making that awful noise. I don't think this rig is ready for a long road trip yet.

 

[Nero]

Is that the rabs module connector under the dash? Thats why the rabs light isn't coming on. Pretty sure chirp is unrelated.

 

[nelstomlinson]

Nah, probably not the right connector since the light is still on. It surely looks like the sketch of the right connector in the EVTM, though.

 

[nelstomlinson]

Darn. The connector I found was for the windshield wiper control, not RABS. I've been thinking RABS, but I noticed there are tone rings and sensors on the front wheels. This old pig has 4 wheel ABS! So I was looking for the wrong connector.

That ugly chirping squeal turned out to be one of the broken studs from a few weeks back. I guess when I was knocking them out, one got stuck in there and I didn't notice. No harm done, near as I can tell.

So, back to looking for the ABS control module, to try disconnecting it and see if my intermittent speed sensor drop out stops.