GMC Hauler - Ques. @ Line Pressure Mod

[MTTwister]

I'm pretty sure that this mod is to splice between the Hi and Low Pressure Control Solenoid circuits.
However - For the '96 K2500 ( pickup) , the ECM pins are C7 - Red/Black, and C15 - Lt Blue / White, on connector C1 ( the brown one in front - at least they're consistent there).

What amazes me ( well it's interesting at least) is that they would have different ECM pin-outs between the 96 Sub, and the 96 Pickup.

Does this have any beneficial effect on downhill engine braking - or just the shifting itself?
Thanks! Enjoy the Day!

[GMC Hauler]

You made me think about that mod, so I pulled out the book and rechecked. I had my pin numbers off by 1 each, but the colors are correct. What I did was look at the 4L80E section for gas (page 7A-271) instead of diesel (page 7A-274). I have it connected correctly, but when I went to do the write up, I looked at the wrong page.

Good catch. tongue.gif

I didn't see any real effect in engine braking with the second gear TCC mod or the shift kit mod.

[MTTwister]

"I didn't see any real effect in engine braking with the second gear TCC mod or the shift kit mod."

Well Hey - let's get with it! There's got to be a way.

Oh & Thanks tongue.gif

[GMC Hauler]

Oh, there's a way. And it may be simpler than a Torque lock.

[GMC Hauler]

I was working on this idea at one time. One idea had me using two VSSB's, but that won't work.

In order to lock the TCC, you have to provide a wide pulse 30hz signal source to (in contrast to a narrow pulse 30hz signal source that would unlock the TCC). This stops the exhaust of fluid from the TCC and applies the TCC. The problem with this, is that the computer looks at torque converter slip speed when it wants the clutch unlocked. If it does not see a certain amount of slip speed, it will set a code P0742. This is the computer's way of checking the TCC actually unocked: engine RPM must be different than transmission output speed by a certain amount. Code P0742 commands maximum line pressure, which causes shifts to be harsh. It also causes the MIL light to come on. What we need to do is look at the reason it is caused, and try to stop the reason, or try to limit the effects of code P0742.

Let's start at probably the easiest way to perform this operation: Readjust line pressure. What we know is that when pressure control solenoid is at max current, line pressure is low and shifts are soft, and vice versa. The PCM pulses the solenoid at ~30HZ. What makes the difference between high line pressure and low line pressure is how long these pulses are. A long pulse equates to high current and subsequently low line pressure and soft shifts, and of course, vice versa. So what we need to do is extend the length of the pulse (PWM) to increase current and thus, decrease line pressure. The PCM has been gracious to provide us the 30 hz signal to trigger our clock (monostable multivibrator), in this case, is a 555 or equivalent (clock on a microchip available anywhere). The clock pulse width then is controlled by support circuitry of the clock (a resistor and a capacitor) which can be varied on the fly. You would have total manual control of the pressure control solenoid. To provide the wide pulse 30hz signal needed to lock the TCC, you can also use another 555 clock tuned to 30hz and the proper pulse width to cause it to lock. That is also simple.

Don't worry, these "clocks" are very cheap (about a buck), and 2 clocks even come on a chip (the 556 version).

Now, this version would set a "limp in" mode, but according to my service manual, the only adverse affect besides high line presssure (which we've cured) is the MIL illuminated. The limp mode resets after you shutdown the transmission. This mod could be done for under $20.

A variation on this mod is to link the newly adjusted line pressure to throttle position. The more the throttle is opened, the higher line pressure can be made, and vice versa. This would take an input from the APP (which can be done safely). This would add about $10 to the cost.

The last way this can be done is another aproach alltogether. Instead of allowing the DTC P0742 to be set, we will prevent the DTC from being set. Using our service manual, we look at all the conditions that set the DTC. The one that we can manipulate is TCC slip speed. TCC slip speed is the difference between transmission output speed and engine speed. The requirement for setting the DTC is that "The TCC slip spped must be -15 to +20 RPM for at least 5 seconds". Of the two inputs used to calculate TCC slip speed, we cannot adjust engine speed, but we can adjust transmission output speed. The signal out from the input and output speed sensors on the transmission provide a AC signal of varying voltage and frequency. The voltage variation is not so important as is the frequency variation, which transmission control is based from. What could be done is to splice into the output speed sensor, intercept it's AC signal, and provide a slightly reduced frequency signal. This is slightly more complicated. Since the output speed sensor frequency is reduced, the PCM thinks that transmission output speed is slower, and since engine RPM hasn't changed, we no longer meet the requirement for setting the DTC, "The TCC slip spped must be -15 to +20 RPM for at least 5 seconds". This version is more complicated, but once implicated, would provide seamless operation.

This "adjustment" could also be done from the output of the VSS with digital citcuitry much easier.

These are untested ideas as of yet.