Manual control of a slushbox torque converter lockup clutch...not how, but why the failures?

Reading through many posts about TC lockup manual control.....I see a lot of failure stories naming "torque" as the killer and lack of holding ability whether it be a stock or aftermarket TC. As I read those posts, I envision a fully locked clutch under WOT being pushed past its relative static hold and just letting go like a worn out manual clutch gives out on a hill. You have to back off for it to grab again, but you've already done some damage from slip. It feels like people blame the engine torque alone for overpowering a TC lockup clutch, no matter the circumstance. Maybe this is likely on a stock TC, but I am skeptical with a fresh, aftermarket TC.

I'm no expert, but I assume the combination of rpm differences at onset of engagement and/or lack of hydraulic clamping pressure are the likely TC killers with manual lockup.

Do you think the hypothetical list below is accurate or bogus for most TC lockup clutch death stories with manual lockup control?:

1) During WOT, engine is below stall speed of converter, so RPM difference between the engine and trans is large. Manual lock up turned on, but slip is excessive, thus too much heat and wear before lock up finally reaches 1:1. A good aftermarket TC may survive this for longer, but still painful and probably the #1 killer.

2) Clutch locked, transmission stays in same gear during deceleration into turn, rpm drops so low that trans pump can't keep up and hydraulic system loosens lockup, throttle is suddenly nailed at corner exit, clutch can't hold until pressure catches back up. No TC should survive this long.

3) During WOT, 1:1 manual lockup complete in current gear and trans makes a shift. RPM difference between the engine and trans new gear quickly spikes and causes clutch slip. Like a manual trans clutch kick to break the tires loose. A good aftermarket TC may survive this for longer.

But if the three scenarios above are prevented, could this eliminate most of the real manual lockup horror stories? Or is there more to it?

I'm dreaming of a manual lock "mode" switch that has some smarts. Its purpose is for road/autocross to safely hold 1:1 within a gear after stall speed is reached and continues holding within that gear until either RPM is too low for the pump or a gear change is needed (locks after stall and remains locked even if it falls below stall, but still above pump min speed). It uses the tach, vehicle speed and gear position to avoid the three scenarios above.

It will kick off with onset of gear changes. It looks at the delta of tach signal and and trans rpm (vehicle speed vs gear selected) to make sure engagement is acceptable. Maybe you can tune a trans computer to do all this for you, but I don't think this is something you would want turning on via computer control in DD duties. I desire to have direct control of tire speed with engine speed within a particular gear, even if I engine brake below stall, then accelerate (like in a manual trans).

I'm not looking for ideas on how to make this happen, but instead establish the criteria necessary to prevent unforeseen failure as I experiment.

For cases 1 and 3, a good converter with a strong lockup clutch will hold without slipping and should last well under that use. In case 3, if you have a good enough converter clutch and it doesn't slip when the trans shifts, you can end up shearing the trans input shaft or other nasty things when it shifts under heavy throttle (imagine if it were possible to shift a manual at WOT without putting the clutch in or letting off the throttle).

For case 2, you're more likely to stall the engine when coming to a stop or dip the revs so low the trans just drops into neutral and the engine revs recover, then the trans will try to re-engage as the pressure comes back. If you can drive a manual and not stall it when stopping, this one shouldn't be a concern.

I've run manual lockup before. As long as you're not ham-fisted with it and avoid letting the trans upshift under more than light throttle with the TCC locked, you'll be fine. I'd tend to avoid locking it when you've got a lot of converter slip occurring as well.

Thanks, real world experiences are highly welcomed, since my knowledge is equivalent to a 14 year old with access to google.

junked wrote: I'm dreaming of a manual lock "mode" switch that has some smarts.

It's nice to know that I'm not the only one who feels this way...Unfortunately, I really have nothing of value to contribute to your thoughts.

Buick Grand National guys used to wire a switch to manually lock up the TCC at the top end of a 1/4mi run. It was generally good for a tenth or two better time.

If 5-600hp turbo sixes can do it on a wimpy weenie 200-4R, why not something more beeftacular?

Also FWIW: I note with interest that Volvo has torque limits on the more powerful engines with the Aisin 5-speed auto, but only in the bottom two/three gears. They'll happily allow the engine to build max torque in top gear with the TCC engaged. And they don't even "lock up" the TCC, they PWM the solenoid so the clutch slips for better NVH.

