Howdy all. First post. I know how these forums work, and have searched back pages quite a ways and with the search engine and have not found an answer, so if I am repeating something covered a lot, forgive me.

1990 F250 7.3 IDI, automattic transmission. Truck runs good, but has some issues with the auto trans shifting eradicially. I can be driving down the road at almost any speed (except when over 70 MPH) and it will downshift for no reason, even when driving down hill, or with the overdrive turned off. I just noticed the other day the tach may be bouncing just before it does this, but its hard to see not being able to watch it the whole time im driving.

I replaced the speed sensor in the rear end, as my speedo was bouncing too. It didnt help it, and it still bounces a little.

Any clues on this?

Thanks

 

Remove the abs control module from behind the glove box and with it unplugged test out to see if it bounces less and stops downshifting ( that was the fix for my truck, took 8 years to figure it out) If not then keep it unplugged, and try another speed sensor on the pumpkin, as far as the sensors go, I really suggest getting them from the autostealer; Just because with Autozone, and Oreillys I've never had luck with their electrical components such as those. Have to usually go through 3 brand new ones right out of the box until I can find one that works correctly. But that's how I've helped people fix the erratic speedo and downshift so far, and it's proven fruitful. Post back

 

Thanks for the reply.
I tried your suggestion of un plugging the ABS behind the glove box, and it seemed to help some, but it still has the same problems. In about 8 miles, some highway, it now only does it in the higher speeds. It may not have been enough of a test drive to determine if it has helped or not, but it is still downshifting it when its not suppose to.

Yes, the speed sensor I purchased was from the dealer. I have read of all the troubes from after market ones, and dont want to be in that same boat.

 

Its pretty much a hit or miss, my speedometer bounces roughly 0.5 MPH since I fixed it, mostly at highway speeds though. I ditched the ABS Module since I plugged it in and made it worse , did you drain the fluid out of your rear end and refill it yet? Just maintance stuff that can be knocked off since it's in the area of work? Old oil could be causing the issue aswell, Last thing I can think of for now if that doesn't work would to be shave down the hole on the sensor so that it can go in a tad deeper.

 

Speed sensor should be on the tail shaft of transmission ... the one on the diff is your ABS / Anti lock sensor .... my truck has the speedo sensor in trans ... are you sure you replace the correct one ? ... just thinkin here...

 

That's the output shaft sensor if I recall, The ABS controls the transmission? Hard to explain, unplug the pumpkin sensor and let your truck give you whip lash

 

Thanks for the reply guys. Im not asleep, just havent had time to work on the truck yet. Next step is to replace the sensor on the trany and see what happens.

Hope to get that done this weekend.

 

The gear oil was my issue on my 94 automatic..od light would come on and truck would fish bite and up down shift. I checked the gear oil in the diff..added 2 qts and it stopped.

 

What are the voltage readings on yout TPS? I replaced my TPS and set it properly...many of my shifting problems went away.