skipc
NAXJA Forum User
- Location
- Georgia & points west and south
This is for all the AW-4 trans guys... and a few Qs for the gurus... 
I've done some detailed looking at the TCU - it seems like the kind of thing I've wished for since I was a kid! A totally programmable trans!!! Anyway, this one seems to have a problem keeping the TC locked. I'll explain.
Background: The TCU gets no input from the Renix computer, only ground speed from the trans sensor (pulses) and throttle position from the second TPS, and from the brake switch. It then decides, based on the gearshift switch, which solenoids to turn on or off, using 2 binary positions for 4 possible combinations, plus 1 for the TC lockup. According to the docs, it should lock in 3rd or fourth (poss in second too, but I've not seen this).
The TC will unlock when the brake is pressed, or the throttle closed. It will specifically lock under accel too - opposite the Ford way. However, recently, mine will lock-unlock-lock cycle at highways speeds on flat ground. Especially when it's just cruising along easily (think tail wind).
Well, the TPS gets a stable 4.65v from the TCU as a reference voltage, and it is a variable resistor with the +4.65v and ground across it, and the wiper is the input signal to the TCU. As the throttle is opened, the voltage drops. At idle, it's about a 4v level, at wide open, about 1.5v. I believe there's a series resistor inside the TPS to prevent full + or gnd at the input.
When there is nearly 4v at the input, the TCU uses it as a reason to unlock the TC (throttle presumably closed). I put a DVM on it at several points. Perhaps someone can see something in my findings below. I also have some Q's at the end...
The idle point seems to vary from 3.96v to 4.15v. Things work well when it's closer to the 3.96 level. It only seems to affect TC locking as you will see - shifting is fine. I can't find any correlation in it's variance with any other thing (temperature, speed, length of time running, etc). When the idle (closed) signal is reading at 4.15, the TC unlock happens at 3.75v. When it is in the mood to read 3.96v-4.02v at idle, the TC will unlock between 3.85 and 3.95v. I don't have precise nums at the 3.85 area because it's tough to do all this and drive too...
Anyway, normal operation at 60 mph opens the throttle to give a reading of about 3.7v +- on flat land. In normal decel and accel, it should stay locked. If closed is 4v, then 3.95v is real close, and all is well. However, when the idle voltage seems to creep up at idle, it'll then unlock at 3.75v - the unlock point creeps down! As you can see, that's close to 3.7v, and using cruise will move the throttle just enough to cycle it over and over. Accel a bit harder and it'll stay locked, but I end up at 80mph... I'll let off the pedal once in a while to see what the closed value reads and if it's 4.15, I can count on the lowered unlock point. The reference voltage is a steady 4.65 all the time. In all of this, it should stay locked, only coming out if I let off almost completely and letting the voltage move to close to 4v.
Bear in mind, it's the TCU causing this, so it's not an internal trans issue... The unlock line is driving the actions.
Any ideas why/how this is occuring? Even if the TPS was loose (it isn't), it shouldn't cause the TCU to change the voltage at which it unlocks.
After looking at sites on making this into a manual type shift electronically (all quite crude too), I have some Q's (some unrelated to the problem):
- Is there a schematic or reverse eng. of the TCU or some description of the input and output circuits? (op amp inputs, open collector outputs, diode protection, etc) so I can figure out what options I have?
- What might cause this interaction between the set point for unlock and the idle level? It's inversely related - as the idle point moves up, the TCU moves the unlock point down. One would expect they would move up or down together at least...
- What happens if the TCU turns on the lines for first gear at high speeds? Do the hydraulics protect it and only let it downshift when appropriate?
- Has anyone tried changing the voltages from the sensor, like adding some resistor network? It's only a resistive divider.
- Has anyone tried running with a fixed value, say 3.6v? It would change the shift points a bit, but a moderate value might work for most accelerations, and bypass this whole issue. I don't push it anyway with 250k miles on it It would make the TCU think light accell all the time, but actual road speed and brakes would override when used.
- Anyone see a problem with adding some external logic (another patch approach) to lock it once in 4th for a few seconds, with unlock from the brake or from leaving 4th? I'd build a simple timer/driver and a logic gate for that.
