Engine/Transmission (1994 - 1998) In and out of lockup

[phillips5]

Used my brothers 95 to pull camper to Verginia and back. Put a up grade plate in the pump 2 weeks ago. The torque could make up its mind lock or no lock when pulling up hills. I was geting 32 psi boost . It mite go in and out at 25 psi maby not go in and out till 30 psi. What desides the lock up booste or tps. Could I put a switch on it and make the dision my self. It drove me nuts.

 

[Joe G.]

Classic dirty TPS symptom. Try using electrical contact cleaner on it before doing anything else. Just drown the shaft and work the throttle linkage back and forth a lot. You could install a manual lockup switch. Pulling with a forced lockup is not really a good idea. If it continues to hunt you can force it to stay unlocked with the switch. The '95 has a 47RH transmission so this stuff is a little different than a later truck with the 47RE.

 

[Howard Durand]

Joe,



Correct me if I am wrong.



It is my understanding that if the TPS is functioning properly, during an uphill pull, when you begin to lose speed, the TC will unlock, then the transmission shifts out of OD. Following that the TC will lock up again with the transmission in third gear.

If you regain your speed, the TC will again unlock while it shifts into OD then the TC will lock up again.

 

[Extreme1]

Originally posted by Howard Durand

It is my understanding that if the TPS is functioning properly, during an uphill pull, when you begin to lose speed, the TC will unlock, then the transmission shifts out of OD.

Right so far...

Originally posted by Howard Durand

Following that the TC will lock up again with the transmission in third gear.

Not on a '94-'95 RH unless you have OD switched off. I don't think they started doing that until later in the life of the second gen like around '99 or 2k.



And stop calling me Joe!

 

[Joe G.]

If the TPS is working correctly it will unlock the TCC if the RPM is low and the fuel is high (lugging). If the TPS is dirty and you are using the cruise you can get into a vicous cycle. The speed is what the cruise is set for, but it taking quite a bit of throttle to maintain the speed and RPM is low. This means the throttle is close to WOT. The TCC will unlock because the engine is lugging. That causes the RPM to rise. The gear ratio is better so the throttle backs off a little. The PCM senses (with some help from the dirty TPS) that conditions are ok so it locks up the TCC again. More throttle and less RPM so it unlocks again. Etc, etc, etc,... If this cycle is pretty quick is will be felt as a hard hammering. That is what once killed my flex plate.



The TCC will not lock up in 3rd if OD is on. If OD is locked out the TCC will lockup in 3rd. If OD is not locked out the TCC will lock up only in OD. Of course, if you have a manual lockup control switch it will lock up in any gear except 1st for a 47RH. The 2nd gear lockup will only hold in decelleration if you have a modified VB. If you start from a stop with your switch in lockup with a 47RH it will lock up as soon as it shifts to 2nd. A 47RE works differently because the switch valve in the VB is different.



I use the 2nd gear lockup everytime I drive my truck. I live on a road that has a very steep crooked stretch so I lock it up in 2nd and turn on my exhaust brake to go down the hill and don't touch my brakes until I get to the stop sign at the main county road.

 

[Howard Durand]

Mr. Extreme1,



I stand corrected.



The "Joe" referred to in my post is Mr. George



Thanks to both of you for the clarification.

 

[phillips5]

I think that the throtle pressure cable for 1-2-3 shift and the od and lockup should be controled by boost pressure not foot pressure. The governer can add fuel when needed even if you don't move your foot and the lockup will not unlock. I have his truck today and going down the road empty in out in out so I unpluged the tps for now. On my 93 I cleaned the tps and bolted it back on upsidedown and pined it at about 40% of the travel and it stays there. Od comes in at 40 and if I am pulling something I can lock out the od till I wan't it .