Truck was not shifting into fourth and fifth gear

Hi CD OWENBY,

I attached the detail about the transmission control system. If the transmission does not engage fourth and fifth then we need to start with scanning for any DTCs. Most likely there is a code for something related that is preventing the shift. It could anything from a sensor to a PCM. The codes will help us narrow it down.

Also, did you replace just the transmission unit or was there anything else that you did with it? Was it complete with shift solenoids or just the transmission? Basically, did you have to transfer anything from the old unit to this one? If so, what did you transfer?

Let's start with this info and then go from there. Thanks

Okay, great. The PCM will flash the overdrive light and prevent the shift if it does not get a signal from the speed sensors, output speed sensor, or the overdrive drum speed sensor.

These are not the only causes of this but this makes the most sense.

I believe this vehicle only has one speed sensor in the rear differential.

Basically, what is happening is it is seeing a speed from these sensors except one (or more) of them and the speeds don't match so it flashes the light and doesn't allow top gear.

https://www.2carpros.com/articles/abs-wheel-speed-sensor-test

Okay. I want to circle back to when you said there are no codes. Do you have scan tool cable of reading transmission codes? It is different on this vehicle and if it is just a code reader, then most likely it couldn't pull transmission codes.

Can you send me the part number and manufacture of where you got the transmission? I just want to look it up and see if I can see what is not on there that may cause this. The main cause is the valve body gasket but it appears this was replaced when you put the unit in it.

I don't want to guess at it because this could take a while and get expensive.

If your code reader doesn't read transmission codes then you can just stop by a parts store and they should have one that reads them. Maybe call them and ask before you drive down.

Let me know this information and we can go from there. Thanks

Okay. If you have a tool that will monitor your sensors in the PCM then we need to start looking at what the PCM is seeing. I suspect you have a sensor issue that you have not replaced yet or wiring to one of them.

If you look at the wiring diagram I attached, this is from Ford and it states on it that the overdrive light will flash if a sensor/actuator or circuit has failed.

I highlighted the sensors that you need to look at and write down what they are reading when the engine is hot. Then monitor your speed sensors that I mentioned above while driving and see if they match the speed you are driving. You may need someone else to drive for you for this part so you can monitor them and write them down or video them while you are driving.

If nothing stands out, send me the results and we can go through them together.

Based on everything you have done, I suspect you have a wiring issue with one of these sensors, so the PCM is not getting a good signal and disengages overdrive. Using the scan tool is easier then checking each of these wires for short to power/ground and checking resistance.

Understood. It is possible but unlikely which is why we need to thoroughly diagnose it because the last thing you want to do another unit and that didn't fix it now you are in the same position. That is why he needs to eliminate everything else. If he does, then it is possible the unit is still the issue. So if he goes through these possible causes that we discussed as you did and they all check out, then you have another person confirming what you are seeing so it would make another unit a little easier to stomach.

Okay. The fact that you just replaced it, there should be a warranty based on where you got it from. Just want to bring that up if you haven't thought of it. All the major distributors these days have a warranty so hopefully it will be covered.

Please keep me updated with the results or if you need more help. Thanks