[T3] Rev limiting motor

[Jim Adney]

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.

The result would be to run the motor at wide open throttle with the 
leanest mixture that would still keep it running. That seems like a 
certain prescription for engine damage.

To do this with accuracy, each relay would either need some kind of 
absolute clock to compare to, which they don't have, or they would 
have to be individually "tweaked." If they were just assembled "as 
is" they would be hard pressed to be within 20%. The D-Jet brains 
have lots of resistors which have clearly been installed as "tweaks" 
to calibrate them, but I don't recall any sign of this in the Rabbit 
fuel pump relays.  

OTOH, it's been many years since I looked inside one of those.

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