My personal opinion? Leave the converter unlocked for performance driving. It's like a slipper clutch on a motorcycle and it allows the engine to come into its powerband even if you're in the wrong gear. It's a feature, not a bug!

The exception of a few aftermarket units there's not a whole heck of a lot of clutch in a torque converter clutch. Just like the OP's proposed smart switch, OEM calibrations don't apply them under high slip conditions and release them under some high load conditions. They also have the ability to close the throttle to reduce the amount of slippage during converter engagement. Of the three scenarios in the OP's post I suspect number 1 is responsible for most aftermarket converter failures.

This is somewhat off topic, but we're talking about converters and lockup, so it's also somewhat on topic.

My 200 hp/zero tq 302/E4OD '96 F-150 4x4 (read: 5600#) feels the need to lock the converter at less than 40 MPH in 3rd. This annoys me for two reasons:

First, it can't pull even the slightest hill at that RPM. The solution I've come up with is to tap the brakes with my left foot, which fools the computer into thinking that we're stopping, so it unlocks the converter. Then it'll drive up the slight grade, usually accelerating. It's like it says, "I'm free!"

Second, whenever the converter is locked (in 3rd or 4th), every time I go from coast to drive (even just to hold speed), it kicks the converter out for a second, then right back in. So, it picks up 400 rpm, then immediately drops back.

I'd really like to know what Ford was thinking when they programmed this stupid thing.

Thanks all for your input.

Knurled, I agree on keeping the converter unlocked during initial acceleration and making use of the torque multiplying feature. But once up to speed (such as 2nd in an autocross) and braking into the first entry, I would love for the converter to lock and let me modulate power to the rear wheels even if I have dipped under the stall speed of the converter. With corner exit, there are options with a manual lockup, first is flip it off and let the car wind up and launch down a big straight (better have sticky meat) to the next turn or leave it locked for the tight turns or slalom a second ahead. I think if I could remain above stall in all corners, there would not be a need to lock.

Bringing a car with high stall to a road race or autocross is not ideal, they are intended for drag racing. But a high stall can make for a fun street ride except when below the stall, then its all rubber band. . This is a general topic, but my own experience is with an LS and a 3600 stall. If I could have several rides, I might build different flavors for each driving experience.

Most TCUs are programmed to lock at partial throttle in 3rd or 4th and drop out if brakes are applied or load is increased past a threshold. Personally, I think (for the enthusiast crowd) the braking part could be replaced in the programming with a an engine RPM minimum to ensure unlock at a full stop or if their is not enough fluid pressure to clamp the lockup. My guess is the braking interrupt is really for safety of most drivers. There you are in steady traffic, uh oh, tap the brakes because taillights ahead, but wait, everyone is accelerating again, hit the gas....if the converter is still locked, the driver will need to give it a lot more to keep up with everyone and it may drop a gear, unlock and wind up just when we are back to tapping brakes for another "taillight wave". BTW, do I get credit for coining taillight wave in traffic or has someone else already heard that one?

My other thought is for the converter to lock whenever it sees both engine speed and input shaft speed (based on speedometer and gear selection) match up. I bet this would infuriate the common driver, but it would make modulating power to the rear wheels in part throttle settings nice. Plus it would always lock at the best time for smooth delivery, just like rev matching. It would still need to drop out if there is a moderate throttle gear change or just heavy throttle in general. To make this livable, the switch most people think of to manually lock would be best morphed into an interrupt to leave the converter loose.

To give an example of everyday driving: You pull from a stop and the car accelerates all the way to 3rd with the converter loose. Then you come up to some curves in the road, so you back out, it locks once the engine speed dips to match the input shaft. You won't be putting your foot into it anyhow, since there are a lot of curves ahead. It feels like a manual in the same gear as your dance back and forth between moderate gas pedal and brake pedal. Oh look, a straight away with no houses and children in sight, but you forgot your interrupt switch and when you punch it, it takes a second, maybe dropping a gear and unlocking, winding up and blow the rear tires off...boo. But wait, you know this road and you didn't forget the switch! You interrupt the lockup just as you are past the apex of the last turn giving you time to bring the engine up to stall. Now you nail it and all the world is right! Maybe I should hold on that third cup of coffee.

