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Transmission Troubles

4K views 15 replies 5 participants last post by  Oveur Engineer 
#1 ·
We have any auto transmission experts in the house?

I've got a 2002 V-6 AWD (U140F trans) Highlander with over 200K miles that's making an intermittent unhappy sound every now and again when starting from a dead stop. Once or twice per week? Hard to describe the sound.

How's this... Sounds similar to the noise of ABS kicking in, but this is with the foot off the brake pedal and over to the gas pedal.

99% of the time it's working perfect. Point is that there's no permanent damage (yet), but it's one of those sounds that makes me think that it'll end up with a slick of ATF soaked transmission parts out the back if I don't figure it out before the problem becomes clear.

I've got a bunch of manual transmission experience and it almost sounds like it's stuck in two gears at once until one of them lets go? Sounds like it's fighting itself inside? Is that even possible with an automatic?

Fluid level is fine and looks and smells normal. Doesn't seem to be temperature related. Doesn't seem to be steering wheel position dependent.

No codes stored, or pending and I put a professional Snap-On diagnostic reader on it and am able to review the transmission info. I didn't see anything there that caught my attention, but I don't really know what I'm looking at.

How sophisticated are the transmission diagnostics?
 
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#2 ·
Can you record the sound and post? Hard to guess at causes. Other than changing the fluid or adding something like Lubeguard, there aren't any other simple things to do. A dealer or tranny shop can troubleshoot more specifically, *if* they have the desire.
 
#3 ·
Yeah, I know I'm not giving a lot to go on here, and having been on the receiving end of nebulous unhelpful "it's making this sound" kind of description, I know how little that adds to the diagnosis. Unfortunately though, I don't think I can record the sound. It happens very infrequently and I can't figure out how to reproduce it at will. I've only ever heard it twice, once when I was driving, and once as a passenger.

I've got a lot of car experience behind me (just not with auto transmissions) and I like to believe I can usually tell the difference between "serious" sounds and "not so serious sounds that won't cause a lot of additional work if left to fester". This one fits into the first category and sounds like one that if I wait until the problem becomes clear as to what is wrong, it'll be too late and the transmission will have grenaded into pieces.

So here I am with all this experience and I feel like the guy who has never worked on a car in his life trying to describe the problem to the mechanic! And with the intermittent nature of the issue, a transmission guy could drive it around all day and it could perform flawlessly. "Seems fine. Didn't do it for me."

That's why I'm hoping that there's someone here who knows the next level of what specifically goes on inside the tranny that maybe could talk through some theoretical issues. For instance...

A) From my investigation, it appears that the transmission is completely electronically controlled. There is an electronic controller that tells the transmission what gear to be in at any given time through the use of solenoid valves mounted down inside the transmissions pan. Is that the same controller that runs the engine, or does the main ECM employ a sub-module that actually does the transmission control?

B) I've seen that there are error codes related to transmission operation such as solenoid valve malfunction. How does that diagnostic work? Do they simply ensure that there is a reasonable amount of current that flows to the valve when energized, or do they actually feedback the current position of the valve in some way?

C) Other than the work involved, would there be any problem with me dropping the pan and pulling out all the solenoids and cleaning them? I've had the pan off for a new filter many years ago, so I'm somewhat familiar with that part. But other than that, I've never done anything to the trans other than put it in "D".
 
#5 ·
LOL... OE, I read this on my phone and didn't see it was your post. Gabe my nube response. Will read your longer post when I get home.
 
#7 ·
Thanks guys.

I did a filter and fluid change around 150K miles just because I thought it was time. I didn't change it because of any problem, I was just doing some preventative maintenance. That was only about 50K miles ago.

I would also think (hope) that if it were stuck between gears it would set a code, but the problem is very intermittent and when it does happen, it only lasts for a second or less. Might not be occurring frequently enough or for long enough duration to set a code. I don't even know if that sort of failure is even possible on an auto trans.

AV - Looking forward to additional input when you get a chance.
 
#8 ·
OE, I'm no slush box expert but have done lots of reading, studied fluid flow diagrams, lurked on trans tech blogs and have experienced my share of bummed transmissions. I've never attempted to tear one apart; seems like a lot of work to even get a FWD/AWD transaxle off the car, much less all the special tools and skill required to properly rebuild one.

However....after reading your posts, I don't think it's a binding issue. The ECU monitors the input and output shaft very closely for rotational speed and rate of change. It also monitors system pressures and fluid temp. It can derive a great deal of information about clutch/band condition, fluid properties, pump condition...

