I am working on a 1971 280SL that has a 123 distributor installed. The owner disliked the burbling and backfiring on deceleration. A little testing showed that the shut-off solenoid on the back of the injection pump was not being activated in a throttle-closed coasting situation. I tested the temperature and throttle switches, and both worked. I then tested for voltage from the "speed switch" to the relay that powers the solenoid and found none. I was tempted to replace the speed switch, but it looked fairly new, so I tested its inputs. It had power, and ground, and the wiring to the relay was good.
So then I tested for RPM input (the speed switch obviously needs to know the RPM if it is going to activate/deactivate the relay like it's supposed to). RPM input is supposed to be on Pin 4 in the speed switch harness, and the diagram in the BBB shows it coming from Terminal 1 on the coil.
Then I remembered that cars with the transistor switching unit had that weird arrangement where Terminal 1 on the coil was grounded, and Terminal 15 provides the coil trigger and thus RPM signal. But this car had the transistor switching setup removed, and the coil connections changed to ignition voltage to 15 and coil trigger to 1. So the green wire that came into Pin 4 in the harness was dead.
My solution was to run a new wire from Terminal 1 on the coil to Pin 4 on the speed switch harness to provide an RPM feed. With this, the speed switch woke up and started sending voltage to the activation relay at 1350 rpm, just like it's supposed to. The car no longer burbles and pops on deceleration, but it does have a cool little "snort" when the RPMs drop and the speed switch de-activates the solenoid.
So the bottom line is, if you put a 123 or a Pertronix in a car with the factory transistor setup, and you want the anti-emissions stuff to work, you will need to provide this RPM signal as described above.
More detail in JeffC's post in this thread, see the "emissions2" diagram.
Cheers,
CT
