rlortie

I know that a lot has been said and done here regarding speedometers (1992) D250's. I would like to know why when my speedometer fails it prevents the auto transmission from kicking in to overdrive? What and where is the linkage, mechanical or electrical that controls this? The speedometer works when it feels like it! It may work for days and then quit, I can be traveling with it dead and it will start working. When it does the transmission also kicks into overdrive. Where should I look and what? Have a problem thinking it is failed gears as when it works it does not skip. If it is the speed sensor on the transmission what should I be looking for on replacement?

I understand that speedometer gears are hard to come by, I found some @ https://www.ebay.com/itm/1990-91-199...rs-Speedometer. Also speed sensors @ https://www.partsgeek.com/ Need advice on which to order that may solve my problem.
Thank you!
rlortie 1992 D250 club long bed.

thrashingcows

Most likely it's you VSS, but also be your CPS, also check the gap on your CPS (Crank position sensor) If the CPS is failing or out of adjustment then the ECM won't know that the motor is running and won't allow the OD to engage. Also when the OD does not work, along with the speedo, does the cruise and AC work? All those functions rely on the CPS to tell the ECM the motor and truck is running so it's OK for them to function,

rlortie

Thanks thrash!!!
Took all the info I could gather including an armful of acronym's and headed for the local diesel shop. Explained my problems as though I was experienced in this field. He in turn headed me to a near auto transmission shop where I met the proprietor. He claims to have a stock of gears in case I should need them. We discussed a plan of attack starting with the VSS spun with a cordless hand drill, and then the rear ABS speed sensor. From there is is on to the ECM, if we do not find the problem by then its time to do some wire tracing.

My hat is also off to Dean Snow and his thread: https://www.dieseltruckresource.com/...er-fix-198605/

oldschldodge

i am having similar issues with a truck of mine.... it all started with noticing the trans wouldn't shift into OD... then noticed the Alternator was not charging either... would not charge or shift in the mornings but in the afternoon with the truck facing north and the sun blazing down on the drivers side. When Dad got off work and headed home it would charge and shift. So i'm thinking the alternator isn't charging and the battery don't have enough juice to shift tranny so i Installed a 1 wire Alternator..... well now it charges.... but tranny still doesn't shift into OD... now noticing the AC does not come on either.... Changed the Crank Sensor(Didn't help), Changed the Intake Air Temp. Sensor(didn't help). soi started to dig into the wiring harness thinking it may be a fuseable link and noticed the Main Blue wire throughout the harness is Melted all the way through the harness..even fried the connection at the engine/tranny harness. So i ended up switching Harnesses out with another one i had. So.... in closing still not shifting and No AC. wait to start light does stay on as well. Dad was mapping a field with a GPS one day and truck truck got closer to the Hot side on Temperature and he noticed the AC kicked on. he took it out on the road and it shifted....but when the temperature came back down it would shift into OD again. any ideas on what this may be?? OH.... does not have the TPS(yes its auto) it has a Potentiometer.

oliver foster

Welcome to DTR.

That sounds like a PCM failure if a Crank Sensor and new wiring didn't help.

I might have one '92/'93 auto PCM left in the shop if you need one.

oldschldodge

Thanks,

Actually i did change it out with one out of another auto truck that i have..... but now that i think about it... that truck was converted to an auto from a manual before i bought it and it had a few wiring issues as well..... hmmmmmm....

How do i get in contact with you to get a price?

oliver foster

A manual transmission '92/'93 PCM will allow all the functions a 5 speed would need, but auto O/D info. would not be one things a 5 speed computer could handle.

You can send me a PM if you need specific info.

oldschldodge

Sounds like it might be the PCM... i sent you a Private Message.

rlortie

I hopefully believe I have finally cured my decade old intermittent speed o meter problem! Being considered a mechanic since my hot rod days in the 50's, I never dreamed of being a "parts changer" but was forced to swallow my pride on this one!
Started by replacing the vehicle speed sensor (VSS)..purchased from oreillyauto.com> This got the speed o to work but still no overdrive. Next I checked the output voltage of the Throttle Position Sensor (TPS), It was reading 2.1 volts rather than the 3.6 volts recommended. I replaced it. Shifting was rough so I reinstalled the original. Now the speed O was slow and lagging below 25 MPH and bouncing all over the dial above 35 MPH but would settle in when decelerating. Shifting was smooth but still no overdrive! Back under the vehicle I replaced the new VSS with the old one, No speed O! Put the new one back on and took another trial spin. This turned out to be a dream come true! Smooth speedometer and the smoothest shifting plus overdrive and passing gear at full throttle.

nonrev

rlortie, sometimes just the act of un-plugging cleaning and applying di-electric grease to your electric connections will solve a majority of electrical problems as 25 plus years of crud can get in there