Crank Position Sensor

What is the Crankshaft Position Sensor?

The Crankshaft Position Sensor CKP https://amzn.to/3R4RQEK is a crankshaft reluctor wheel triggered hall effect sensor that allows the computer to know the speed and position of the crankshaft in it's rotation.  This is required for proper injector timing for engines with sequential fuel injection systems and helps the computer know if the engine is misfiring.

The most common symptom that is first noticed with a faulty CKP is that the engine won't start or will start hard.  Usually it will set a DTC, sometimes this happens right away, sometimes it takes a LOT of cranking before the DTC sets.  As always a DTC doesn't always mean that the sensor is bad, it could be the wiring/connectors/grounds/power/etc as well.  This is why diagnostics are important.  Problems with the CKP can also cause a rough idle, stalling, misfires, backfiring, etc.

One quick way to test if the sensor and wiring are working at all is to pull the sensor, check for wear marks on the front of the sensor, and bring it close and farther away from a steel object like the sub-frame on the A/S vans, you should hear the fuel pump turn on if the sensor/wiring/etc is functioning properly.

Here are a few things to keep in mind:

1. Make sure to check the wiring and connectors when replacing the CKPS

2. As the crankshaft bearings wear, it can cause an issue with the reluctor being too close or actually hitting the CKPS, this can also happen with a replacement sensor.  You must have at least a 0.030" air gap between the sensor and the reluctor wheel, this can be checked with the proper gauging compound.  A cheaper less accurate alternative would be to put a dab of grease on there of the proper thickness(feeler gauge and a popsicle stick to get it flat).  Install and see if you have any contact, if so add additional shim.

3. Always do a CKP System Variation Learn procedure using a proper scan tool when you replace the sensor/engine/timing cover/PCM/balancer/shims/etc.  If you don't do the re-learn, you might get false misfires.

4. When replacing the sensor, the sensor tip and reluctor should never touch, sometimes the sensor will require 1 or 2 0.5mm thick shims GM 12565102 (check dealer) or https://amzn.to/3KxQsrS and might cause false misfire detection or DTCs to be set if there is not enough space between the two.  You could also make your own cut from an aluminum can but they are not expensive and should be readily available. TSB #02-06-04-059(shown at the bottom of this page)  Some replacement sensors come with a pack of shims included.

5. Replace and lube the o-ring when the sensor is removed, torque is 8Nm or 71 lbin.

How do you do the CKP System Variation Learn procedure?

1. Connect the proper scan tool, generally only the Tech2(or tech2win) or other higher end scanners have this option though some cheaper options seem to be adding the feature lately.  Please confirm before you make a purchase.

2. Within the scan tool, select the crankshaft position variation learn procedure.  This includes stepping on the brake and going to specified RPM.

3. Follow directions on the scan tool.

Where is the CKPS located?

It's located on the bottom passenger side of the timing chain cover.  Lay under your van and and look up, you will see the crankshaft pulley with the belt on it, and right behind that on the plastic cover will be the crankshaft position sensor.  You can see pictures of the cover and pulley on the Distributor page.

Description from the 1996 FSM:

The Crankshaft Position Sensor (CKP Sensor) is located in the front engine cover and is perpendicular to the crankshaft target wheel. The air gap between the sensor and the wheel is preset and not adjustable. The target wheel has three slots 60° apart and is keyed to the crankshaft. As the target wheel rotates, the slots passing by the sensor create a change in the magnetic field of the sensor which results in an induced voltage pulse. One revolution of the crankshaft results in three pulses (3x signal). Based on these pulses, the VCM is able to determine crankshaft position and the engine speed. The VCM then activates the fuel injector and provides a spark to the Distributor. The relationship between the crankshaft position sensor and the target wheel is crucial. The sensor must be exactly perpendicular to the target wheel with the correct air gap.

Why do we need the CKP sensor?

The requirement for the CKP sensor arrived with sequential fuel injection. Prior to that the injectors would fire in batch mode, all cylinders get fuel at the same time, so the computer didn't need to know which cylinder needed fuel.

With sequential fuel injection systems, the computer needs to fire the correct injector for the 1 cylinder that needs fuel at that time, so we have the crankshaft position(CKP) sensor for that.

It's basically the same for the ignition coil/distributor combo. With the distributor based system the computer doesn't need to know which cylinder is firing because the gear driven distributor will point the rotor at the correct distributor cap terminal. The computer just needs to know that a non specific cylinder needs spark and the rotor position dictates which cylinder gets that spark. With the later 1 coil per cylinder(not available on our vans), the PCM needs to know exactly which cylinder to provide spark to.

A bonus of the computer knowing the crankshaft position and speed, the computer can detect misfires and know exactly which cylinder(s) misfired.