It uses this info to orchestrate clutches/bands during shifts. The ECU also adjusts throttle (not 1st Gen HL), fuel, VVTi and ignition timing to modulate (usually reduce) torque from the engine during the shift. This allows the shift to occur as rapidly as possible without engine flare (clutches/bands with insufficient release/application overlap) or RPM sag (binding caused by too much overlap). You don't mention any odd tach movements or engine flare/bogging.

Your description sounds like clutch/band chatter, where it slips/catches rapidly. This can be caused by low application pressure, thinning clutch/band friction material, contaminated fluid (usually water), or clutch plates that have varnish build up. Or a combination of all, in older trannies,

I've had two Honda's chatter. Worse in cold weather. Didn't set codes on either. For both, a rebuild (soft parts) followed within weeks of them doing that, as I couldn't stand it. The transmissions may have lasted another 50k before 1st totally failed, for all I know.

I've also read where a sticky pressure regulation solenoid (really a linear motor), or a leaky application solenoid caused that chatter. Usually first gear chatters, where torque is highest from dead stop. Next most common is 1st to 2nd chatter. Next common is torque converter lockup chatter.

Since transmission work seems like long tedious activity and taking apart a valve body is fraught with horror stories of lost springs, misinstalled check balls and incorrect assembly torques, I've shied away from tinkering with them.

As I posted in another thread, my HL had this 1-2 shift shudder in hot weather, which was completely solved by Lubeguard. To this day, I have no surety whether it was a marginal solenoid/valve or a clutch pack that was a bit varnished. That was , well, 80k or more miles ago.

If it were my transmission, I'd pop for $10 worth of Lubeguard Red and see what happens.
 
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#9 · (Edited)
AV, Yeah if it comes to the point where the solution requires removal of the trans from the car and complete disassembly, I'm not going to do that myself. I'm "capable", but there's no way I'm going to do something like that on the family truckster. In the cold. Without a lift. Without something else appropriate for SWMBO to drive.

I'm just hoping that this thing will make it to the Spring and I may pull the pan off and have a look around. Here's what I'm expecting (and hoping) to find... Nothing. Nothing out of the ordinary anyway.

I'm not expecting any chunks. I'm not expecting any significant particle accumulation in the bottom of the pan or in the filter. I'm thinking it'll look the same as it did the last time I was in there. Reason being is that WHEN IT WORKS, IT WORKS PERFECT. No slip, no shudder, no excessive flare, no bogging, no weird tach operation, no nothing. It's not like it slips a little on some dead stop starts and more on others. It works perfect... Until it doesn't.

Because of that, my engineering Spidey sense tells me that it's a controls issue, not a wear issue. I mean, after over 200K soccer mom miles, anything on the car is allowed to have issues. But this one doesn't seem like a "I'm tired and just want to go to bed" thing. This one seems like a "Oh wow, my knee just locked... That's not supposed to happen!"

So you talked about lurking on trans tech blogs... Have you got a recommendation for somewhere I may be able to quietly learn enough to be dangerous?
 
#10 ·
And about the diagnostics, I see (in the U140F) there are five shift solenoids in the and a temperature sensor. But I don't see a pressure sensor anywhere.

When I had the good scanner hooked up to the OBDII there were a couple tests that I could run like manually commanding the solenoid valves to change state, but I don't remember a pressure reading provided back to the ECU.

So about my first basic question... Do you know for sure if the same ECU that controls the fuel injection system also responsible for controlling the transmission? Or is there a sub-module that controls the tranny?

I've heard that there are diagnostics derived by comparing input and output shaft speeds and it'll flag an error if there is an inappropriate relationship between the two, but I would be very surprised if the short time duration of this problem would be enough to set one of those flags.

Here's another thought... If I'm slipping a clutch on first gear, Could I test for that on a uphill dead start where I hit the throttle hard?
 
#12 ·
OE, once again you are forcing me to look deeper into my '02 HL innards--it's been so reliable, that other than curiosity, I've never had to really study the manuals. You are correct; no pressure sensors for the U140F. Confused that with my Honda's which use several, and which also drift out of spec as the miles gather. Maybe a reason Toyota doesn't use them.

The U140F shows to share the same engine control module with the engine. It appears to derive everything it needs to know from fluid temp and shaft speeds, plus the use of modulated solenoids vs on/off solenoids, to permit the ECU to compensate for worn parts and simply command more or less actuation until it gets the target transfer behavior during shifts.

Also makes sense that brief rapid clutch chatter might go either unnoticed or ignored until it created a large enough discontinuity in shaft speed/acceleration.

