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

[Chris Ice]

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". :)

> If you mentioned whether your AFM was a rising-voltage or falling-
> voltage model, I missed it. However, most of the Vc-style AFMs I
> have seen have a rising voltage with rising airflow. In this case

Yes, this is also rising volts with airflow.  I measured 3.2v at 
~900rpm, ~7.5v at 5000rpm.

> 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. :)

> My suggestion is to carefully measure Vc with Vbatt at slightly
> different levels. If it is always constant, get an adjustable
> regulator to match that voltage exactly. (And if not, pick a
> likely value from the observed range and go with that.)
> Whatever values your converter box outputs are *relative* to the
> Vc voltage, so if Vc is going to change at all, the signal you're
> sending down pin 7 has to change in ratio to the Vc value.

Got it!  All makes perfect sense.  Will do some measurements this 
weekend and let you all know.

Now, a dumba*s eletronics question.  Assuming we're talking a *constant* 
Vc, how can I reproduce it?  Is it simply a matter of putting resistance 
between pins 9 (Vbatt) and 8 (Vc)?  Some reference materials I have show 
the a functional resistance check of the following:

6 to 8 -> 260 to 520 Ohm
7 to 8 -> 200 to 1k Ohm
8 to 9 -> 140 to 280 Ohm

I attempted to recreate this, but I think I ended up making a voltage 
divider and...well...didn't work.  I also put just 200 Ohms across 8 and 
9 and that didn't seem to do the trick either.

(meanwhile, I'll verify Vc with the car running, as well as determine 
whether it's constant or relative to Vbatt)

Chris


_______________________________________________
Diy_efi mailing list
Diy_efi at diy-efi.org
http://www.diy-efi.org/mailman/listinfo/diy_efi