Rev Limiters (was RE: Knock Sensor Q)

[cory.langford@icbc.com (Cory Langford)]

So from the sounds of the last flurry of messages, it seems the Fuel
Pump Relay takes a signal from the Knock Box (pulsed ground) and uses
this to keep the relay portion of the system closed and thus the fuel
pump works.  When a specific RPM is reached the Knock Boxs cuts the
signal, the FPR opens and the fuels stops flowing.

It also looks like on old MK1 there was a different FPR that had a
similar circuit inside, but with out a knock box the signal was taken
from the Coil ground. This would result in NO rev. limiter as the coil
is not smart enough to cut out at a specified RPM :)  BUT, it did solve
the safety issues of having the Fuel pump continue to run when the
engine was dead.


So if all this is correct, would not the simple solution for the new
Knock Sensor people who want to remove the Rev. Limiter be:

Take the wire that supplies the signal from the Knock Box to the FPR,
cut it and wire the FPR into the Coil Ground.  Then when the Knock Box
wants to cut the signal it can because it is not connected to anything.
The FPR continues to work, and has the same safety features as the MK1
system.


Could be missing something here, but it does not seem that complicated,
then again the easiest way is to install the Digifant-1 from a G60 and
have the chip programmed to rev limit where ever you like ;)




-----Original Message-----
From: drew 
Sent: Friday, May 24, 2002 5:26 AM
To: ATS - Patrick Bureau
Cc: Brian Honnold; scirocco list
Subject: RE: Rev Limiters (was RE: Knock Sensor Q)



Patrick, I'm running a similar setup in my 87 Jetta winter beater.  I
slapped some RD pistons in a JH block, threw on the JH head with a G
grind, and grafted a knock sensor ignition in from an A2 GLI.  I kept
the
stock CIS, and the cat-free from factory 87 makes for some fun exhaust
options without having to worry about emissions woes (not that I do
anyways.)

The kock box I used came from an 86 8V GLI, and the fuel pump relay was
the stock 87 Jetta CIS relay.

I didn't think about rev limiters too much when assembling the system.  
As a matter of fact, I didn't think about it too much when I installed
the
16V instrument cluster with its 8K tach and 260km/h speedo, either.  For
some reason I developped a tendency to bounce of the 8V's top end when
the
tach needle told me I had another 1000rpm to go before hitting the
red...
:)

I generally shift when the power starts to drop off, which is just about
6K anyways (limitation of the G grind?) however one morning I was in a
tight spot and kept into 3rd gear at WOT, and it kept pulling hard until
about 6200, when the engine started to buck like mad.  I dropped into
4th
and it started pulling again - my guess is I hit a rev limiter function
that was cutting fuel to the motor.

My fuel pump relay is straight CIS (GX/MZ engines: 321 906 059H -
compare
with CIS-E relays for RD/PL: 191 906 383 C)  Your 85 Scirocco should
have
come with the "059 F" part - heaven knows what subtle engineering change
prompted the shift from F to H.

My expectation, as I believe others have confirmed, is that the knock
box
cuts the coil pulse to the relay, which then in turn cuts power to the
pump.  This "protection" mechanism, which keeps the pump from pumping an
accident scene full of fuel, has been present in CIS fuel systems for a
long time - I'd have to go back to the wiring diagrams to check, but I'm
pretty sure it was there in 81+ german-built cars.  Definitely there in
84+ cars.  Running either a 16V or 8V knock box (I'd go with the 8V box
if
you're running an 8V engine, personally) with your stock fuel pump relay
should result in a rev limiter - it's just where it is that's the
question.

HTH,  

Drew