"Bucking" in 1,2,3 only

[SubaSteevo]

I have been trying to drive in 3 around town to save gas as some people have suggested, but there is a "bucking" unless I give it gas. When I give it gas it is fine, but once I coast it begins to "buck" (feels like its going fast/slow/fast/etc). However, there are no problems in D. Anybody have any ideas as to what this could be?

[Subafreak]

Do you have the resistor unpluged? If so I had the same problem with mine. Usually in 3rd if you let of the gas you would have an engine braking effect, and in drive it would just coast, but after I unpluged the resistor it would do the same thing you describe. Not sure why but it is annoying.

[SubaSteevo]

Thanks a lot, I do have the resistor unplugged. I didn't even think of that. Does anyone have an idea why it does this?

[svx_commuter]

Well when the resistor is unplugged the line pressure is always at maximum value. When it is plugged in the line pressure will go down at lower engine speed lighter TPS loads. So it seems to me that some clutch pack can't slip as easily as it's supposed to.

[gl1674]

Well, there is a valve called "shuttle shift valve" in the throttle body. It is controlled by line pressure and it determines whether the overrunning clutch should be engaged.

Overrunning clutch controls engine braking.

The factory design was to disengage overrunning clucth when throttle is wide open. They detect wide open throttle by line pressure. Line pressure is normally controlled by TCU using a duty solenoid A. TCU does take throttle position sensor as main input to control line pressure.

At idle engine speed line pressure is kept low (60psi), shuttle shift valve not engaged, overrunning clutch is engaged.
Whenever throttle is applied, line pressure goes up, shuttle shift valve flips, overrunning clutch disengages.

With the disconnected dropping resistor there is nothing to keep
the line pressure low, it will depend on the output of oil pump, i.e. the engine speed.

What will happen is at idle the pressure low enough to engage the overrunning clutch, but as engine revs up the pressure goes up, flips the shuttle valve, it disengages overrunning clutch, engine revs drop to idle and the process repeats itself.

Don't ask me why they designed the transmission to disengage the overrunning clutch when "throtlle is wide open" - I am puzzled by this myself.

After several miles of drining, my 94 "bucks" until I downshift to 3rd for a mile or so. Then, going to D works fine. Any ideas? And, what resistor are you talking about that can be unplugged?

[SubaSteevo]

If you look between your battery and the side of the car there is a metal thing (for lack of better ways to describe it) attached to the side. It says "Jet" on it I believe. You might also want to clean out your throttle body and check the MAF, as they are the cause for many common problems. Just do a search and you should find plenty of info on any of the above.

[Earthworm]

Have you checked/changed your ATF lately?

I'm hoping your tranny cooler is not plugging up and slowing the fluid flow. Perhaps there's something wrong with the torque converter?

[oab_au]

Originally posted by gl1674
Well, there is a valve called "shuttle shift valve" in the throttle body. It is controlled by line pressure and it determines whether the overrunning clutch should be engaged.

Overrunning clutch controls engine braking.

The factory design was to disengage overrunning clucth when throttle is wide open. They detect wide open throttle by line pressure. Line pressure is normally controlled by TCU using a duty solenoid A. TCU does take throttle position sensor as main input to control line pressure.

At idle engine speed line pressure is kept low (60psi), shuttle shift valve not engaged, overrunning clutch is engaged.
Whenever throttle is applied, line pressure goes up, shuttle shift valve flips, overrunning clutch disengages.

With the disconnected dropping resistor there is nothing to keep
the line pressure low, it will depend on the output of oil pump, i.e. the engine speed.

What will happen is at idle the pressure low enough to engage the overrunning clutch, but as engine revs up the pressure goes up, flips the shuttle valve, it disengages overrunning clutch, engine revs drop to idle and the process repeats itself.

Don't ask me why they designed the transmission to disengage the overrunning clutch when "throtlle is wide open" - I am puzzled by this myself.

First the resistor is a parallel feed to the A solenoid, it just changes the range of voltage that feeds the A solenoid. Without it the voltage to the solenoid ranges from 0 to 5volts, when the resistor is connected it supplies a pedestal voltage that the TCU sits on, instead of a 0 to 5volt range it is .5volt to 5 volt range, a reduction in working range.

The TCU will still vary the pressure the same as normal, with it disconnected, just supplies a slightly higher pressure at the top end. The voltage that the resistor uses, is supplied by the
TCU, the level that the book says is 5V to 12V. This says to me, that this voltage to the resistor may also be varied by the TCU, as the modulated signal is.

The other point is that the over running clutch is operated by the shift solenoid no.3, that is controlled by the TCU, the shuttle shift valve holds the clutch out of operation in some conditions and really just locks the 3/4 free wheel sprage.

This solenoid is used to do a few jobs, depending which valve is supplied with pressure at the time. When it is in 3rd the 3 solenoid is turned on when the TCU detects a closed throttle, when accelerating the clutch is off, when cruising the clutch is off. The clutch is turned off to allow smooth changes, only turned on when needed.

A faulty throttle sensor can cause the non-existant or intermittent operation of the overrun clutch.

Harvey.

[SubaSteevo]

so what you're saying harvey is that because the resistor is missing, the signal from the throttle sensor isn't being restricted, so the overrun clutch acts weird, correct?

[oab_au]

Originally posted by BurgundyBeast
so what you're saying harvey is that because the resistor is missing, the signal from the throttle sensor isn't being restricted, so the overrun clutch acts weird, correct?

No Mate, the disconnected resistor only raises the line pressure a bit.
The TCU looks for the closed throttle signal, to engage the over run clutch for engine braking. If the throttle position sensor does not produce the signal, (faulty, dirty, maladjusted), the TCU will use other inputs to do the job, but not as smoothly as it should, if at all.

This is all an educated guess, using the available information.

Harvey.

[Andy]

Had the same problem, resistor knackered, replaced resistor, problem gone, nuff said.