I just bought a 1992 D350 11ft Dumping Flatbed. Miles are unknown as the odometer stopped at 155K. 12V cummins auto. Tranny is a 518A from what I have read which is a 727 with an overdrive. Non-Lockup converter. Truck is 2wd, weighs 7900lb empty, and looks to be stock besides a straight pipe.

Truck takes a little throttle to get moving, which sounds about right with the stall speeds and non-lockup converter. Runs through the gears just fine, but seems to lock and unlock the converter on the highway. I know this is impossible since it is a non-lockup converter. Is it really just hunting for overdrive?

So far I have replaced the vss. The speedometer was bouncing around when I got the truck. The new VSS solved this. When I had it up on the rack, I fixed just about all the wires that connect on or near the transmission. There are 4 connectors on or near the transmission.

-Vehicle Speed Sensor 2 wires on tail shaft
-Tranny Temperature Sensor on the tranny cooler line. 2 wires. Mine had a jumper connecting them. I presume this was to bypass the temperature safety sensor. I left it as it.
-3 wire plug that I am not sure what it was for on the side of the tranny by the shift linkage. I believe it is the for the kickdown solenoid. The wiring for this plug had been cut and twisted back together. The bare connection were sitting there blowing in the wind and shorting out on the tranny cooler line. I soldered and heat shrunk these wires back together.
-2 Wire plug just above the previously listed plug. It was had brittle spots that I heat shrunk also. This plug was for the control solenoid.

I thought for sure that the steady speedo that the vss fixed and the 3 bare shorting wires fixed would solve my issue. Not so much. Took it out again and it was doing the same thing. See the attached video. That was with steady throttle. You can hear the tranny "locking" and "Unlocking" based on the sound of the engine load and you can see it on the speedometer. Is my torque converter failing? Am I slipping in and out of overdrive? Is this caused by the TPS and I need to do the POT mod? Previous owner said it had been like that for the 2 years he had it and he pulled some hefty loads with it. I have read until my eyes have bled without a defined answer. I do not want to throw parts at this, just trying to properly diagnose.

http://s272.photobucket.com/user/JoeyDi84/media/null_zps17740352.jpg.html
http://i272.photobucket.com/albums/jj179/JoeyDi84/379224D7-EE64-4FC0-B062-B0EA84DE98AE-5081-0000080DF7D7D110_zps7502d74b.mp4
---AutoMerged DoublePost---

Tested my TPS like was stated here...
https://www.dieselbombers.com/89-93-...ensor-tps.html

At rest it was .03 volts and at WOT it was .17 volts. Doesn't even sounds like I can adjust it as the range should be 2.25 to 2.75 volts. My range was .14 volts. Sounds like I need to replace the TPS or do the POT mod.
http://i272.photobucket.com/albums/jj179/JoeyDi84/214336FD-7E0D-4129-BC38-281AA47FC0F6-5295-00000829D207D32C_zpsba68afb0.mp4
---AutoMerged DoublePost---

So before I go any further with the pot mod or the tps replacement here is what I found today. When tracing tps wire I noticed the crank sensor was not plugged it. I plugged it it. Next I tried to start the truck and about halfway through the crank it shut down everything. Turning the key wouldn't do anything. Thought my battery connection were bad so I wiggled them. Still nothing. Unplugged the crank sensor and still nothing. Hooked up a jump box and she fired right up. Plugged in the crank sensor and the truck seemed to run fine, but the voltage meter jumped around. Wiggled the battery connections again and she settled out. Took her for a ride and the check engine light came on. It was the for the following codes, including the 42 twice.....

12 Direct battery input to PCM was disconnected within the last 50 key-on cycles (normal)

42 Fuel pump relay control ckt, Auto shutdown relay control ckt, No ASD relay output voltage at PCM, Fuel level sending unit volts out of range, Fuel level unit No change over miles

24 Throttle Position sensor out of range

42 Fuel pump relay control ckt, Auto shutdown relay control ckt, No ASD relay output voltage at PCM, Fuel level sending unit volts out of range, Fuel level unit No change over miles

46 Charging system voltage too low

55 End of error messages (If you get this only, no errors were found)

So from what I understand the crank sensor controls overdrive too. Do i also have a bad crank sensor?

---AutoMerged DoublePost---

Cleaned the battery connections and cleared the codes. Started the truck back up and a few seconds later the check engine light came on with the same codes. Shut her down, disconnected the crank sensor and fired her back up, no more codes. Replace the crank sensor? I'm going to search on how to test it now.

 

Well it could be all those things or just one, it could even be the PCM (computer). There are some test procedures for the cps and tps, as well as for the computer.

There are also work arounds for at least most if not all the functions of the pcm. It controls; charging, a/c, OD, cruise control.

I would start with the cps testing / replacement.

some good info here...
89-93 Tech Articles - Dodge Cummins Diesel Forum

specifically for the cps Testing Crank Position Sensor? - Dodge Cummins Diesel Forum

Hope it helps and Good luck

 

My truck ended up being a mess. There may not be anything wrong with the tranny but there is with what controls it. The vss was bad and I replaced it. The tps is out of ranges and need to be replaced or pot mod. But then I realized the crank sensor had been unplugged and an external voltage regulator added. My options are to replace the tps (or pot mod), unhook the voltage regulator, replace the bad crank sensor, hope the ECM isn't bad, and hope I can fix the "modified" wiring harness or I can just forget about it. Since the truck already weighs in at 7900lbs and I will use this truck for hauling, very rarely on the highway, with less than 1000 miles a year, I plan on putting a manual switch on the overdrive solenoid and manually controlling the overdrive. This is the best option for my situation.