Rev Limiters (was RE: Knock Sensor Q)

[jkarlsso@metabolex.com (Jonas K.)]

Hi all, 

I perused the wiring diagrams in the A2 bentley last night, in search of an
answer to this question.

Here's what I found:

All the A2 cars have the following pin designation on their FPR's:

30: Always 12v
15: switched 12v
87: output to power fuel pump
1: goes to coil/ignition control unit
31: either ground or to knock box.

On the low compression cars, terminal 31 goes directly to ground, and in the
High compression cars, terminal 31 goes to terminal 10 on the knock box.

So the knock box does _not_ trigger the FPR's activation, that's still done
by the coil, the box merely provides the ground, which it can interrupt "at
will". 

I forget who said the knock box gave a pulsed ground, but from these
diagrams it seems that the knock box provides a full-time ground until the
rev limit is reached.

This is why Honnold's solution worked. He tee-ed the wire from terminal 10
on the knock box into the ground wire for the FPR. Thus, even when the rev
limit was reached, the FPR is grounded. Elegant.

When I installed the knock box for my 2.0 (aba/JH) install, I left the wire
from terminal 10 unconnected. It'll work fine.

A neat experiment in the real world would be for someone with a 16v (keeping
it scirocco specific), who knows what it's like to bounce off the rev
limiter, to tap in a ground for the wire from pin 10 on the knock box.

Then do back to back test with this tap _not_ connected to ground (rev
limiter enabled) vs. with the tap grounded (rev limiter disabled).

Let us know what happens when the knock box is pushed beyond the preset rev
limit, and how high it will rev. Also let us know at what point the motor
blows up. :-)

If no-one steps up to the plate, I'll just have to wait a couple of weeks,
then try it on my brother's POS discarded beater 86 GTI.

Jonas