Overdrive problems

[CLiberty]

<!--StartFragment --> [font=verdana, arial, helvetica]I have been having problems with the overdrive in my truck, trans is a 518 non/lu, stock with 176k on it. I have all the service records from the orig owner and the truck was well cared for, trans serviced approx every 20-30 tho, fluid level is full and fluid looks brand new, nice and red,not burnt/signs of slippage. When i first got the truck (172k?) it would drop out intermittently on the hiway and then pop back in on it`s own, without rhym or reason. I did all the usual stuff, i. e. cleaned & adjusted the tps, ran new tps wiring using shielded cable and put a filter cap on the tps ckt. All of those things had the OD working fine till about 3 months ago, it started dropping out again, except now when it drops out it will not go back in, but when it is working the 3-4 shift is nice and positive, doesnt feel like it`s slipping. It is temperature dependent and will only dropout after the truck has been fully warmed up, or about a 10-15 mile ride at hiway speeds. When it does drop out I can get it to re-engage by pushing on the go pedal a little harder (raising line/TV valve psi?) but evetually it simply stops engaging into 4th. I have re-cked all of the wiring/inputs to the PCM and all ck`s good, i. e tps/speed snsr/crank snsr all ck good. So what i did was tapped the ground side of the OD solenoid and measured with a DVOM. As I had feared, when not in OD I have system voltage present and @ around 35-37 mph the voltage drops to . 5 indicating that the PCM is commanding the OD sol to apply. I`m thinking I have some sort of internal hydraulic leak or worse yet the OD assy is going south. My question is, if the OD unit fails will the trans nuetral out until it drops back to third (it doesn`t do this, just feels like it`s downshifting to 3rd when it happens), and if there is the possiblity of an internal leak, do any of you folks have an idea where to look. I was looking thru my FSM and I can see where the the OD solenoid gskt,VB separator plate, and the 3-4 accumulator can be a source of leakage but i don`t have much experience with these units so I would like an "experinced" opinion of where I might concentrate my efforts. Other than the OD problem the trans works fine, no slippage etc. TIA!![/font]

 

[Chris and Sam]

had something similar happen to my 93 d250 cummins, and it ended up being the TPS (throttle positio switch) located on top of the injection pump. changed that and my problems were and are still gone, and O/D works great.

 

[MGinn]

I had problems with my od. Nobody could find poblem as it was intermitin. I fixed it myself. One of the sensors external was leaking oil. I would blow it out with air and it would work for a while. Once i discovered that i changed it out. No problems since except blowing apart the revirse band.

Mike

 

[1stgen4evr]

OD Problem

It is almost certainly not the OD unit itself. My opinion on the PCM and TPS are already stated (totally negative) but you can put a switch on the OD and that will let you know as to the functionality of the unit itself. The OD is wired hot and the computer grounds the solonoid to activate OD. You can do that function with a switch. Some do this and use it as a splitter. I have one truck set up that way.



There are more places to explore than you can imagine and some have just given up. If you find the cause, by all means post the result as it might benefit someone with the same problem.



James

 

[CLiberty]

It is almost certainly not the OD unit itself. My opinion on the PCM and TPS are already stated (totally negative) but you can put a switch on the OD and that will let you know as to the functionality of the unit itself. The OD is wired hot and the computer grounds the solonoid to activate OD. You can do that function with a switch. Some do this and use it as a splitter. I have one truck set up that way.

There are more places to explore than you can imagine and some have just given up. If you find the cause, by all means post the result as it might benefit someone with the same problem.

James

The thing is, when i ran the wire off the ground side of the OD sol, I also ran a pigtail off of that line to a switched ground, grounding the OD sol does nothing... grrrrr.

 

[cerberusiam]

If you are seeing the voltage drop the PCM is telling the trans to shift so it is probably an internal issue. Since the OD solenoid is electric and they are notorious for failures and problems it could just be getting worn enough that it doesn't react the same way anymore. Wear on the trans could be dropping the line pressure enough the TV pressure is too high for the shift to occur.



There is an adjustment on the trans that will set internal pressures and OD circuit pressures to control the shift point. You might want to play with the pressures a bit and see if that doesn't bring it back to normal.



Set it up with a mystery switch and see if the shift happens firmly and quickly. That will tell you if you have an electrical issue or an internal problem.

 

[1stgen4evr]

Maybe

"The thing is, when i ran the wire off the ground side of the OD sol, I also ran a pigtail off of that line to a switched ground, grounding the OD sol does nothing... grrrrr. "



This is so simple that I will say 'don't be insulted by the question'. You did check to know that the 'hot' wire is actually hot?



There is also a high heat switch that disallows the OD when the transmission temp gets too high. Now I am not sure what the wireing logic on that circuit is but it may be responsible for the problem. If that sensor goes through the PCM however, what you are doing with the mystery switch should show that the problem is inside the transmission. You are doing these test with the truck at the fault temperature aren't you? Working backward from the transmission should get you to the source of the problem, BUT, don't feel like the lone ranger here. While there are some that will tell you quick like that the problem can be solved, some pretty good mechanics have tried and finally just said to hell with it.



James



In fact, if you have enough money, any thing can be solved. It might be cheaper to buy a different truck. I did.

 

[Moab4x4]

It does sound like the OD solenoid to me. However, it could be the temp switch just mentioned. The voltage for the solenoid goes through the switch on the transmission cooling line and then to the solenoid.

When my transmission gets toasty (like on the 4th of July) my mystery switch doesn't do anything but when it cools off and the switch closes, all works again.



Your diagnostic procedure mentions heat as one of the factors. Check the voltage out of that switch.



You mentioned that it works again for a bit after more go. If the switch is good (IE cuts out at the proper temp... no I don't remember what that is... ~280?) then could you have a plugged cooler? More go = more cooler flow until the entire system heats up?



BTW. good job on the electrical diagnostic logic.

 

[CLiberty]

I did not mention that I have already by passed the trans temp switch, it`s looking more and more like an internal prob. I`m going to borrow some psi gauges from work this week and see what kind of pressures I`m getting at the OD port. . grrr.

 

[Doc Holiday]

Did you ever solve this problem?

 

[CLiberty]

Sure did! see here, it`s been running fine ever since.

https://www.turbodieselregister.com/forums/showthread.php?t=136566