Also seems reasonable that one of the solenoids or passive accumulators in the valve body intermittently fails to respond properly. I.e. a stop, the ECU wouldn't know the application pressure for the clutch was inadequate, until you stabbed the throttle, it begins to slip and the output shaft sensor shows erratic acceleration.

Possibly an intermittent electrical connection where the harness enters the transaxle? I.e. lower than normal application current to one of the solenoids?

The base condition of clutch packs and application pressure can be tested by doing a brake stand. Full mash of the brakes and then full mash of throttle for 5 sec. RPM's will rise until it reaches the torque converter "stall" speed. The service manual should state what RPM that is, although I'll darned if I can find it. The stall speed provides a cursory evaluation of torque converter, fluid pump and 1st gear clutch/band condition.

If I get a chance this weekend, I'll do a brake stand on mine and report. I have 226k mi on the clock.

The service manual *should* also state the nominal resistance values of each solenoid.

As for reading materials, Toyota has excellent technical overviews of their transmissions. So did Honda. A few of the sites I lurk on:

http://www.gearsmagazine.com/magazine/currentissue.aspx

http://www.brakeandfrontend.com/

http://www.sandyblogs.com/techlink/pdf-newsletter-archive.html (you can learn a great deal from TSB's from any manufacturer)

http://www.searchautoparts.com/motorage/article-list/powertrain-pro

http://motor.com/category.asp?category=magazine

Youtube videos...like:

https://www.youtube.com/watch?v=jBHJZ_x0neI
 
#13 ·
As for Lubeguard, my engee-neer instincts whisper that an additive which makes base fluid a little slipperier might help with somewhat worn solenoid and accumulator bores/armatures. No harm I'd think--my '02 seemed to benefit-- unless the fluid pump was at the ragged edge of the service pressure range and couldn't keep a worn clutch from slipping. My U140F seems very happy with Lubeguard in it. My son's Ranger 5-speed slusher also benefits from it (it had very delayed engagement).

One caution in logic; the ECU's seem programmed to do whatever they need to perform perfect shifts. My Honda's went from perfect shifting to a once a week chatter that I didn't immediately recognize, to violent chatter by week two. It was like the ECU had worked furiously to compensate for wearing clutches, then fell of the cliff, reaching the limits of its control loop.
 
#14 ·
AV, Thanks for the additional info and links. Up until this point, I've been in the same boat as you in that the transmission has just worked and I haven't had to even think about it. In fact, I was hoping the live out the rest of my years and never need to know anything about what goes on inside one of them fancy boxes...

I didn't write anything down, but I think a stall speed test was listed as one of the OBDII diagnostics I could initiate with the Snap-On scanner. I didn't test anything when I had it hooked up, I was just poking around looking for something that didn't look right.

I'd be interested in knowing the stall speed of yours, and I'll do the same on mine. Let's just hope that neither of us bust anything on our old gals!!

I'm figuring (optimistically?) that if there was an electrical issue the ECU would (should) pick that up. I know there are diagnostics for the solenoid valves, but I'm assuming they're rudimentary current level tests. They can tell open, short, or out of range resistance by monitoring the current. But of course, I'm only guessing.

Clearly I got some reading to do...

JMSinMD, Thanks for the help as well. If I get to the point where I need a shop, it's nice to have some recommendations. Here's to hoping that I don't reach that point!
 
#15 ·
My "stall" test produced ~2500 RPM. Engine fully to temp, in Drive, full brakes, mashed accelerator.

Oh, and no unusual noises. Nothing broke.
 
#16 ·
I have an update. I've come up with a way to reliably reproduce the issue.

Start from a dead stop. Slowly accelerate through first gear into second gear and then come back to a dead stop (before it shifts to third).

The next time I start moving again, it makes the noise in question. The noise starts just after the vehicle starts moving, and disappears after the first second or so. After that the noise goes away and everything goes back to sounding OK.

But here's the trick... It only occurs when you come to a dead stop from SECOND gear only. Doesn't do it if you have made it to third or forth before you come back to a stop. Only happens when you shift from first into second and THEN stop dead.

It's definitely something fighting against itself inside the tranny. Two clutches or band brakes are engaged when only one should be. And it's got something that occurs as a result of the sequence of "stop-1-2-stop".

I'm sure that's enough info for someone who really really knows how these transmissions to work to unquestionably say. "I know exactly what that is. It's the ____ sticking or not actuating correctly." So we got anyone here that knows the system that well?
 
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