mbeach-
I imagine you have the snow up there to try this, I fortunatly haven't got the white stuff down here NE yet.
If you want to monitor what the TOD ecu is doing I think monitoring these wires would be the most important:
1. TOD to Solenoid - On a 2000 it is the light blue wire that exits the TOD ecu from pin 4
If you hook a multimeter to this wire and monitor voltage it should hover around 0-1V most of the time and jump when the TOD dislpay shows power to the front wheels. An ocsiloscope would confirm if this is indeed a PWM signal.
2. Front and Rear vehicle speed sensors- On a 2000 TOD ECU the front is pin 24 (green/white or grey/white), the rear is pin 23 (green/yellow or grey/yellow) and the reference for these sensors is pin 15 (green or grey)
The TOD ecu's logic probably looks at a differential between these two sensors and then decides how much torque split to enact. We could probably gain alot of insight on the function of the ecu by doing the same thing. I assume these two are probably PWM signals as well. If you use an oscilloscope with two channels of imput you can see this PWM differential occur when the rears slip.
If you want to look at the circuit diagrams yourself you can use Tone's maintaintence cd (if you don't have one just e-mail him or post and I'm sure he'll hook you up) The most useful pages are:
Driveline/axle > Driveline control (TOD) > Basic diagnostics flow chart > Checking failed pin > Connector pin assignment
and
Body and acceserories > Wiring diagram > circuit diagram 19 (the second 19)
I agree, at this stage a spreadsheet would be very helpful here, the way I'd set it up is with columns for this data:
Front speed sensor duty cycle
Rear speed sensor duty cycle
TOD-solenoid duty cycle
TOD-solenoid rms Voltage
What the solenoid is doing (0 front, 50/50 etc.)
Alright, so after we get this information together (hopefully it will confirm what the Terracan guys have discovered) it is a matter of making the modifications
I talked to a few of my EE collegues and they all agreed that substituting our own controlled PWM signal to the clutch instead of the ECU's signal would be the best route. This could certainly be done with a 555 like your tech said and it would be fairly inexpensive. Though I personally don't have the indepth know-how to build the actual circuit, but I know people that do, and you sound like you might have the knowlege and tools.
After thinking about the actual mechanics of modifying this system I think we have three basic options:
1. Modify the signals coming into the TOD ecu
This basically tricks the TOD ecu into thinking the car is doing one thing, and we get the desired result from the TOD ecu outputs. So if we modify the front or rear speed sensor signals before the ecu, then the ecu thinks there is slipping and we get lock-up. The trooper guys have kinda done this by cutting the front sensor wires. Then the ecu sends power to the front axle, but also throws a code. This method would produce the correct dash display. Though it scares me because if something goes wrong the TOD ecu would get fried....$$$
2. Modify the action inside the TOD ecu
I think this is the approach you and your tech are talking about. It would be the most elegant because it would all be contained inside the existing black box. If you can score an extra TOD ecu it could be taken apart and investigated, reprogramed almost. However, this approach would be hard to duplicate for the electronically timid among the VX community. Though it seems to be the most technically interesting.
3. Modify the TOD ecu's outputs
This would consist of modifying the signal from the TOD ecu to the clutch like I mentioned before. It is not the most elegant because it wont display correctly on the dash display but it would be the least risky and easiest to duplicate. The risk is minimized because you couldn't hurt the ecu (since the modification is after the ecu, you would need a dummy load though), and the soleniod is probably robust enough to withstand any minor malfunction we casue. The electronic circuits could be contained in a project box that would require cutting only the wire going from the TOD ecu to the solenoid. This makes it easy to install and produce for members on this board (might as well spread the wealth ehhh), you would have to cut one wire, install the knob, hook up power and ground and you'd be set. I think this approach is the one I'll persue.
Well thats my take on the whole deal right now, throw back your opinions on it and we'll keep brainstorming, this shouldn't be that hard. Plus, once the research is done and the modification tested there are plenty of guys on this board that could benefit from the knowlege obtained.