I just had to replace the crankshaft sensor on my 87 BonnevilleV6 SFI. I noticed that in the book the crank damper has 2 sets of interruptor rings on it and the sensor I replaced has 2 windows. But my crank damper has only 1 ring on it instead of 2. Was there a service bulletin or something that changed this? Does anyone know the difference in the 2 systems used?

Yes the earlier design (think the code "3" engine) had a single interrupter ring on the balancerwhile latere (code "C") engines had the dual design. Think 87 was the crossover year.

The single ring crank sensor has a 3 wire connector, the dual ring has a four wire connector. Also the Ignition module is keyed differently on the harness connector.

There should be a complete description of both in the 87 factory service manual.