Brake Switch / Unlock Torq Conv???

[Thomas C Potter]

The genuine FORD Manual says the brake pedal switch both activates the brake lights and unlocks the TCC, say while crusing along in 3rd or 4th gear. Mine does NOT unlock the TCC. What does yours do?? I was hoping this would be an easy way to disable the TCC until I want it to lock.

On a similar note, we have a 'trans fluid thermal sensor' that might be an easy tap. Does anyone know the resistive range of this sensor, is it high or low when cold, and where is it located?? I have not looked yet.

TP 03 Blue 300B

[EbonyMarauder03]

I don't know about the brake light switch unlocking the torque convertor. I do know it is used to unlock the shifter. The Trans Shift Solenoid is what keeps you from going from Park to any gear without your foot on the brake.

[TripleTransAm]

So you're saying your TCC *never* unlocks with brake application? I'm assuming you mean unlocking with a light tap on the brake...

Here's a test: the same switch that actuates the brake lights also feeds the PCM to determine TCC unlocking and also to shut off the cruise control. Set your cruise to 65 mph or so, and while going up a hill (so you know the accelerator is being applied), slowly apply pressure on the brake pedal until you feel the car jolt from the cruise control getting cut off.

This is the point at which your brake lights will light up as well, and should be the point at which the PCM orders the TCC to unlock.

With the brake switch properly adjusted, the slightest of pressure on the brake pedal will register. If you have to move the pedal any sort of distance, your brake switch is faulty and/or refuses to return to a centered position (tough to describe in text).

When I was having my switch woes on my way to Hershey, I would actually get to the point of having the brakes attempt to slow the car down, while the accelerator tried in vain to keep the speed up. Once the switch did register, the car would decelerate awful quick, as I was already fairly hard on the brakes at that point. This is the extreme scenario... I had reported flaky brake light operation for months and months before stumbling on the solution in the parking lot at the Hershey Hotel.

While you're at it, have a close look at your brake light operation.

I said it before and I'll say it again: Ford electronic components and schematics leave me scratching my head wondering "WHY in the world did they design it this way?". Sorry, I love this car, but the electrical department needs a few months off to rethink their design philosophies...


Another thing to keep in mind is that some transmission misbehaviour was also eventually blamed on a harness problem.

[JohnE]

The Torque Converter always unlocks for the brake input. It also unlocks when you coast. under normal driving, it slowly locks up. With the stock tranny programming it is never 100% locked. The TCC is modulated to generate a very small slip. Good aftermarket programming changes this to 0 slip during lockup.

If you really want to prevent the tc from locking, get a custom program from someone like Dennis.


The tranny temp votage could be tapped into, but you're better off installed a separate sensor. An OBDII scanner can read it directly from the cars computer.


Here's some logged data. The Torque Converter lock Control command is TCC. (Dowload the file, change the extension to .csv, then open with excel)

[TripleTransAm]

A few weeks ago, I was shocked to see that my brake pedal application would also NOT disengage the TCC! In other PCM-controlled automatics I've driven (mostly G.M. I have to admit), I could keep throttle pressure to maintain cruising speed and tap the brakes to see the TCC unlock (RPMs would surge slightly).

Not in my MM.

So I took the opportunity to snoop things out with my newly installed Autotap. Looks like the MM's stock PCM calibrations unlock the TCC completely the moment one lifts off the throttle completely, but not with the brake pedal (I have to investigate more scenarios before saying this with 100% certainty).

Here's the plot of a regular acceleration away from a light, going 1-2-3 before backing off (slower traffic) then getting back on the throttle at cruising pressure:

http://www.tripletransam.com/mm/tech/chambly1.jpg

I included some labels to show what was going on.

A) I've accelerated to the normal relaxed 1-2 shift point. Note that the PCM order to the transmission shows up a bit before the transmission actually upshifts and brings about an RPM drop. This is due to the transmission going through the motions of actuating changes within itself to bring on the next ratio. Note the RPM doesn't drop that much (I'm still well below the stall speed of the converter, so the upshift doesn't affect my RPM much).

B) Here's a shocker. The PCM begins to flicker the TCC clutch on and off at a duty cycle of about 47-48% (meaning it's locked up 47-48% of the time, resulting in lots of slippage). I always thought this was an actual upshift with the TCC completely unlocked (since it was kind of mushy) but in reality the mushiness is the half-a$$ed TCC application! So the RPMs drop somewhat as the TCC loads down the engine somewhat.

C) Here we have a full blown 2-3 upshift. Instead of disengaging the TCC completely to avoid a super-harsh BANG upshift, the PCM simply slips the TCC even MORE during the upshift. Because the TCC is still partially applied during this upshift, this is why the 2-3 upshift is SO FRICKING NOTICEABLE. Again, note the delay in the RPM drop, due to the latency in the transmission's reaction time to the upshift command(s).

D) Thanks to our wonderful stock calibrations, the moment the tranny is firmly in 3rd gear, WHAMMO! We get full TCC lockup (a quick climb to 100% duty cycle of lockup). The period of time between C and D is that wonderful bog that occurs at around 25 mph in gentle acceleration. Essentially a quick combination of a 2-3 upshift with TCC partially locked and a full bore TCC lockup.

E) Here I backed off the throttle due to slower traffic... no brakes applied, but the TCC completely unlocks and the RPMs drop to whatever speed the PCM commands as minimum speed while the car is moving (perhaps to feed the power steering adequately?) and possibly some reverse torque reaction from the drivetrain of the moving car.

