3B (internal) relay
3Bpin37 is flyback (EC36pin23). Injectors are powered from the ECU via this pin (see MembersPage/PeterJensen/AudiSwiring, 3B is different from the AAN/ABY/ADU in this respect).
3B ECU internal relay (standard, unless requested otherwise):
Relay coil powered from:
- mot55/27 +12V when ignition is ON (EC36/25 ECU supply pin)
- GND
Relay output switch "normally open" (closed when ignition is ON = ECU is powered up)
- 3B MOT55/pin18 (VBATT constant +12V, even alive when ignition is off)
- 3B MOT55/pin37 (EC36pin23). Injectors of the 3B harness are supplied from the ECU via this pin.
The idea is NOT to draw much (2A+) current from 3Bpin27 because we fear that might damage the ignition switch.
Relay problem => conversion to "external" relay that lives in the harness, not inside ECU, like 99% of the cars - as it makes sense.
If the internal 3B relay is stuck or open does not power pin37 reliably:
- replace the relay inside the ECU (needs MUCH more work and risk than adding a relay in the harness)
- or RECOMMENDED: add a new relay in the harness powered by GND and the ignition switch (the wire that goes to 3B MOT55/pin27 to power these:
- 3B MOT55/pin18 VBATT pin (disconnect from "node30" BATT+12V)
- 3B MOT55/pin37 flyback/inj+ (essential if the internal 3B relay is open, and recommended even if stuck: => no dependence on a failed relay)
- optional: 3B MOT55/pin27 can be powered by the ignition switch directly (tapping the MOT55/pin27 wire)
- or powered by the the new relay (recommended), by cutting the MOT55/pin27 wire.
Obviously, if 3 pins of the ECU (pin18, pin27 and pin37) are all powered by the new relay, which is switched by ignition switch (the wire that originally went to pin27) than failed relay inside ECU cannot power anything with ignition-off (that could drain the battery).
IMPORTANT: MOT55/pin18 and MOT55/pin27 wires might be cut, while MOT55/pin37 wire is not cut, but "tapped" !!! (or not touched at all)
- verify that MOT55/pin37 is always connected to injector+, maintain that constant connecion (even when ignition is off)
- tapping into GND is also necessary for the relay coil to work, obviously
- tapping into 3B MOT55/pin37 is recommended but might not be necessary if internally shorted to pin18 and installer is convenient about dependence on a constant-shorted relay - perhaps own engine)
Minimum solution:
- 3B MOT55/pin18 VBATT pin is NOT powered directly by battery
- cut the wire (as in the movies), and connect to a relay switched by the NO=normally open circuit
- relay coil powered by the tapped 3B MOT55/pin27 and GND