I know little to nothing about transmissions, except how to put them in. The transmission on my 1990 2x4 F-250 7.3 IDI Automatic has been acting weird for a few months, so I figured I'd see what ya'll thought about it When I'm driving and I hit about 30-38mph the transmission tries to shift from 3rd to 4th. While trying to catch 4th the engine speed revs high then drops low repetitively until I hit about 40mph. At this speed it normally catches 4th. At about 43-50mph it begins trying to shift from 4th to 5th, and once again the idle starts bouncing. At about 52mph it catches 5th and its smooth sailing from there Trying to maintain speed between 30 & 52mph is extremely aggravating though Some input would be greatly appreciated.

 

the transmission you have is actually a 4 speed with a locking torque converter which is controlled electronically.
drive = 1st gear of course
the first shift you feel is 2nd gear.
then you will feel a semi shift for your 2nd type of shift.though this is not a gear change,it kinda feels like one.this is the torque converter locking up.
shortly following (almost instantaneous sometimes with the lockup of the TC,if your going downhill/starting out real easy) you'll feel your final 3rd gear change to 4th.this is the overdrive/fuel economy gear.

do you have blown fuses?
do you have a flashing OD off lite?
have you pulled codes?
do you have a working speedometer and tachometer that seem to work flawlessly?
how many miles are on the trans,is it original,and what has the maintenance schedule been?

right now,with the amount of info given,it sounds as if the toque converter is failing to lockup and stay there.

 

I'm pretty much new when it comes to transmissions so I had no idea where to start.

1) I haven't checked the fuses.

2)My overdrive button works well. I actually use it to make it where I can drive below 50 occasionally when I'm in town. So no it is not flashing.

3) I'm relatively sure with the age of the truck there is no way to pull codes from it, though I could be wrong.

4) The speedometer works fine. Though the tach is a little messed up. it doesn't lift from zero until about 1200. after it picks up it seems to read accurately.

5) And as for the maintenance I haven't done any on it yet. A little before i bought it the previous owner had the filter and fluid changed. I've put about 9,000 miles on it.

6) The truck has 83,000-183,000-283,000 miles on it. I really dont know but the odometer is functional

I was also thinking it may be the torque converter.

 

well the tach needs to be working 100% so you'll want to check the wires and test the tach.
when im cruising around at most of our 45 mph speed limit secondary roads around here,and let up ff the throttle when going downhill,she'll unlock the convertor and drop the rpms below 1k.
so if the tach didn't work under 1200 rpms my trans would be going nuts too.
at 45 im just around 1400ish.
what else is odd,is that im seeing OD kick in just after TC lockup much sooner than you.
my TC locks up around 35 mph,and she finds OD around 38-40 (only higher speeds if im taking off faster than i should for fuel economy,or of course if im loaded up,she'll wait to kick in latter,or until im done reaching my cruise speed and back off.)
so i also wonder about your TPS adjustment.
for starters,since you know the tach isn't working fully,you'll want to begin investigating there.
there is a computer for this tans,and it can store codes for you.it's devoted for the trans,aside from the engine.
the signals from these sensors tells the brain how to shift the trans.this is why they all need to work properly.

 

Well I greatly appreciate the advice. I'm pretty new to the diesel thing. I mostly work on gas vehicles (pretty good at it). I'm eager to learn about diesels though. I love a challenge. My first question would be where to start with the tach. On gas motors it runs off the distributor. Well since I lack a distributor where would It be located on the motor? I would assume a problem in the instrument cluster would not affect the operation of my transmission. Second, where would the cpu for the transmission be, and how would I pull codes from it?

 

I second the tach as being a very likely culprite, or rather the tach sensor.



Its the big 1" nut on the top cover at the front of the engine with two black wires coming out of it. Try taking it out and inspecting the magnetic pickup probe for any signs of metal filings, sludge buildup or frayed wires. Sometimes a good cleaning can get them working again other times you just have to replace it.

 

That is very helpful!!! I'll be checking it out soon, thanks.

 

I took the advice, checked out my tach sensor and replaced it. My transmission now works like new. Though, it was an act of God for the idiots at the parts house to find it, but they finally got it ordered. I told the guy behind the counter I needed a tachometer sensor and he replied, "What's a tachometer?" Bahhhhhhhh


Thanks for the help I'm very appreciative

 

whats a tachometer lol.dontcha just love it.glad it all worked out for ya in the end though.
now,a little info for ya;
the trans should be flushed every 20k (heavy duty use/towing) - 30k (normal use.)
Mercon or Mercon 5 ATF.
having a trans temp gauge doesn't hurt,so you can keep the fluid @ the pan ideally around 175 if possible.keeping her under 200 is ok though as long as she gets flushed regularly.

 

Thanks for the update. And holy crap.......that parts guy was dense.....LOL

 

I have no idea why he works at a parts house. But in this college town there isn't a knowledgeable person in sight when I go into a parts house. A quick way for me to get into a bad mood up here is to go to the parts house.

And thanks for the advice. I just drive the truck around campus and back-and-forth to jobs. It's the best work truck ever. Two tool boxes in the bed, and the back of the cab is full of power tools, cords, and straps. I'll be flushing the tranny soon

 

Oh... And my water temp gauge wasn't working before I replaced the tach sensor. But with the pic you posted I knew exactly where to find the sensor so I cleaned up the connection and now it works perfectly. The truck is now working like a brand new machine, and I don't have to worry about it overheating. I couldn't be happier.