Difference between revisions of "NG 900/9-3 Crankshaft Position Sensor"

Revision as of 23:57, 24 November 2009

Overview

The Crankshaft Position Sensor is part of the Trionic engine management system. It generates a signal used by the ECM to calculate RPM and engine TDC. Typical life of the sensor is 100-150K miles. The original sensor is manufactured by Bosch, dealer list price $141, or ~$100 online. Crankshaft position sensors are simple inductive sensors, consisting of a magnet, a wire coil, housing, cable and connector. Generic equivalents like the one pictured above typically cost ~$50.

No-Start

If the sensor does not have a valid signal during cranking, the engine will not start. Some cars may start with a cold engine, but may not restart when warm. It usually gets progressively worse until the engine fails to start cold or warm. If the sensor fails during start, no CEL is generated.

Engine Stall

If the signal from the sensor is lost with engine running, the Engine Control Module (ECM) will cut the fuel. If the signal is weak or intermittent, the engine may appear to miss or stall when it reaches a higher rpm. A CEL and fault code may not be generated until the condition gets worse.

Electrical Test

Coil Resistance

The resistance of the sensor coil can be measured across connector pins 1 and 2, and should be within about 10% of the nominal value (At 20C temp). Both pins should test "open" to pin 3. If not, the sensor should be replaced. However, resistance is not always proof of a good sensor.

Access

Sensor Connector

The connector for the crankshaft position sensor is located under the engine cover, tucked below other wiring and hoses behind the engine. The cable is usually held in place with a plastic cable tie which has to be cut before the connector can be unplugged.

Turbo Hoses

• Disconnect the BPV vacuum line (1)
• Remove the BPV and hoses (2)
• Remove the Charge Air Pipe (3)
• Plug the open pipes with a towel

900SE Turbo

The turbo downpipe is located directly in front of the sensor and makes access to the retaining bolt more difficult. Removing the fan makes more room and takes about 15 minutes.

900S Non-Turbo

There is more space to work, and the sensor is also accessible from below.

Battery and Power Steering Hose

Remove the battery. Unclip the power steering suction line below the battery tray and remove a single 10-mm bolt holding the pipe above the fan. Do not disconnect the p/s hose clamps!

Radiator Fan

Remove two 8-mm bolts holding the fan and unplug the electrical connectors. Push the suction line toward the engine and pull the fan out between the power steering line and the front of the engine compartment.

Removal

Sensor Retaining Bolt

Use PB Blaster or liquid wrench to loosen the torx bolt, which may be rusted. Remove the bolt using a ratchet with an extension, and a T-30 torx bit. Use a new and sharp torx bit and gently tap it into the bolt head with a hammer to loosen rust and help the end of the bit bite into the bolt.

Tool Clearance

On the 900SE turbo, pass the extension under the downpipe. The picture is after removal of the fan, and shows a ratchet with a 6-inch extension. (With the fan in place, the extension would have to be shorter to allow the ratchet handle to clear the fan.)

Crankshaft Position Sensor

Remove the sensor from the engine block, with the old o-ring and the support collar. Unclip the sensor cable from the coolant pipe on the side of the engine, and remove the sensor. Save the bolt, the support collar and the two clips.

Replacement

Sensor

The sensor on the right is a generic replacement from a chain auto part store, for about $55. Clean the sensor seating surface. Install the new sensor in the engine block with the support collar and a new o-ring. Tighten the retaining bolt to about 6 lb-ft. Do not overtighten.

Support Collar and O-ring Seal

Note the relative position of the o-ring and collar. If the new sensor does not come with one, the collar may be reused.

Sensor Cable

Attach the cable to the same coolant pipe with the two clips. Route the cable around the back of the engine in the same location as the original. The cable is fairly rugged and has a braided wire shield under the insulation, but should not be bent at sharp angles, "kinked", or pinched by the clamps.

Connector

Plug the sensor connector in behind the engine, and secure the cable slack to the fuel rail harness with a plastic cable tie.

Fan and Battery (if removed)

Replace in reverse order,

• Radiator fan, two 8-mm bolts and two fan connectors
• Power steering fluid suction line, clip and 10-mm bolt
• Battery, hold-down bracket, 13-mm bolt
• Battery cables

Remaining Components

• Turbo pipes and hoses, hose clamps
• Turbo BPV vacuum line
• Engine cover

Information

Retaining Bolts Spec

Torx T-30, M6x100

6 lb-ft torque

Tools

• 3/8 drive Ratchet
• 6-inch extension (or shorter with fan in place)
• T-30 Torx socketed bit
• 8, 10, and 13-mm sockets
• PB Blaster penetrant
• Wire brush

References

Saab EPC 3A-0420

Haynes Ch 4A, section 15

Sensor Types