I did a bunch of reading, thought I had a way to diagnose but now I can't find the topic again. Anyway: trans started dropping out of OD, then wouldn't go in at all. The TC has been slipping for months, got so bad I couldn't pull a trailer. Just got a new 47RE, billet TC, servos, hogged out VB, etc. Very reputable builder who dyno tested it before shipping and made sure everything worked properly.

Put the new trans in, everything works nice and smooth, but still no OD. TPS voltage is .80, which isn't great, but I've seen it as low as .60 and still have OD. Batteries, battery wiring and terminals are all recently new and clean. The tach works, speedo works. Top speed of about 65mph @2600rpm. I installed a TC lockup switch to see if I could narrow it down. TC will lock with the switch at any speed, but still no 4th gear. I'm just praying it isn't the PCM, and I can't remember if the TC locking only on the lockup switch rules that out or not. I really don't think it's the trans or anything internal because the old one did the same thing. I need this truck for work every day, I'm at my wits end. Anyone know what it could be?

 

Hows the throttle position sensor look?

 

I think its the TPS sensor. I would start by changing that. New tranny and all, that's what I would check, unless it your OD switch that is causing this issue. The PCM doesn't really control the tranny. Are you sure that you TL Switch is properly wired? All you need is a good ground.

Check the wires at the connector for the TPS, might have a bad connection or broken wire.

 

I had the same problem on my 47 RH stock transmission. I changed out the TPS; first one I bought didn't help I pulled my hair out; kicked around checked wires; checked everything I knew to check!!!!
Went back to Oreileys & got another one the next day.
That one fixed it!!!! It's been in here for 4 years now.
That taught me that just because its new don't mean it's fixed & you can rule it out....

Hope it helps!!!!

 

Don't change it. Check it. The tps and throttle valve both effect lockup and od. Both are computer controlled functions. Check the voltage of the signal wire at the tps.

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What is the TL switch? You mean the lockup switch? I only put that in to see if it would lockup even though it won't shift into overdrive, and it does. The problem was around before I put the switch in, but it won't lockup without it.

I checked the TPS according to the directions at Dodge Diesel Maintenance Tips. The only thing that didn't necessarily check out was probing the connector with the key on/engine off, the black and purple wires are supposed to read 5 volts, and I only got 5 at one of them. I forget which, I believe purple. The other read nothing. I've done this test quite a few times over the years, but I can't remember if it's always been this way or not.

Also, the OD switch on the gear selector appears to work, at least, the light in the dash comes on when I push it. Doesn't really matter though, because turning the OD on doesn't mean I have OD since I don't.
Also, there is no trans temp light on in the dash either. I did get locked out of OD once while plowing several years (and three transmissions) ago, and the light came on first. Eventually it cooled down and worked again, but that's not the problem here. Could be a bad temp sensor though, if I knew where to look for it.

 

temp sensor is in the vb also known as the gov. press. sensor. I would surely hope it would be new along with the gov. press. solenoid. considering you just had it rebuilt. You can check temp with a scan tool any reading 275* and up o/d will be inhibited. Ive seen trans temps show almost 300 with a bad sensor and no codes or light set for trans temp. The bad sensors would show around 160* or more with a cold engine and trans.

 

Hmm. Well, I probably shouldn't, but right now I'm trying to rule out everything internal. Since the transmission is all new, and this problem was present before I put it in.

 

I feel pretty confident you have a tps issue. Its very common to have bell crank wear which causes erratic tps voltages.

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If you want to find out if it is the trans or not you could use a jumper wire. Going to need a Wiring diagram so you know what wiring are grounds and voltage.