ttuling

Either result would be interesting if you had the sort of transmission hunting which goes away at more than light throttle.

The reason I'm asking is that if such hunting is in fact repeated locking and unlocking of the torque converter clutch as commanded by the TCU (marcodant's VAG-COM readings tomorrow should tell), it is conceivable that an underreporting MAF sensor could be the cause, if

(1) the engine torque is calculated from air flow reported by the MAF sensor, and
(2) the TCU uses the engine torque reading to decide when the car is coasting (requiring the converter clutch to unlock).

Suppose the MAF underreports in a way that once the engine rpm drop after locking the clutch, the calculated torque suggests to the TCU that the car's coasting. The TCU will then unlock the converter clutch, so the car can coast with less drag. (Unlocking during coasting, at least partly, is normal converter clutch behavior, and has been confirmed with VAG-COM and a seemingly normal A8 transmission.) The clutch unlocks, engine speed and measured airflow rise to where the torque value makes the TCU lock the converter clutch, and the cycle repeats. A simple subtractive offset on the part of the MAF sensor might do this.

There's a lot of assuming and guessing here in my question, and I look forward to seeing data which steer us in the right direction, whatever it may be.

Tom

tozoM8

At highway speed during coasting if I apply the break, shouldn't it lock the TC to help slow down with the engine break? My hunting even worse after breaking and then accelerating. If I drive faster than 70 mph in 4th gear, it is not hunting at all even if I let it coast. Mine hunts only if the rpm is lower than 2000 in any gear.

tozoM8

Because I am taking my tranny a part twice a week, I did a few changes. My car was perfect after the valve body sealing, but a few days later I've lost 2nd gear. I took the valve body out and I couldn't find anything wrong with it, so I swapped the solenoids from my old 97' valve body and I think it hunts more now, but I don't want to mislead anybody. I don't think is the toddle pos. sensor, because if I keep it above 2500 rpm, it will not hunt and the TC will not unlock. I've tried yesterday on the highway a few times. I think the TC unlocks because it receive a signal from the TCU for whatever reason, and then the rpm drops, fluid pressure drops, and when you step on it again it has to build the pressure again to lock the TC.

ttuling

after having gone into a partly locked state while coasting. At least that's what VAG-COM shows on my 2001, and it's typical of most cars.

Tom<ul><li><a href="https://forums.audiworld.com/a8/msgs/73293.phtml">https://forums.audiworld.com/a8/msgs/73293.phtml</a</li></ul>

ttuling

Just to make sure we're talking about the same thing, does your engine speed go up and down about 100-300 rpm (or more) over and over again at certain throttle openings and loads? That's the sort of thing I'm pursuing.

Or is it a one-time increase then decrease in engine rpm when you reapply the accelerator after coasting or braking? That's normal as the converter clutch reengages, though it should be prompt, smooth, and barely discernible. If this is what you feel more with the 1997 solenoids, I wonder if the 2002 torque converter clutch pressure regulator valve is somehow more linear or responsive. (Yes, I know that doesn't explain the improvement you got from adding the gasket material.)

And how is it worse when you accelerate after braking? Bigger rpm swings?

I suspect that if you use VAG-COM to look at the torque converter clutch state above 2000 rpm, you'll see it unlocking at least partly when you coast, but at higher rpm the difference between a locked and unlocked torque converter isn't as big -- an unlocked converter slips less at high rpm.

It seems odd that there would be fluid supply problems affecting only reengagement of the torque converter clutch even when the transmission engages reverse or drive promptly when selected at idle on the hottest days. I am, however, very open to being convinced.

Tom

PaulW

It's funny, my car did it a little today, if I lift off the gas on the highway at 55 MPH plus, it will drop from say 2000 RPM down to 1400 RPM or so.....then stay there for a short (1 second) to maybe 3 seconds, then "thump" the driveline and come back up to speed, about 1800 RPM because vehicle speed dropped.

Then I tried it again, and it would just drop 100 or so RPM (like your car does, one that works correctly), give it gas and power comes right on.

And most of the time it does the "thump", but sometimes it acts normal.

And you know what? I have a guy who is going to sell me a new MAF for $200 shipped, I'm putting it in, worse comes to worse, I'll have a spare!

CA_A8

Doesn't this sound like the TC disengages and re-engages? This could be due to low hydraulic pressure as well as an external signal (not likely)

ttuling

which if I understand correctly involves 100-300 rpm fluctuations over and over again at constant light throttle. I think his stands a chance of being MAF related. Yours, yours is just weird. Do you know if that's what tozoA8 was experiencing also?

It would be interesting to see VAG-COM logs of what the TCU is trying to do with the solenoid and pressure regulator valves during the rpm drop and recovery. Possibly a sticky N218 pressure regulating valve causes the TCU to:

(1) initially unlock the converter more than it wants to, as it drops N218 current excessively to get the sticky valve to move as indicated by rpm across the converter going up (so far as I know, going from a fully to partially locked converter clutch is the the only change commanded when going from light throttle to coasting),

(2) measure that the clutch unlocked too far via excessive rpm across the converter, and

(3) overcompensate with a thump in the locking direction, again because the valve isn't moving smoothly

Just a guess.

I'll be surprised if a new MAF sensor fixes the rpm drop &amp; thump. And I don't know if it's applicable to your car, but for a 2001, you can get a rebuilt MAF sensor at the dealer for under $200.

We should use different words for the different kinds of hunting.

Tom

ttuling

since the transmission controller commands the torque converter clutch to go from fully locked to partly locked when changing from cruising at light throttle to coasting. It is also likely something inside the transmission isn't responding smoothly to the controller's signal.

The URL below records such torque converter clutch behavior in a nonthumping transmission.

Tom<ul><li><a href="https://forums.audiworld.com/a8/msgs/73293.phtml">https://forums.audiworld.com/a8/msgs/73293.phtml</a</li></ul>

tozoM8

This is what I am talking about, but it never happens over 2500 rpm.