Been running a 5k ohm pentiometer in the brick for as long as I've owned it, as the P.O. didn't replace the TPS. Ran perfect, up until the other day.




After rebuilding my throttle linkage, adjusting the TV cable, to where all works as normal, I noticed the trans wasn't shifting into OD. The 518 has been pretty much no issue otherwise.



Thoughts as to what to look for ? I'm going to check the connections at the transmission, making sure no wires worked loose.




God this is gonna suck driving 3 hours with no OD.


Any ideas would be appreciated.

It's a simple circuit. Two wires at the trans. One wire is 12 volts key on, ground the other wire to energize the solenoid. A simple temporary toggle switch install will get you O/D. You just have to remember to turn it off when you slow down.

Rolled under the truck today, and found that the new kickdown cable was disconnected, and had been for at,least a few days if not more.


The new,cable end,does not,grasp the pin, and pops off. Took another, used.lever off of a spare trans, and replaced the lever. It held,for,about 10 miles, and nothing changed. I still have no 4th gear. Either,there,is,a,short,from the computer, or the valve burned out from the disconnected cable.


I don't know, as I've never studied up on how these transmission works.

EDIT:

I didn't know that there was an overdrive fuse that controlled the solenoid.


I'm checking that out now.

The kick down has nothing to do with O/D.

As previously noted, I don't know much,about,how these transmission work. I started reading,the,fsm, but time isn't,on my,side.

The fuse for the od unit checked out fine.. The of circuit is either shorted, or the solenoid gave out.


Not comfortable driving,3.5 hours to Carlisle, so ill have to delve into this at another time.


Thank you AJ


Appreciated

Tman is the speedo working? the reason I ask is mine had no od when the the plug that goes in the vss went bad

Yes.. Speedo works. When I can, I'm going to Jack up the back on stands, disconnect the lead wire to the od solenoid, have the wife start the truck, and get it spinning north of 35mph. If the PCM, and activation circuit is working, the plug should get 12 volts to it. I have to confirm the wiring diagrams beforehand

This will confirm if it's the od solenoid, or the circuit that activates it.which looks to be 2 sensors (coolant and distance????) that can interrupt the circuit as well

The plug will always have 12v going to it. The ground side is what triggers the OD solenoid.

Thanks!

Can someone help me read this diagram ?


The OD switch on the dash is the first thing that came to mind, as it interrupts the circuit to the OD on the trans.


I'm trying to understand how this switch works.


Power comes into the switch through g5 20db/wt which is the blue wire white tracer. Isn't the switch supposed to allow power to run through it to the t6 20 or/wt all the time, then disconnect it when the switch is pressed momentarily to disconnect ground to the PCM, which then woudn't activate the overdrive ?


I'm a bit confused as to how this circuit works. Anyone know how to read these better than I ?


I'm trying to diagnose whether or not the OD switch on the dash isn't allowing the feed through to the Orange white wire. When I test it, it doesn't allow it to feed though. I put a jumper in there to see if I could manually have it power the orange white, but it changed nothing. Probably not the best thing to do, but I would like to have 4th gear back, as this really sucks not being able to drive over 65MPH.


Thanks for any help.

The 20DB/WT wire goes into the indicator light which gets it's ground from the Engine Controller and isn't connected to the switch at all other than being in the same housing.

The switch appears to be a momentary contact pushbutton in the N.O. position which connects to the Z1 splice on one side and the engine controller on the other. I have no info on what the engine controller (ECU) does.

Is this schematic one of the sheets on the CD you sent me. I'll look at it and see if it lends any clarification.

If I'm right about the switch being a M.C. pushbutton my guess would be that it activates a latching circuit in the ECU which both lights the indicator and activates the solenoid. Search the internet and see if there are any schematics of the ECU to be found.

Edwin

The diagram is from the 1992 SM I sent you. When the ignition sw is off, no power on blue wire. Ignition sw on, blue powers up. When you ground the Orange wire with test lamp backfeed, each time you touch and release, it either turns od off light on and or off. Touch once or light on. Touch again, od light off, and will continue to do so, each time.

So yes it's a momentary sw.

I suck at wiring stuff like this.....Amen

It's OK. I went to a speshul skool to learn how to read these diagrams.

At least we have them. Some of the TV's and Radios I worked on had either nothing or a sticker about 2" x 3" to cover the whole thing and had to be examined under a magnifying glass.

This thread here



https://www.dieseltruckresource.com/...rdrive-320078/


Unfortunately, I don't have a tps anymore, as about 6 years ago, I dumped it for the $6.00 radio shack swap...

Check your crank sensor. I lost my OD and it turned out the crank sensor was out of adjustment. I know it may not make sense, The alternator was still charging so enough of a signal was getting through just not enough for the OD. I also deleted the TPS with a potentiometer years before this. This happened about 6 years ago and its still working fine.