All 900's and later 700's with the B230F/B234F (1989-on) or B230FT (1990-on) will have a crank position/rpm sensor bolted on top of the bell housing behind the head.

The early sensors were known to be weak, especially when hot, and should be replaced if they are suspect. They can be identified by p/n 1389399 with a yellow band near the electrical connector. An improved sensor was introduced in late 1993 production. Although they originally had the same part number, you can identify them by a white band or dot instead of a yellow band. The improved sensor ultimately became p/n 3547847.

The sensor heads of the newer ones are reportedly much less failure prone, but any damaged wiring can still cause problems. The cable has a braided shield and if this is damaged then noise can disrupt the signal. The improved sensor uses new mounting clips to prevent chafing against the coolant tube behind the block. The rpm sensor can become fairly brittle so you must be careful when removing it (do not pry) -clean around the sensor, remove the bolt (10mm), apply a little penetrating oil if you have it, twist the entire sensor gently to break the bond then twist back and forth while pulling straight up to remove it. Apply a little grease on the outside when reassembling. Also apply dielectric grease in the electrical connector to prevent moisture and corrosion problems. Don't overtighten the bolt (spec calls for 6 ft-lbs).

There are tests for these sensors:

o Check for diagnostic trouble codes at the ODB unit (see the FAQ on how to do this). You're interested in looking for either of the following codes:

Socket 2 (ECU) 1-3-1 "Ignition system RPM signal missing on starting"

Socket 6 (ICU) 2-1-4 "RPM sensor signal absent intermittently"

o Remove the rpm sensor and visually check for damaged wiring, especially near the sensor head.

o Check the resistance of the circuit through the rpm sensor. Do this at the ICU (ignition module) connector (disconnected). Only probe the pins at the side of that connector to avoid damage. For both the EZ116K and Rex-i systems use an ohmmeter across pins 10 (red) and 23 (blue). As a quick test you can also do this at the rpm sensor connector on the firewall (pin 1 red and pin 2 blue), although resistance values may be a hair lower.

For the older style rpm sensor you should get 240 +/- 25 ohms at 25degC/68degF
For the newer style rpm sensor you should get 170 +/- 30 ohms at 25degC/68degF

You can also check that pin 11 (black) at the ICU has continuity (near zero ohms) to pin 3 at the rpm sensor connector and to the braided shield on the rpm sensor cable -carefully probe with a pin at a shallow angle if needed.
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Dave -not to be confused with a real expert, just goofing around at this