DFlaJax
Active LVC Member
Any suggestions on why after coasting above 40 mph and stepping on gas even lightly to maintain speed, tranny shifts to 3rd momentarily before going back into OD? Even downhill?
Strange shift
- Thread starter DFlaJax
- Start date
NoLimit95
Dedicated LVC Member
Does the trans do this when it is cold or after it warms up good?
NoLimit95
Dedicated LVC Member
Has to be it most likely because TC lock up is at 39 mph.
No its not. Its steady state slip and based on load, temp and throttle position. Ive read the stock tune for hours on end and there is no command to fully lock the TC in fourth based on speed.
NoLimit95
Dedicated LVC Member
Normal driving at lower speeds is usually 39 mph. Maybe your 98 trans is different or have you studied all years?
The torque converter lock up schedule doesn't start until 37 mph in 3rd or 4th gear as commanded by the ECU. This has been discovered by many to be too LOW of an rpm range which places too much load on the engine. In other words, it's almost like shifting a 5 speed in to 5th gear at 37 mph, causing it to bog, or stumble. The same thing can happen in a Mark VIII-especially 93-94, and 95-98 base models with 3.07:1 gears out back.
You see, the 4R70W transmission in our cars has that W on the end meaning "wide ratio". Couple that with low rpms, low speed, heavy car, freeway gears (3.07's) and a high rpm DOHC V8, and at 37 mph, you get an engine that can hardly make power since at such a low mph, the engine is barely at 1200 rpms, way below the power band of these engines. Especially if the ignition components are old/weak.
The lock up is also reversed as soon as the ECU picks up a signal from the brake switch(BOO). The next time you feel the shudder, lightly place your left foot on the brake pedal without lifting your foot from the accelerator, and see if the shudder goes away. Just press hard enough to turn on the brake lights, but not hard enough to slow down the car. This will command the converter to unlock fully.
Ive only read the code for 96-98 patterns and then for auto Mach1s (no steady state slip) and that of Marauders to figure out what I wanted mine to do. None of them command a lock in fourth at 39 mph. Its closer to 60 and still allows for some slip except on the Mach1.
Now if you are going under 60ish and not on the throttle then the RPM will go down to what the RPM would be if it were locked. Then when you push the throttle a little it will slip the TC and climb several hundred RPM and then to a partial lock after it sees it dont need so much slip and then do a full (almost full) lock when at the computers target speed. That could feel just like a downshift and then upshift.
Its like when Mikes car was at the track and he thought it was upshifting then downshifting then upshifting again on the big end of the track but it wasnt shifting, it was locking then unlocking then re-locking the TC again. I think his issue was the fact his hi-po TC cant do a partial slip and am digressing from the point of this thread........ :lol:
Now if you are going under 60ish and not on the throttle then the RPM will go down to what the RPM would be if it were locked. Then when you push the throttle a little it will slip the TC and climb several hundred RPM and then to a partial lock after it sees it dont need so much slip and then do a full (almost full) lock when at the computers target speed. That could feel just like a downshift and then upshift.
Its like when Mikes car was at the track and he thought it was upshifting then downshifting then upshifting again on the big end of the track but it wasnt shifting, it was locking then unlocking then re-locking the TC again. I think his issue was the fact his hi-po TC cant do a partial slip and am digressing from the point of this thread........ :lol:
NoLimit95
Dedicated LVC Member
I was just sayin, with the way he was explaining his issue (lagging around at around 40 mph) that the TC is locking and unlocking, sh!t, idk lol Anyway, I didn't mean when pressing the pedal at any speed that the TC would lock up at 39 but at a normal lagging rate of speed, it is commanded to do so, really at 35 mph, not 39. It's just my experience with 3 Marks that it usually is right at 39 mph when the lock up occurred.
I wished sometimes that I could attend a safety meeting without having anxiety.
OP, drive normal and quit coasting around at 40 on a hill or turn the OD off and whatever is going on will stop.
DFlaJax
Active LVC Member
Thanks guys for an abundance of tranny info. I used 40 mph as an example, but it doesnt matter what the speed is (65, 70, 80), whenever I let off the gas (whether 1 sec or 1 min), it coasts in OD, but when I reapply throttle, tranny shifts to 3rd before going back into OD. You can imagine what rpm's do at highway speeds. Hope this info narrows things down. Again thanks for the help.