Clutches and Reference Sensors…

In the past year or so I have come across an interesting situation regarding failed reference sensors that involve the clutch – something to put in your mechanical head and use when the time is right…

The situation first came up when a friend was racing his 924S SP2 PCA car at Watkins Glen.  On Saturday, the car suddenly lost power and shut off and he coasted into the pits. The crew diagnosed a bad reference sensor and replaced it in preparation for the enduro on Sunday.  He started the enduro the next day only to have a second “shut down” failure, again traced to the same reference sensor.  Instead of messing with it at the track, it went into the trailer and came home to Florida.  When we looked into it, we found that the second reference sensor was damaged – by the clutch coming apart.  A piece of the destroying clutch disc had contacted the reference sensor, cutting and denting it.

Since then we have found three others – street cars – at the shop that were victimized in the same way. The clutch disc starts coming apart, and a piece of thin metal gets outside the circle of the disc and cuts up a sensor.

Reference sensors are not the easiest things in the world to get out of the car.  Not only are they difficult to access on the top of the bell housing next to the firewall, but if they have been in there a while, they may be stuck in place.  However, when a piece of the clutch nicks the sensor, they may be difficult or impossible to remove from their mounting – the end is too deformed to come through the hole.  This means that the entire mounting piece has to be removed to get the sensor out, which then means that you will have to reset the clearance to the flywheel to reinstall it.  Not difficult, but adds to the complexity of the repair.

Other Failures

Of course, reference sensors will rarely fail at the flywheel unless something gets loose in the bell housing.  Instead, the connectors fail where they plug into the harness at the back of the head and intake.  Check there for issues, like cracked or broken plastic in the connectors.  If the sensor side of the connector is bad, you have to replace the entire sensor; if the harness side is bad, then a replacement of the harness wiring is in order.  Lindsey Racing has a harness for the reference sensors (among others) that replaces the shielded cable all the way to the DME (computer) under the dash.  While installation can be a bit challenging, it can be done at home.  (Note that you will need to dislodge the pins for several of the connections in the plug on the DME, and you will need a wire terminal tool to release the pins from the connector.  They look like this.  

You pick the correct tool for the pin size, push it over the pin to release it’s tiny catches, and pull the pin out of the connector.  The replacement slides in place and clicks into position – and that is the most difficult part of the job.  I have installed three or four of these kits in cars, and it isn’t bad at all.  Just make sure that you run the wires along the harness and tie everything up so that the cables don’t get near any hot stuff under the hood.  Use the bracket that was standard issue on the car to keep the connectors in place on the intake – they keep the plastic as far away from the heat as possible, thus helping keep the plastic from degrading.  I see many cars with that bracket empty or missing and the sensor connectors lying on the bell housing or just stuffed behind the head – not a good idea to keep heat-related failures to a minimum.

The reference sensors tell the computer that the engine is running and at what speed, with an indicator for Top Dead Center to help with timing of fuel and spark.  Too many cars have died an early death due to a “broken fuel pump” or “no spark, unknown issue” when a simple reference sensor or connector was the culprit.  If the sensors don’t report TDC and speed, the DME won’t tell the ignition to fire or the fuel pump to pump.

“What does the clutch have to do with whether the engine runs?  It shouldn’t.”  Yes, it does. If there is damage on the end of the sensor, the clutch is probably at fault.  Check it.

Happy motoring!