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USA Fiat 500 DTC CAN-B and A3 cable error

Posted: 15 Feb 2018, 04:36
by Milo C
My first time into the forum.
End goal, transplant USA engine/drive train into (4) early 1970's vehicles.
Very steep learning curve and no help within a 1000 miles!
Foundation vehicles: USA delivery Fiat 500/500X/500 Abarth

What hardware is supported using which connectors / cables?

In reference to a USA FIAT 500 1.6 , does one even need to use the A3 cable for any hardware modules or functions?
If so, there appears to be a problem in that it appears that the A3 cable is intended to interface to the CAN B (50 kbps) bus.
If so, it appears that the CAN+ and CAN- are reversed.

This is documentation for “standard” OBD port used in the USA and on the ELM327 connector.
Please see FIAT OEM manual 2012-2015 pp 8203 "Computer Data Lines" (attached)

Documentation for the interface cable given in the MULTIECUSCAN web for the FIAT 500 1.6L
Indicates either direct ELM or Cable A3 are to be used.
The direct connect supports the CAN-C 500kbps link.

On the A3 cabled link to the CAN-B bus, is there a problem?
See the pin-out below.

I don’t have a schematic of how the BCM interconnects the two CAN buses or how the BCM firmware transfers the independent CAN-B devices onto a single bus for the DTC port.
If the bus drivers are to specification, they all could be tied common at the BCM and the bus impedance would be reasonable and not require any processor or hardware assets.
Fiat documentation infers that data packets on the CAN-B side are passed through onto the CAN-C bus and I assume this is done by software and has some significant collision avoidance mechanisms to prevent slow speed, low priority traffic from generating problems on the CAN-C communications side.

FIAT side
DTC end ELM327 end
Pin# Pin#
CAN-B+ 1 ---------------------- 6 CAN-
GND 4 -----------------------4 GND
CAN-B- 9 ----------------------14 CAN+
(NC) 12----------------------- 7 ISO K-Line
+12V supply 16--------------------- 16 +12V

Anyone care to clarify?