- Anyone gotten into this further than me so far?
Well, thanks in advance,
Skip
I've done some detailed looking at the TCU - it seems like the kind of thing I've wished for since I was a kid! A totally programmable trans!!! Anyway, this one seems to have a problem keeping the TC locked. I'll explain.
Background: The TCU gets no input from the Renix computer, only ground speed from the trans sensor (pulses) and throttle position from the second TPS, and from the brake switch. It then decides, based on the gearshift switch, which solenoids to turn on or off, using 2 binary positions for 4 possible combinations, plus 1 for the TC lockup. According to the docs, it should lock in 3rd or fourth (poss in second too, but I've not seen this).
The TC will unlock when the brake is pressed, or the throttle closed. It will specifically lock under accel too - opposite the Ford way. However, recently, mine will lock-unlock-lock cycle at highways speeds on flat ground. Especially when it's just cruising along easily (think tail wind).
Well, the TPS gets a stable 4.65v from the TCU as a reference voltage, and it is a variable resistor with the +4.65v and ground across it, and the wiper is the input signal to the TCU. As the throttle is opened, the voltage drops. At idle, it's about a 4v level, at wide open, about 1.5v. I believe there's a series resistor inside the TPS to prevent full + or gnd at the input.
When there is nearly 4v at the input, the TCU uses it as a reason to unlock the TC (throttle presumably closed). I put a DVM on it at several points. Perhaps someone can see something in my findings below. I also have some Q's at the end...
The idle point seems to vary from 3.96v to 4.15v. Things work well when it's closer to the 3.96 level. It only seems to affect TC locking as you will see - shifting is fine. I can't find any correlation in it's variance with any other thing (temperature, speed, length of time running, etc). When the idle (closed) signal is reading at 4.15, the TC unlock happens at 3.75v. When it is in the mood to read 3.96v-4.02v at idle, the TC will unlock between 3.85 and 3.95v. I don't have precise nums at the 3.85 area because it's tough to do all this and drive too...
Anyway, normal operation at 60 mph opens the throttle to give a reading of about 3.7v +- on flat land. In normal decel and accel, it should stay locked. If closed is 4v, then 3.95v is real close, and all is well. However, when the idle voltage seems to creep up at idle, it'll then unlock at 3.75v - the unlock point creeps down! As you can see, that's close to 3.7v, and using cruise will move the throttle just enough to cycle it over and over. Accel a bit harder and it'll stay locked, but I end up at 80mph... I'll let off the pedal once in a while to see what the closed value reads and if it's 4.15, I can count on the lowered unlock point. The reference voltage is a steady 4.65 all the time. In all of this, it should stay locked, only coming out if I let off almost completely and letting the voltage move to close to 4v.
Bear in mind, it's the TCU causing this, so it's not an internal trans issue... The unlock line is driving the actions.
Any ideas why/how this is occuring? Even if the TPS was loose (it isn't), it shouldn't cause the TCU to change the voltage at which it unlocks.
After looking at sites on making this into a manual type shift electronically (all quite crude too), I have some Q's (some unrelated to the problem):
- Is there a schematic or reverse eng. of the TCU or some description of the input and output circuits? (op amp inputs, open collector outputs, diode protection, etc) so I can figure out what options I have?
- What might cause this interaction between the set point for unlock and the idle level? It's inversely related - as the idle point moves up, the TCU moves the unlock point down. One would expect they would move up or down together at least...
- What happens if the TCU turns on the lines for first gear at high speeds? Do the hydraulics protect it and only let it downshift when appropriate?
- Has anyone tried changing the voltages from the sensor, like adding some resistor network? It's only a resistive divider.
- Has anyone tried running with a fixed value, say 3.6v? It would change the shift points a bit, but a moderate value might work for most accelerations, and bypass this whole issue. I don't push it anyway with 250k miles on it
It would make the TCU think light accell all the time, but actual road speed and brakes would override when used.- Anyone see a problem with adding some external logic (another patch approach) to lock it once in 4th for a few seconds, with unlock from the brake or from leaving 4th? I'd build a simple timer/driver and a logic gate for that.
- Anyone gotten into this further than me so far?
Well, thanks in advance,
Skip
