Hey guys,
Just a quick question that is probably newbish, but what actually tells the tranny ECU which gear is selected in an auto (L,2,D,OD)? Is there diff pins that detects a signal from gearbox? is it the actual position of the metal rod peice in the engine bay?
Say the car was in D, is there any way of "tricking" the ecu to think it's in L?

your gearstick is connected to a little lever on the tranny via a push/pull cable.

Lever on tranny is where all the smarts are.

your gearstick is connected to a little lever on the tranny via a push/pull cable.

Lever on tranny is where all the smarts are.

Ok, so this "lever" (the one that runs under the airbox i'm assuming) selects which gear to use, but say in D, you start off in first and when you accelerate it goes through the gears. Is this controlled by the transmission/lever itself? or does it send info to the tranny ecu? is it possible to intercept this "info" and change it to say, the gear stick is in L, not D?

ok, i could be completely wrong - but this is my interpriation of the transmission in a gen1 (which is very similar to a gen2 auto too)

firstly, the TCU is the thing that selects the gears - uses solenoids, etc to select the gears...

the gear stick moves the neutral safety switch - that does two things... tells the TCU what position the gear stick is in, and stops you from starting the car in gear (P and N only)

the gear shifter doesn't actually select the gear in the gearbox - it's basically a huge switch that tells the TCU what gear you want... it does engauge the park gear though

in D, the TCU just knows (it's been programmed to) that it has to get up to top gear for fuel econ. the whole trans is pretty much software driven - and although you tell it you want L or 2 - the TCU is really the thing that is selecting and holding those gears for you... it's all computer controlled (except park)

like on newer hyundais (i use hyundai as an example cos my dad has one) instead of having L, 2, D then an OD button... his gear shifter is L, 2, 3, D... putting his in D allows the car to go into overdrive (unlike ours where you have a button for it)... but the point i'm trying to make - the gear shifter doesn't do much really - it just sends a signal to the TCU to tell it what gear you want - or if it should follow what's been programmed into it for gear changes (eg. in D)

having said that... i reckon you can make the TCU think L is selected while the car is in D...

hope this helps - and i'm also open to corrections... this is my understanding of how gen1 and gen2 TCU controlled transmissions work...

Cheers,
Bill

ok, i could be completely wrong - but this is my interpriation of the transmission in a gen1 (which is very similar to a gen2 auto too)

firstly, the TCU is the thing that selects the gears - uses solenoids, etc to select the gears...

the gear stick moves the neutral safety switch - that does two things... tells the TCU what position the gear stick is in, and stops you from starting the car in gear (P and N only)

the gear shifter doesn't actually select the gear in the gearbox - it's basically a huge switch that tells the TCU what gear you want... it does engauge the park gear though

in D, the TCU just knows (it's been programmed to) that it has to get up to top gear for fuel econ. the whole trans is pretty much software driven - and although you tell it you want L or 2 - the TCU is really the thing that is selecting and holding those gears for you... it's all computer controlled (except park)

like on newer hyundais (i use hyundai as an example cos my dad has one) instead of having L, 2, D then an OD button... his gear shifter is L, 2, 3, D... putting his in D allows the car to go into overdrive (unlike ours where you have a button for it)... but the point i'm trying to make - the gear shifter doesn't do much really - it just sends a signal to the TCU to tell it what gear you want - or if it should follow what's been programmed into it for gear changes (eg. in D)

having said that... i reckon you can make the TCU think L is selected while the car is in D...

hope this helps - and i'm also open to corrections... this is my understanding of how gen1 and gen2 TCU controlled transmissions work...

Cheers,
Bill


Wow thanks for such a detailed asnwer! That's what i wanted to hear too :D. But my next question is, where exactly is the solenoid/switch that sends the signal to the TCU telling it which gear is selected? Does a haynes manual go into detail about this?

...But my next question is, where exactly is the solenoid/switch that sends the signal to the TCU telling it which gear is selected? Does a haynes manual go into detail about this?
All the sensors are inside the transmission.

They are linked to the control electronics via a plugin harness.

The OD on a 1st gen is electronically switched, thus the button.


No, the workshop manuals only have a very thin chapter on auto trannys.
Basically only show you how to check that its working ok - error codes etc.

Auto is a very specialised item, if its busted its a changeover recondition job.
Impossible for a backyarder to do any repairs, so no point wasting paper.

All the sensors are inside the transmission.

They are linked to the control electronics via a plugin harness.

The OD on a 1st gen is electronically switched, thus the button.


So would it be possible to intercept the signal coming from the transmission to the TCU by re-wiring it?

Umm, what exactly are you trying to do??

Yeah, if you've clued up on electronics there's no reason why you can't
intercept the tranny's signals and generate your own to fool the ECU.

The ECU would have sensors to detect what is going on where/when
then use valves to change the hydraulics to actually do something.

If you have the time and equipment, you could always breakout your CRO and
do a signal trace on all the wires in the ECU hookups, then work out what does what.

is it a value that gets sent to one pin on the TCU, or is there a diff pin for each L,2,d, od that just detects a signal? I think i'll get a haynes or gregories and do abit of testing on the wires to the TCU :).

The cable from the gear selector goes to the inhibitor switch located on the transmission. Through this switch the position of the gear selector is tramitted to the TCU and other areas of the car. Six wires are connected back to the TCU to let it know where the gear selector is. The other wires are for power, connection to the reverse lights, Ignition and starter motor. I can add a chart if people are interested in where each wire goes to.

Gear selection is done by the TCU via 2 control solenoids called switch control solenoid "A" and switch control solenoid "B". These solenoids hydraulically control the selection of gears. The TCU can detect if the transmission has selected the correct gear because it has many feedback sensors. There are two pulses generators located in the transmssion. One detects the speed of the kickdown drum and the other measures the speed of the transfer shaft.

The TCU also receives input from the throttle position sensor (TPS), overdrive and power switch, oil temp switch (prevent power mode being used whent he transmissiion is cold), kick down switch, accelerator switch, speedo reed switch (car speed), and Ignition coil (engine speed).

A very complex piece of equipment.