F) Just before this point I've gotten back on the gas (see the little spike in the blue RPM curve). The PCM quickly assumes its fuel economy role and locks up the TCC pretty quickly.

G) The TCC is fully 100% locked up here, and I'm accelerating gradually as the slower traffic has turned off to a side road.

H) I'm approaching a stop sign, so here I back off the gas completely. Again, the PCM completely unlocks the TCC (back to 0% duty cycle). No brake application at any time so far, so there doesn't seem to be a link between brake application and TCC unlocking, on my car.

I) Speed related downshift from 3-2 is ordered, as I'm slowing down (I probably have the brakes on by this point).

A) Another speed related downshift from 2-1 as I'm coming to a complete stop.

[JohnE]

H) I'm approaching a stop sign, so here I back off the gas completely. Again, the PCM completely unlocks the TCC (back to 0% duty cycle). No brake application at any time so far, so there doesn't seem to be a link between brake application and TCC unlocking, on my car.


The torque converter is unlocking when you decelerate. If it didn't you'd hear a clunk every time. The clunk would be the lashes in the gears, when they switch mating surfaces.

If you had your car reprogrammed the TCC could stay locked to slow down, until you reach a certain slow speed per gear. This would give great engine braking. Most people don't like the clunk though.


Your statement makes it sound like the you don't unlock upon braking. It's just that there's more thought into the programming there you're used to on your car. Why don't you try logging this: Drive down a highway at a constant speed. You should see the TCC at 100% duty cycle. Keep you go foot consant. With your other foot, tap on the brake pedal. Push enough to turn on the brake light, but not enough to slow down. I think you'll find the TCC go to 0%. This should be obvious by the tach, because rpms will rise to maintain the same speed on an unlocked converter. I would be surprised if your tc doesn't unlock for brake input.

Another item that unlocks the torque converter is traction control. This helps lessen the power to the ground and prevent slams from the brake modulation from effecting the drivetrain.


John

[TripleTransAm]

The torque converter is unlocking when you decelerate. If it didn't you'd hear a clunk every time. The clunk would be the lashes in the gears, when they switch mating surfaces.


It's not that bad, John. My GTA has a full 0-100% lockup, no partial thing like on the MM. No clunk. A definite positive "shift" when it engages, as if there was an extra gear in the tranny, but no audible noise whatsoever in the cabin or when listening with the window open. And no shudder as the drivetrain lash is soaked up. Now, I have to admit I'm a big fan of General Motors transmissions and drivetrains (except for the metric THM200), so maybe Ford has a bit of work to do in that respect, but I don't recall hearing anyone on here with harsh ride characteristics when their chip changed the TCC unlocking to speed-related.



Drive down a highway at a constant speed. You should see the TCC at 100% duty cycle. Keep you go foot consant. With your other foot, tap on the brake pedal. Push enough to turn on the brake light, but not enough to slow down. I think you'll find the TCC go to 0%. This should be obvious by the tach, because rpms will rise to maintain the same speed on an unlocked converter. I would be surprised if your tc doesn't unlock for brake input.


That's exactly what I'm saying, John. While I didn't have the Autotap hooked up when I tried this, I certainly didn't see any RPM increase. Was it at 100% when I tried it? I don't know, but I will find out... perhaps the unlocking is a partial unlocking, so that if it's already at 50% it doesn't bother dropping to 0% at that speed.
So far, the only scenario I've come across that unlocks the TCC is lifting off the gas completely, but I will try some more captures when my laptop battery is charged up.

Gotta keep in mind there's 3 years' worth of changes that could happen between both our calibrations...

[JohnE]

Mine's no longer stock. But I'm fairly certain that the tc should unlock for the brake input. That's an extra safety measure. You'll know when you log next time.

[TripleTransAm]

I'm curious to see this myself.

But what prompted me to follow up on this thread was when I tried things out the day after this thread popped up. The scenario: going slightly uphill, the car had gone through it's annoying 2-3 upshift / TCC lockup combo which bogged the motor. Since this was a residential sector, I only needed a small amount of throttle pressure to maintain speed or perhaps even accelerate slightly up to the speed limit.

Based on my logging yesterday, I'd be willing to bet the TCC was at 100% here (since I only saw 45-70% duty cycles in 2nd gear, with 3rd showing a quick climb to 100%).

On any other car I've driven with a TCC, tapping the brakes *should* have unlocked the TCC showing a sudden rise of RPM in such a scenario (going slightly uphill, still accelerating gently, especially with a converter as loose as the MM's). But no dice... this is what really threw me off.

I'll definitely be logging this sometime this weekend... but the tachometer shouldn't lie.

[TripleTransAm]

Tried it again without logging, just now. I was applying only enough throttle to maintain speed, in 3rd at about 30-35 mph. Based on yesterday's logs, this should have put the TCC duty cycle at 100%, verified by the tachometer reading about 1200 RPM.

I applied the brakes gently at first... no change in RPM or engine note (that hum that happens when the engine is smothered by that 2-3/TCC-lockup combo). I applied more braking pressure... no increase in RPM, even to the point where the car began to slow down and I saw the corresponding RPM decrease as well. I managed to bring it down to around 1100 RPM or less, all the while maintaining the same throttle pressure.

If the brake pedal was to unlock the TCC, I would have seen a jump to about 2000 RPM at that point (conveniently verified when I left home this morning and noted engine RPM under a similar load at that speed, as the engine was too cold for TCC operation).

It's not a matter of my brake switch being off, as most of you will remember how psychotic I am over my brake light performance since dealing with a faulty switch until just after the Hershey 2003 meet.

So there we have it...