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Old 11-02-2009, 05:27 AM
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iancp5 iancp5 is offline
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Join Date: Aug 2006
Location: U.K.
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Originally Posted by Qui-gon View Post
I have dissasembled the interface and drawn out the circuit. The Tx line from the ECU uses part of a standard Maxim 232 Rs232 level convertor. The ECU side of the level converter has an extra pull up resistor giving the line in from the ECU a nominal voltage of +6v. This is with respect to ground which is common to both the ECU and the PC it is connected to. This part is most likely standard TTL to rs232 conversion, ie inverted and voltage converted. The Rx or input line to the ECU is slightly different using a single transistor with a diode clamp. I would guess that in the ECU there is some kind of differential bus tranceiver to either a RS422 or 485 microcontroller interface. As for the communication protocol used it is probably home grown or using a subset of one of the known ones.
With regard to your original idling problem try playing with the throttle body bleed past screw which is the one visible through the plastic cover over the throttle cable on the kickstand side.
The service port is a 3 wire connector located under the seat just behind the tank. Probably identifyable by a rubber blanking plug in it to keep dirt and moisture out.
Hope this helps
Do you know which of the pins of the service port is output?
I was planning on trying some standard diag s/w to see if it can interpret the signal but on the only wiring diag I can find (2005 User Manual) it only identifies earth as the middle pin. The outer 2 pins aren't named.

As far as I can find the ECU is a Magneti Marelli IAW 15P not that this helps much as there seems no information on this to be found using Google. A lot of Magneti units use K line (KWP or ISO) protocol so standard s/w might work to read it. Not sure about writing. I'll try the bleed screw but I'd like to be able to read and adjust the ECU myself. Mainly to experiment with getting it to start easier from cold but the initial issue is stalling. The idle was set at 1800 in summer by the dealer but with the cooler air now it's dropped and will stall sometimes when idling. Maybe the IAT's gone - I should check that but it'd be nice to see everything on a screen rather than poking around with a multimeter.
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