[ischmitz]

Above post ^^^^^^^ is why the Bentley manual scares me; it suggests pin 11 and pin 21 are identical. That is far from true. More likely the authors didn't know which pin does what.

Rick has it right. Pin 21 puts out a pulse for each spark event. This is used by the tachometer to display RPM. Pin 11 carries the fuel pulse length. The duty cycle of the signal is evaluated in the tachometer to decide when to turn on the shift light.

Further, if you mix these wire up when connecting the tachometer you will inevitably destroy your DME. First the car will run lousy and next it will quit altogether with no fuel. The reason for this is that the signal on pin 11 comes directly from the fuel driver. And when you hook that wire up to the tachometer input the pull-up resistor of the tachometer will drive current into the DME and destroy the fuel driver. This is an expensive repair.

Ingo