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Electronics and Sensors

The advent of distributorless ignition systems has added yet another family of sensors to the inventory of underhood electronics, the crankshaft (CKP) and camshaft (CMP) position sensors. These sensors serve essentially the same purpose as the ignition pickup and trigger wheel in an electronic distributor, the only difference being that the basic timing signal is read off the crankshaft or harmonic balancer instead of the distributor shaft. This eliminates ignition timing variations that can resultcrankshaft position sensor from wear and backlash in the timing chain and distributor gear. It also does away with timing adjustments (or misadjustments as the case may be).

On 1996 vehicles with Onboard Diagnostics II (OBD II), the crankshaft position sensor is also used to detect variations in crank speed caused by ignition misfire. If the computer senses enough of these, it will illuminate or flash the Check Engine or Service Engine Soon light to signal the driver he has a problem. General Motors uses a variety of crankshaft position sensors. One is a Hall effect crank position sensor that reads a notched metal "interrupter" ring on the back of the harmonic balancer. This was first used on the early 3.8L V6 Buick Sequential Fuel Injection (SFI) engines (and turbos) with distributorless Computer Controlled Coil Ignition (C3I). The crank position sensor provides an on-off signal to the ECM that the ECM uses to monitor engine rpm and crank position. The system also uses a separate cam position sensor in place of the original distributor to inform the ECM about valve timing. This enables the ECM to determine the correct firing sequence which it then uses to control both injector and ignition timing. Ford uses a similar setup on its 5.0L V8 with distributorless ignition.

Another type of crankshaft position sensor GM uses is the "combination sensor" which you will find mounted on the front of the 3.0L and 3300 V6. GM calls it a combination sensor because the crank position sensor contains a pair of Hall effect switches that generate two separate signals. There are two notched interrupter rings on the back of the harmonic balancer. One ring has three notches which causes one of the Hall effect switches to generate three crank position signals every revolution. The other ring has only one notch, which causes the other Hall effect switch to generate a single "sync-pulse" signal that the ECM uses to calculate rpm and ignition timing.

Another variation of the combination sensor is the "fast start" system used on GM's 3800 engine. A pair of Hall effect switches are mounted by the crank pulley while a cam sensor is mounted over the timing gear. One crank signal generates 3 pulses per revolution while the other generates 18. This allows the coil module to "sync" with the engine more quickly so the engine will start almost instantly.

The third type of crankshaft position sensor GM uses is a magnetic pickup that reads slots machined in a "reluctor" ring in the center of the crankshaft. This setup is used with the Directmagnetic pulse sensor Ignition Systems (DIS) on the 2.0L, 2.5L and 2.8L engines, and the Integrated Distributorless Ignition (IDI) on the 2.3L Quad 4. The crank reluctor ring has six equally spaced slots 60 degrees apart. A seventh slot is spaced 10 degrees from one of the others so the crank sensor will generate an extra "sync-pulse" every revolution. The ECM then uses the information to calculate proper ignition and injector timing. This type of sensor must be carefully positioned so the air gap is within .050 in. of the crankshaft reluctor ring.

Whether a crankshaft position sensor is the magnetic type or a hall effect switch, most problems can be traced to faults in the wiring harness. A disruption of the sensor supply voltage, ground or return circuits can cause a loss of the all-important timing signal resulting in an engine that cranks but won't start. When troubleshooting a suspected crankshaft position sensor problem, you must follow the diagnostic flow charts in the manual to isolate the faulty component -- otherwise there's no way to know if the problem is in the ignition module, computer, wiring harness or sensor.

On older GM applications, a trouble Code 12 while cranking would indicate no reference signal being generated. On older Ford applications, a Code 14 would indicate a problem with the crank position sensor signal, which Ford calls a "PIP" (Profile Ignition Pick-up) signal.

On OBD II vehicles, a faulty crankshaft position sensor may set a code P0338 or P0339. A faulty camshaft position sensor may set a code from P0340 to P0349.

Magnetic sensors can be checked by unplugging the electrical connector and checking resistance between the sensor's two terminals. On the 2.3L Quad 4, for example, the sensor should read between 500 and 900 ohms. On a Saturn 1.9L engine (whichCam Sensor - will not stop the engine from starting has a high failure rate of crankshaft sensors), the resistance specification is 700 to 900 ohms. Always look up the exact specifications for the vehicle because the specifications will vary from one application to another. Testing the resistance of a magnetic cranksahft position sensor with ah ohmmeter will tell you if the sensor is within specifications. If it reads open or is shorted, it must be replaced.

An "intermittent cranks but won't start because of no spark" problem may occur if a magnetic crankshaft position sensor changes resistance excessively when it heats up and cools down. Some increase in resistance is normal when it heats up. But if the resistance suddenly jumps or drops outside of specifications, the sensor is bad and must be replaced.

The sensor can be HOT tested by removing it and immersing the magnetic tip in a pan of hot water. Watch the resistance as it heats up. Then remove the sensor and watch the resistance as it cools down. DO NOT attempt to heat the sensor with a propane torch or open flame as doing so will damage the sensor.

A magnetic crank position sensor should also produce an alternating current when the engine is cranked so a voltage output check is another test that can be performed. With the sensor connected, backprobe the connector and read the sensor's output voltage while cranking the engine. If you see at least 20 mV on the AC scale, the sensor is producing a good signal. If you have a scope, you can also observe the sensor's waveform. It should produce an alternating current signal that changes frequency and amplitude with engine rpm.

Hall effect crankshaft position sensors typically have three terminals; one for current feed, one for ground and one for the output signal. The sensor must have voltage and ground to produce a signal, so check these terminals first with an analog voltmeter. Sensor output can be checked by disconnecting the coil and cranking the engine to see if the sensor produces a voltage signal. The voltmeter needle should jump each time a shutter blade passes through the Hall effect switch. If observed on an oscilloscope, you should see a square wave form.

If your diagnosis reveals a faulty crank sensor, the only option is to replace it. With Hall effect sensors, the sensor must be properly aligned with the interrupter ring to generate a clean signal. Any rubbing or interference could cause idle problems as well as sensor damage. Magnetic crankshaft position sensors must be installed with the proper air gap, which is usually within .050 in. of the reluctor wheel on the crankshaft.

On many engines with distributorless ignition systems and sequential fuel injection, a camshaft position sensor is used to keep the engine's control module informed about the relative position of the crankshaft. By monitoring cam position (which allows the control module to determine when the intake and exhaust valves are opening and closing), the control module can use the cam position sensor's input along with that from the crankshaft position sensor to determine which cylinder in the engine's firing sequence is approaching top dead center. This information is then used by the engine control module to synchronize the pulsing of sequential fuel injectors so they match the firing order of the engine. On some applications, input from the camshaft position sensor is also required for ignition timing.

The camshaft position sensor may be magnetic or Hall effect, and mounted on the timing cover over the camshaft gear, on the end of the cylinder head in an overhead cam application, or in a special housing that replaces the distributor (in the case of some of the GM applications). Operation and diagnosis is essentially the same as that for a crankshaft position sensor. On engines with Variable Valve Timing (VVT), a camshaft position sensor may also be used to monitor the relative position of the cam to determine if cam timing is advanced or retarded. The PCM uses this information to monitor the operation of the VVT system. The relative timing of the overhead cam(s) may be changed at higher rpm to increase power.

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