Chirping down shift

[ROSKOE115]

So today I was hauling a trailer and thought as I was coming to a stop at the red light that instead of riding the brakes so hard I would down shift it. Well I put it in the 2nd hole (no pun intended) and it didn't do anything. It was acting like it was in drive still and so I slowed down more to where I could shift it into 1st and it didn't do anything for a bit then slammed the gear chirp the tires and it felt like it was in lock up. This was a shaft snapping shift no doubt.

So what my question is, since I have a new VB and TC is the PCM not communicating with the electronics in the VB or what? I am still learning the auto tranny world but I thought I would run it across the best brains in the industry. Thanks for the help.

 

[RockinMC]

Manual low has alot higher line psi than D 1st gear, so when it down shifted with no throttle applied it caused the chirp. If you're running a VB the pressures are even higher, enhancing the "feel". Get a brake controller and use trailer brakes.

 

[sstockton]

The added line pressure from the new valve body most likely caused the Transducer to read out of spec, and caused the PCM to throw the trans into "Fail Safe" mode aka limp mode. In limp mode you will have 3rd gear starts (which can burn up a trans really fast, especially when towing) and you will also have the issue of manual 1st acting like park. I can't explain that part, but thats what happened to me.

If this happens again, and you experience 3rd gear starts, shift into neutral and then you can use your gears manually.

The permanent solution to this problem is to:
A) install a resistor in the orange/black wire that runs between the trans and the PCM.
B) install a potentiometer that feeds the same wire. Run the power to the Potentiometer from a 12v keyed power source, and adjust it down to approximately 4.67 volts. This is the route that I went and it has worked great for me.

I'm out doing some stuff on the truck right now, so I will get you a pin number on that wire so that you know exactly which one needs to be used. I'm not sure exactly which resistor you need either, because I went the other route.
Sean

 

[sstockton]

Correction, solid orange wire, in the middle block on the PCM. It was kind of hard to tell an exact pin number. But thats the one you want.
Sean

 

[BgBlDodge]

If you downshift in to to low of a gear and the computer senses that it will cause an over rev condition it will not allow the truck to downshift. Once your speed finally dropped low enough to where it wouldn't over rev then the computer allowed it to downshift. I'll bet your tach whacked 2800-3200rpm when it finally downshifted.

Don't downshift an auto. It's not good on them and they don't like it. Get a trailer brake controller. You can get a cheap one for $50-75 and it'll work just fine. We've been using one for years with no issues.