Hi again folks, been monitoring this for a while, but in response to a question from Fanl,
I have some info to share:
First of all some facts as I know them (I'm pretty sure about this as Ive spent a long time looking at the circuit diagrams and am an electronics engineer, but if anyone disagrees feel free to question this):
- On the 156 the speed signal comes from the 4 wheel sensors and goes directly to the ABS ecu.
- the ABS ecu uses this signal directly to deal with its braking requirements, so there is NO affect on the ABS braking performance caused by cutting any wires coming out of the speedo. The ABS is self contained in this respect.
- the ABS unit then sends a single speed signal to the Speedometer, which uses it to display the speed
- The speedo then sends 3 (buffered) copies of this ABS-derived speed signal out to the following places:
a) the engine ecu. This is the famous wire that we are discussing cutting on this thread. The purpose of this signal is to allow the ecu to eliminate a jerky fuel cutoff when you take your foot of the accelerator and the car is moving at speeds above 20kmh. When it knows the car is going at this speed it switches on a special servo that lets the engine rpm die down more slowly. The problem we are all having is something to do with this system not working properly! The big question is -how, why, what changes to make this happen?
b) the aircon control ecu - this needs a speed signal to compensate for the amount of air entering the car when travellling at speed, I don't know exactly how this works, I imagine its only related if you use the 'auto' setting which I never do...
c) the Selespeed ecu on Selespeed cars... this is used to determine when to automatically change down gears when the car is slowing down.
The reason there are three copies is just to prevent a fault or short cct in one unit from affecting the signal to the others. Although none of these signals performs a safety-critical function.
