[Diy_efi] Re: Bosch L-Jet AFM question...

[Alexei Pavlov]

cice at trub.com wrote:
> 
> Chris Conlon wrote:
> > Pin 8 is almost certainly "Vc". Older Bosch AFMs, which are powered
> > by Vbatt rather than regulated +5, often output a lower voltage back
> > to the ECU as a full scale reference value. From what I've seen this
> 
> This is circa 1981 unit.  Assume this qualifies as "older". :)

Yes, older are those used with L-Jetronic ECUs.


> > leaving the Vc pin open or (essentially) shorting it to Vbatt will
> > cause just what you've described. In one case the ECU thinks the
> > full scale output range is 0v to 0v, and so even 1v translates
> > to a huge airflow. In the other case, pulling Vc up to +12v would
> > make any airflow signal seem less by comparison to that higher-
> > than-usual voltage than to the usual voltage (7-9 volts).
> 
> Got it!  So basically, the AFM must tell the ecu what it's reference
> voltage is via pin 8, and the volts sent as the afm signal are relative
> to this voltage.  Kinda cool, but frustrating. :)

Yes, in fact the L-Jetro connects the AFM reference voltage to a
resistor
(current source) which charges a capacitor ..., you see.

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