Overriding an Electronic Automatic Transmission

[rfdesigner]

Originally Posted by Tygen1

I set up some switches to control my shift solenoids. If I start to use them, I need to keep using them until I turn off the car because the PCM forces limp mode. Essentially I am cutting off the PCM's comunication and just sending twelve volts to what ever solenoids need to be active for each gear change. What I found is that the pcm's shifting was pretty much the most effcient for my car. Now if I could lock the TCC in second gear then I could probably improve over the stock pcm shifting, however my TCC is not very robust and it's too much of a risk because I'd have to accelerate pretty briskly to take advantage of the locked TCC which could kill it.

I'm beginning to think about designing a whole transmisson control unit based around the PIC.. have it do all my shifts etc. The more I read and research the more I realise I'm simply not going to get this no to throw a code, and the one it will throw (P0700) causes a limp mode where it won't change gear any more, it will clear if you swithch off the car and switch it on again.

What I've realised by reading and experiment is this:

ABS system has wheel sensors in.. this gives the car knowledge about the wheel speed.

N2 & N3 speed sensors in the gear box tell me about the internal speeds of two of the planetary gears, not the speeds either side of the torque converter.

The way the TCM works out if the TCC is on/off/slipping a bit is by conparing the wheel speed, selected gear and the engine speed. For which is makes use of the CAM-BUS. (serial, vehicle wide, communication bus)

In other words to fake the correct signals I need to fake the CAM-BUS.

The biggest problem there is that no manufacturer releases this kind of information. There are some codes they release for fault diagnosis, but not enough detail for someone to design their own system. I'd need to do it by measuring each code... but with a vehicle wide serial bus there will be a lot of codes that are seen but don't have any relevance to the TCM.

Best solution is homebrew TCM.. but then I still need it to listen to at least some of the CAM signals. Mainly because the gear selector talks to the TCU over the CAM-BUS (it's tiptronic). I could also do with reading the throttle and engine speed, so I can then make sure that if I lock the TCC I can unlock if my right foot becomes a little heavy.

Derek

[rfdesigner]

Just had a completely different idea on getting this to work.

1. Lock TCC manually
2. TCM throws the P0700 code: Torque converter slip/general transmission error code, and refuses to shift gear... fine by me for a few miles.

...some time later...

3. Unlock TCC manually
4. pre programmed PIC automatically clears code P0700 immediately.
5. TCM wakes up and decides which gear it should now be in.

Would need testing to make sure the TCM really behaves like this but I like it as a much simpler solution than trying to fake signals.

Can't be that hard to get a PIC to send one ODBII code can it?

Derek

[Phantom]

Search for the ELM I know they are on amazon and can do PCM calls scanning ect, but it needs to be manually controlled, also I'm not sure if they handle CAM-BUS.

[mwebb]

the alex pepper obd2 scan tool
OBD-2 Vehicle Explorer Scan Tool Browser

does communicate to the CAN bus
=================================
and
many of the newer cars are using the DSG transmissions which are more like standard transmissions , there is no torque converter , there is two clutches which are controlled by the TCM ...

it may be easier to trade in the older cars with hydraulic auto transmissions to replace them with DSG equipped cars

[texanidiot25]

Instead of manually locking, have you looked into trying to reprogram the computer to lock at a lower point?

It would be interesting to try this on an older late-80s early-90s car, with simpler transmissions and electronic torque converter. I'll use the 200R4/700R4 as my example since I've been in the process of building one.

Converter locks up when it gets a signal from the transmission that it's in 4th, and that the motor vacuum is with in a certain range (engine load). It's a simple circuit, and is easily manipulated. All it takes is a 12v source to switch the converter manually, and it's still a very simple setup with a 3 position switch (Locked, Unlocked, Auto) so you can either manually locket, keep it unlocked, or let it operate like normal.

Give a call to summit, and get a manual valvebody kit, and you have full control of the transmission's shift points. Besides that, you can simply play with the governer weights to lower shift points if you wanted to do something quick and dirty.

This way you have a trans that you can either lower the shift points, or take control entirely to shift it most efficiently, and have the converter under your control too. Hell, if you really wanted to mess with it, wire the converter to a foot switch and treat it like a clutch..

And, all of these things can be done with the trans still in the car, and very easily under a day.

[rfdesigner]

Well this project has gone off the boil

Simply put:

The Transmission control module is hooked into the CAM-Bus

Everything on the cam-bus does a roll call at start up..

To reset the TCM I need to cycle the power (which means it's expecting everything else to also power cycle)

If I make a PIC controller to replace the TCM it needs to read and undertand CAM-Bus signals, particularly the throttle setting and road speed... that isn't something for which there is code already written that I can download.

However I look at this it's just more trouble than it's worth. It might save me £1000 in fuel over three years but it's going to cost me so much time and effort that I virtually need to generate a complete product to make it happen. The better way is to work a bit of overtime or make a product that will sell, and a W210 torque converter override module, isn't going to sell that well.

Best solution:

By a manual car next time... the new Merc E250CDi is pretty awsome, same power as mine but consumes 36% less fuel, just have to wait for the prices to drop a bit, no point in spending £25k to save £1k/annum

Derek