On my KJ I could unlock the TC clutch by just a slight push on the gas pedal.

Seems some on here are looking at it as performance thing. Not what it was designed for.

Your detailed description of how you want a lock up converter to work describes almost all of the characteristics I dislike about badly programmed ones. I don't want to know I have a lock up converter. I want it to wait until I'm darn good and ready to be stable before it locks, and I want it right the berkeley off the moment my foot twitches on the gas pedal. I have an "off" switch on my Camaro so the converter doesn't irk me.

If you are trying to use it as a compensation for a slick converter, I don't think it will last very long at all.

One item not mentioned yet, newer stuff doesn't just engage/disengage. The solenoid is controlled through increasing PWM to lock it gently.

This means more wear, less shock, and it doesn't always feel like a shift.

In reply to Streetwiseguy:

Does your dislike to the characteristics described apply to both a stock and aftermarket converter? With a stock converter that is typically above the factory stall speed anytime your foot is touching the gas pedal, I agree, those characteristics would be of little benefit and really more of a nuisance. That gets back to the comment I said about having different flavor cars for each driving experience if I could.

In short, yes, I am trying to compensate for a slick converter. Which I don't view as a bandaid for a slick converter, but finding a path to make the most of it.

In reply to tuna55:

Luckily in my case, the transgo shift kit installed with the higher stall converter has a PWM delete :) I think the PWM engagement is for people who would be happier with a CVT transmission.

In reply to Streetwiseguy:

Have you ever driven a VW DSG or Porsche PDK? If so, do you dislike how they drive?

I think the behavior you want is exactly how a recent Mercedes box shifts when in sport. My mom's R-class (not AMG) will almost immediately lock the TC, then leave it locked until you come to a stop. This coupled with the transmissions ability to skip gears on up/downshifts, and its willingness to shift often removes the vast majority of the sloppiness traditionally associated with automatics.

Edit: And since there are so many gears, instead of unlocking the TC, you just downshift. Instead of a TC unlock, it can downshift from 5 -> 3, for example.

Driven5 wrote: In reply to Streetwiseguy: Have you ever driven a VW DSG or Porsche PDK? If so, do you dislike how they drive?

Nope. I've never met a drivetrain computer yet that doesn't do something in a fashion I don't like.

I get pissed off at the rain sensing wipers in my Volvo because they are never the right speed.

snailmont5oh wrote: Second, whenever the converter is locked (in 3rd or 4th), *every time* I go from coast to drive (even just to hold speed), it kicks the converter out for a second, then right back in. So, it picks up 400 rpm, then immediately drops back.

A lot of setups unlock the TC as soon as you let off the throttle (and then re-lock when you apply throttle). My Jeep used to be this way and it drives me crazy. I was able to re-tune it to stay locked though. I wish I could kill the unlock when hitting the brakes, but it doesn't appear to be possible on this ECU.

I think you got that backwards. When the multiplication of the TC is no longer needed, (Cruising speed) the clutch will engage/lock for improved fuel mileage. In most cases only in top gear. A slight touch of the throttle will disengage the clutch, as I mentioned earlier.

The first thing you have to understand about a converter clutch is how small it is compared to a manual transmission's clutch. The second thing you have to understand is that on both sides of the clutch material (the apply piston and the converter case) are basically stamped sheetmetal and if you crank the hydraulic pressure you can easily run into distorting the pieces. By the time you fix either one of those issues you're pretty deep into machining totally new parts and what in effect happens is you end up with a mostly new converter. Torque converters are already way more complex than a manual transmission's clutch assembly so by the time you build a mostly-redesigned one and machine it all and put it together you're talking way into 4-digit dollar figures.

So basically it's just money. If the stock converter clutch can't handle a lot of torque you have to drop a bunch of money on one that can. About the only thing you can do at reasonable cost is get a converter with a 'billet' front case half that won't balloon/distort so you can run a little higher pressure, and maybe put a different clutch lining material inside it. Back in ~2007 i bought one like that for ~$300 at shop cost when a stock converter was ~$100. Triple the price but still affordable. Look at the prices for aftermarket lockup converters for diesels to get an idea what it costs to get a 'serious' lockup converter built.