Re: In search of a SunPro (& Duty Cycle?)

[rdh@sli.com (Robert D. Houk)]

   ...
   I got another message asking about what the duty cycle is used for.  Duty 
   cycle indicates the percentage of time that a given regularly cycling signal 
   is active.  A signal that is cycling active for 1 second and then inactive 
   for 3 seconds has a 25% duty cycle (active 1 second out of four total seconds 
   in the cycle).  A signal that is active for 0.025 seconds and then inactive 
   for 0.075 seconds also has a 25% duty cycle.  The diagnostics in the Audi 
   ECUs have been known to use measurement of duty cycle, and other than inter-
   preting the output of a dwell meter, the SunPRO CP7678 is the only device 
   I've seen on the market that has this feature.  This can be used on the idle 
   stabilizer valve, OXS frequency valve, EHA, speedo sending unit, etc.

And specifically, for '83 UrQs and other Audis with the K-Jetronic injection,
the ECU modulates the "Frequency Valve" to in turn modulate the system fuel
pressure in the fuel injection system, thus "fine tuning" the instantaneous
air/fuel ratio. Nominally, the Frequency Valve (FV) should sit at 50% duty
cycle, which indicates that the rest of the mechanical fuel injection sys-
tem has perfectly guessed the right air/fuel ratio. A lower duty cycle in-
dicates that the computer is leaning out the mixture, a higher duty cycle
indicates that the computer is enrichening the mixture. In normal operation,
at the closest you can manage to "steady state" driving, the FV's duty cycle
will be bouncing plus/minus several percent.

You can monitor the FV while driving to get an indication of what the com-
puter thinks the rest of the engine is doing (With K-Jetronic, the fuel
injection is basically a static mechanical system system; the "ronic" part
is the ECU (computer) which is monitoring the O2 sensor to determine the
instantaneous air/fuel ratio (how well the mechanical system is metering
fuel) and then feeding back into the "mechanical" fuel injection system by
dicking around with the internal fuel pressure of the mechanical injection
system (i.e., the fuel distributor) via the FV and "bleeding" off or by-
passing fuel pressure around the distributor. Just in case anyone was cu-
rious...).

Specifically to the Hitachi ECU used with the '83 UrQ, and possibly to
other Audis of that era, at wide open throttle, the computer will force
the FV duty cycle to an "open-loop" 70-80% "rich mixture" for "performance"
purposes. (On my particular beastie, with my particular 7678 meter, it runs
78.9% for about three seconds, then drops to a steady 70.0%.)

The Hitachi (and possibly other) ECU has a convenient "self test" mode in
that at idle (idle throttle switch closed) if you manually trip the wide
open throttle switch the ECU will switch to "open loop" 50% duty cycle (my
unit reads 49.9%), which allows you to verify half of the computer manage-
ment sensors/circuitry/operation in one easy test . . .

So there are some of the fun games you can play with your 7678 meter. It's
almost as much fun (pegging the duty cycle reading) as playing with the
boost guage . . .

					-RDH