Shifting issues

[tbrown5079]

Did a 4.8 from a 06 GMC Sierra pickup into a 56 Chevy truck. All components from that vehicle. Motor runs strong, no check engine light, but when shifting into 3rd it seems like the transmission is confused. It will seem like it is in neutral then when you give it more gas then it downshifts. If you pick up more speed then let off the throttle, it will slowly shift into third. Took the pan down and both solenoids are firing. Today I calibrated the electronic speedometer but that didn't help much. At least I know I am getting a speed signal from the ECM. Is my next step to check the shift points in the tune?

[aknovaman]

Sounds like a low pressure or low fluid issue. Post up your file and tire/gear ratio.

A data log will help too.

[tbrown5079]

Checked the fluid cold but not hot. Will do that tomorrow. Don't want to get it too full. 215/75/15 tires. 2.73 gears. Thanks for getting back.

[BR3W CITY]

Trans level cold is useless, needs to be warm and have been in P,R,and D to make sure every fluid path has taken fluid. If the level is good then, and the fluid looks good and doesn't smell burned or like hot sh!t, move on.

Do you know what size tire the ECM was told you were running? A significant differential between the input size and the actual size will not only throw off the speedo, but more importantly, throw off the shift points and the lockup draaaastically.

As an example; my '66 runs a 325/50/15 drag radial. Thats around a 28" tire. I had to toss on my old front rollers, a tire thats only 225/60/15 or a 25.6" tire. That difference caused hugely annoying issues with the VSS/ISS reporting speeds @ rpm that didn't fit the computers logic. This usually means things like ending up locked up in 4th only going 40 and lugging like all heck....or the opposite, randomly unlocking and re locking, almost feeling like your going up and down a gear constantly.

If the tire size in the tune is correct, then (assuming we've made sure no other mechanical problems are affecting it) we would start to look at the trans tune setup. Most tuners don't really touch trans settings when they should, and 75% of the ones that DO mess with them do it wrong. If it does come down to troubleshooting the tune, then simply reflashing/resetting a base GM trans cal should sort things out quickly.

[tbrown5079]

That makes sense to me. I will get the level set on the fluid then talk to a tuner. Thanks for your input.

[lowrodderchev]

Holy crud I was just having this same issue I guess I know were I need to start