Top Speed Limiter - by Jim Conforti
Jim Conforti's explains Top speed limiter. Here's Jim's explanation, reposted from the BMW Digest:
OK.. let's look at OBD-I and OBD-II...
First, in BOTH of them, Vehicle Speed is measured by receiving the "A" signal from the vehicle speedometer. and once a preset limit is reached, speed limiting goes into effect..
How that happens differs..
In OBD-I generally two banks of cylinders are disabled.. first one.. then if the vehicle speed continues to increase, the other..
In OBD-II (specifically Siemens MS41 and MS42) the system cuts ONE cylinder at a time until the speed increase stops.. up to ALL cylinders.. it does so in a pattern that makes the speed (and rev) limiters much smoother on the engine and you.
Note.. that the REV limiter is basically the same function, only that the current RPM is compared with a pre-programmed limit to activate the same FUEL CUT functions.
Jim