Speed Control Module

79GLIDE

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Been trying to get the speedometer working on the '93 since last Fall. Anybody ever found the engine compartment speed control/cruise module to be the culprit? I've had the instrument cluster and speedometer circuit board exposed. No evidence of circuit interruptions. Typical odometer displays mileage when speedometer quit. ABS light is on. I haven't messed with the frame mounted ABS unit. Any help appreciated.
 

79GLIDE

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Odometer shows the mileage when the speedometer quit working. Display works, just doesn't add any miles.
 

snicklas

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Is the transmission working correctly? If I am looking at the correct one in your sig, it has an E40D….. it needs the signal too.. If it is working and he speedo isn’t, I think on the OBS trucks the speedo signal goes to the TECA first, then the dash.
 

79GLIDE

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I've been shifting the E4OD manually since the speedometer quit. Replaced the VSS and inspected the instrument cluster/speedometer internals. Towing a trailer from IA to AZ and back in a few months. I'd like to get it figured out before then.
 

snicklas

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Is the O/D Off light dimly lit when the truck is running? There is a relay and a fuse to the TECA. I would start checking to see if it has voltage and is actually working. I wouldn't drive it with the TECA out of service, that is a quick way to kill an E40D....

Because for the Transmission and the ABS to be happy, they have to get a good speed signal. I'm not sure the exact path that signal takes, but the TECA, Speedo and ABS all use that signal.

Maybe @trackspeeder or @david85 might have some insight....
 

david85

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Are you able to get 1st gear and 2nd gear, using the shift lever? If not, it means you have no 12V power to the transmission, which is the most common electronic malfunction.

With no 12V power, you will get:
2nd in manual 1 or 2
4th gear in Drive
Reverse will work normally but may engage harder than usual
Torque converter lockup will not work at all

Although if you can get 1 and 2, You may not be able to get 3rd or 4th, since you need a speed signal for the computer to command higher gears.

Sicklas is correct that there is one sensor for ABS, Transmission and Cruise control. I kinda doubt it's the cruise control though.

Have you checked for the Overdrive cancel indicator?
 

79GLIDE

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I get all gears shifting manually. I towed my enclosed trailer to Virginia and back in September shifting manually. It's mostly just a pain not having the speedometer and it's input for automatic shifting. I don't think it's the PSOM. I didn't find any evidence of burnt circuitry.
 

david85

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Electronics can fail without any visible indication on them. Semiconductors just let out the ghost sometimes (which is why so many of us prefer mechanical fuel injection).

This is a strange one for me. So when you start from a standstill, how are you able to get through all the gears? In drive, do you still get 1st gear at 0MPH? How does the transmission shift as you accelerate? Does it automatically go from 3rd to 4th, or do you have to use the OD cancel switch? (does the OD Cancel switch illuminate?)

@trackspeeder , is there any way to test the PSOM?
 

79GLIDE

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From park, you can get 1st by shifting to it directly. If I'm towing I turn off OD straight away. Once I'm rolling I advance through the gears manually at 2200-2000 RPMs, then hit OD. When slowing or stopping, it's possible to shift down to 2nd manually, but not 1st. The trick to getting 1st when decelerating or stopping, is once you're at the RPMs desired move the selector down to 1st, then quickly shift to 2nd and back to 1st. This works to get 1st every time. That locks the transmission in 1st until you move the selector. I figured this out by accident. When I towed the trailer to VA, I was having to come to a complete stop, shift in to park, then to 1st to get that gear to get rolling. It was a pain.

I'll probably have to bite the bullet and send the PSOM off. I'm trying to exclude all other factors first. I don't think the PSOM from my '90 or '94 are the same. Maybe the one in the '90 will work??
 

BeastMaster

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Does your speedometer display the mileage on some sort of seven segment LCD display? If so, that's a PSOM . VSS comes from a sensor mounted on the rear differential "pumpkin". The PSOM (part of the speedometer assembly in the instrument cluster ) may be programmed for which rear end gear ratios and tire size. Programming is done by manipulation of the reset and select pushbuttons above the speedometer display.

Both the E4OD and ABS get their VSS signal from the PSOM, as the electronic circuitry applies the calibration factor so that the other things that need the signal that the transmission vss pickup used to supply will still get the signal they need. My PSOM feeds both my E4OD and ABS. I do not have speed control but I would strongly suspect it would get it's speed information from the PSOM. This signal is a frequency.


Or, Is the mileage shown mechanically on a number of rotating wheels with numbers painted on them? If so, you have the old style where you have the vss sensor on the transmission and the calibration/scaling is done by using a special little plastic gear in the transmission.
 
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79GLIDE

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Correct. The '93 is digital. Both of my '90's are analog. My '94 is late year, so PSD, and 5 speed. I replaced the VSS mounted to the differential. I'm going to have to send off the PSOM, or go to the salvage yard. I'm still suspicious of the ABS signal, which is in line from the VSS. I'm also suspicious of the speed control module, as the cruise has never worked on this truck, despite sound fuses.
 

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