[T3] Rev limiting motor

[Jim Adney]

On 8 Mar 2012 at 22:18, Hal Sullivan wrote:

> On Mar 4, 2012, at 8:21 PM, Jim Adney wrote:

> > On 3 Mar 2012 at 22:24, Hal Sullivan wrote:

> >> On CIS cars, the rev-limiting function is built into the fuel pump
> >> relay.

> >> Slick piece actually -- teh relay only activates the fuel pump if
> >> there's a signal from the (-) side of the ignition coil, meaning that
> >> the engine is turning over.  It also monitors the engine RPM, and if
> >> it gets "too high" (I think 6000RPM) cuts power to the pump...

> > Are you sure about this? I've never heard this before, and I've dealt
> > with those relays. It's certainly possible to build such a feature
> > into the relay, but that doesn't strike me as such a good idea.

> Yes and no -- some CIS fuel pump relays have a rev limit built into  
> them and some .. don't.  When I posted the other evening I had a brain  
> blip .. remembered about the rev limiter, forgot that it isn't on all  
> of them.  I used a rev-limiting relay in my engine-swapped Westfalia  
> (1.8L GTI engine, "basic" K-Jetronic), but the relay currently  
> installed in our Cabriolet (1.8L, KE-Jetronic)  doesn't have a  
> limiter .. and the car is supposed to have one.
> 
> http://www.cabby-info.com/electrical.htm (down at teh bottom)
> 
> http://forums.vwvortex.com/showthread.php?4878510-What-s-the-rev-limit-on-a-Mk2-GTI-8V

Okay, it's clear that VW did this. I didn't follow all the 
discussion, but I was puzzled by one statement that stopping the fuel 
pump would cut the engine off instantly. I know that's not true, but 
it would certainly provide a rather soft rpm limit. I'm still 
concerned that it would do so by leaning out the mixture under WOT 
with possible engine damage. OTOH, surely VW thought about this and 
assured themselves that it wasn't really a problem.

The key may be that in the CIS (K-jet) system, the airflow is always 
measured, so, as the limiting kicks in, the airflow would also drop, 
meaning that there would be less of both air and gas, hence less 
leaning out than I expected.

Certainly under the D-jet system this would not be the way to do it.

-- 
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Jim Adney, jadney at vwtype3.org
Madison, Wisconsin, USA
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