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Discussion Starter · #1 · (Edited)
Hello folks, need some serious advice... My fathers truck has been awesome and any issues I had were easily resolved, until the O/D death light started flashing.

2003 7.3 4R100 with PHP tuning. The same tuning has been on the truck for a few years. It has single shot injectors so I don't think I can just get a stock tune to try.

We don't drive this truck much, so that is why there is so much time between incidents below.

Late last year O/D light started flashing although everything seemed perfectly fine. We took it on a couple trips camping and what not, figuring it was higher mileage and probably due for a trans anyway.

Shortly after, it started holding first gear to redline or until you let off the throttle, then it would shift to second (I think, it appears now it might have been just shifting straight to 3rd after the speed got high enough). Codes below from the old trans.

P0733 - #3 Incorrect Ratio
P0741 - TCC Slip
P0743 - TCC Circuit

Instead of throwing money at the 175k hard miles transmission, we decided to get a dealer rebuilt trans and swap it in.

It drove great when I left, then when I got back home about 75 miles and came to a stop, it held first again and would go all the way to redline or until you let off the throttle and it would then shift.

I pulled the column apart and checked for chaffed wiring from the shifter, I found one very small rub, not through to the wire. Wrapped it up and put it back together.

I then hooked up my scanner and pulled the codes and got a ton of codes, including those old ones that I assume were stored. I also set up my scanner to monitor shift commands. I went ahead and cleared since there were old codes from the batteries going dead, Transfer Case not being plugged all the way in, etc. I took it for a drive and It did the same exact thing, would hold first until I let off or until the PCM commanded 3rd. It did command 2nd when it should, but wouldn't shift. After about 5 minutes, the O/D light started blinking. I got back and have one code now, P0781.

If I manually shift the trans into second, it will go into second.

I have tried my best to look at the harness connections and wiring at the plugs and they all look good with no brittle insulation.

I'm at a total loss at this point. To me I think were looking at a wiring harness issue since the same thing happened with two different transmissions with all new sensors and what not, but I'm not positive.

Is there any advice or input anyone can give me on this? Thank you a ton.
 

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Something isn't adding up. I agree that the problem is most likely external to the transmission and that the original trans was not the problem.

Code P0731 means that third gear ratio is wrong. But you say it is shifting into third gear, but not second. Can you monitor gear ratio while driving? I know that Forscan can do this. Then we will know what gear it really is using.
 

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Discussion Starter · #3 ·
Something isn't adding up. I agree that the problem is most likely external to the transmission and that the original trans was not the problem.

Code P0731 means that third gear ratio is wrong. But you say it is shifting into third gear, but not second. Can you monitor gear ratio while driving? I know that Forscan can do this. Then we will know what gear it really is using.
Gosh darn it, I apologize. It code P0781, 1-2 shift.

I used Torque Pro and was able to see when it was trying to command a gear change. It would switch to 2 on the app, but not shift until it commanded 3rd.
 

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Discussion Starter · #4 ·
Gosh darn it, I apologize. It code P0781, 1-2 shift.

I used Torque Pro and was able to see when it was trying to command a gear change. It would switch to 2 on the app, but not shift until it commanded 3rd.
Unless I manually shifted it to 2nd, then it would show 2 on the app, and clearly shift to 2nd gear from a stop or from 3rd or 4th. If I'm no mistaking, manually shifting it would in a way "bypass" the harness since its a cable straight to the trans? I Could be wrong, may be wrong, probably am wrong 🙃
 

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That makes a lot more sense.

When you manually shift to 2 the transmission hydraulically applies the intermediate band. This will create second ratio with or without the intermediate clutch, which needs to be applied by the shift solenoids.

This tells me that the intermediate clutch does not work. How could it be bad in two transmissions in a row? I have no answer for that, but that is what it looks like. I recommend taking it back to the dealer. The trans should be under warranty.
 
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Discussion Starter · #6 ·
That makes a lot more sense.

When you manually shift to 2 the transmission hydraulically applies the intermediate band. This will create second ratio with or without the intermediate clutch, which needs to be applied by the shift solenoids.

This tells me that the intermediate clutch does not work. How could it be bad in two transmissions in a row? I have no answer for that, but that is what it looks like. I recommend taking it back to the dealer. The trans should be under warranty.
Thank you for the response! I really appreciate your help. If it did have a bad intermediate clutch, would it work intermittently? I took it for another drive, and it did the first gear hold for about half the drive, then the O/D light started flashing, but then it started shifting perfectly normal! My father was telling me when he took it into town a couple days ago, it shifted perfectly fine for most of the day, then when he was getting ready to head back it started doing the 1st gear hold again. This thing is throwing my mind for a loop. Lol
 

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I think you need a pro to diagnose this. I can't do it without having the truck.

The intermediate could have a bad seal. Sometimes a damaged seal gets turned a bit and works again, then turns back and leaks.
 
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