In the world of engine controls, if we ever were to classify two sensor inputs to be the MVPs of an engine management system, it would probably have to be the crankshaft (CKP) and camshaft (CMP) position sensors. All life for ignition control and fueling begins with these two fundamental sensors.
Think of what you first check when you have no spark or no fuel in a no start diagnostic process – CKP and to a lesser degree CMP. If the Powertrain Control Module (PCM) has no clue that the engine is being turned over, nothing happens with fuel and spark. To better understand these two MVPs of engine management, let’s look at their theory of operation, weak links and diagnostic tips when it comes to problems in the service bay.
Sensor Designs
There are four sensors I want to discuss here: Variable Reluctor, Hall Effect, Magneto Resistive and optical.
Variable Reluctor Sensors: The simplest and most popular design is the magnetic sensor, or as it is often referred to, a Variable Reluctance (VR) sensor. Unlike other varieties of sensors, the magnetic style needs no power source to do its thing.
Its principle of operation revolves around a permanent magnet surrounded by a coil of wire. When the magnet’s field surrounding the coil is varied, that changing magnetic field induces an.JPG?auto=format,compress&fit=max&q=45&w=250&width=250)
In Distrubtorless Ignition (DIS) and Coil On Plug (COP) ignition, the signal must be synced to the position of the crankshaft in order for the correct ignition coil to be triggered. Typically this is accomplished via uniquely spaced gaps, an extra gap or missing gap to signify the rotational position of the crankshaft. The signal produced is an analog sine wave.
Hall Effect Sensors: Hall Effect sensors utilize a permanent magnet mounted on the sensor’s body. Next there is a semi-conductor wafer that creates a small Hall voltage signal when exposed to a magnetic field. Another circuit inside the sensor assembly amplifies this signal to turn on a transistor.
The transistor inside the sensor assembly then provides a ground for the ignition module or PCM supplied signal voltage (typically 5 volts), bringing the incoming signal voltage to ground. Metal vanes/interrupter blades on the crankshaft (or camshaft) rotate into the gap between the Hall Effect sensor element and the magnet, blocking the magnetic field from the sensor, turning the transistor off and allowing the sensor signal voltage to remain high.
Magneto Resistive Sensors: A Magneto Resistive sensors (MR for short) is in a way a bit of a mix between the Hall Effect and VR sensor. It also uses three wires (power, ground and signal) and a magnetic field, and produces a digital square wave signal. The magnet is positioned between two magnetic reluctance pick-ups (sensing elements MR1 and MR2).
The magnetic field changes in the area of MR1 and MR2 as the crankshaft mounted reluctor wheel rotates. Each tooth of the reluctor wheel reaches MR 1before MR2. Both MR1 and MR2 produce the same voltage signals, but the MR2 signal is just a few milliseconds later than the MR1.
Signals from MR1 and MR2 cause an internal circuit called a differential amplifier to produce the MR differential output which contains a square wave that toggles above zero for MR1 and below zero for MR2. This signal is used to switch a transistor-like device called a Schmidt Trigger off and on. The Schmidt Trigger then produces a 0-5 volt digital output on the sensor signal circuit leading to the PCM. The PCM does not supply a DC bias pull up voltage on the input circuit as with the Hall Effect sensors.
Optical Sensors: Optical sensors began showing up in many of our service bays in the LT1 GM V8 engines back in the 1990s, and were widely used in several GM models, including the Corvette, Firebird and Camaro. Some Asian models produced by Nissan and others used variations of this type of sensor.
The optical position sensing element on the LT1 engine applications was built into the pancake looking distributor housing (complete with cap and rotor) mounted behind the engine’s water pump and driven by the camshaft. Along with the sensing element was a super thin rotating metal disc (called a track disc) with tiny notches cut in it.
Common Failures
Now, I’ve seen one ASE A8 or L1 test question that comes up regarding the proximity of the sensor to the cam or crankshaft. It sometimes reads like this: “A variable reluctance (VR) crank sensor is too far away from the crankshaft. Could that cause a cylinder specific misfire?”
And of course, the answer would be no. You’d have no spark.
Make sure there is nothing holding them back from the proper proximity. There has always been wiring issues with anything duplicated in the aftermarket and VRs are no different. I’ve read more than a few case studies where reverse polarity of the wiring due to harness swaps and/or connector mismatch repairs caused problems.
You wouldn’t think a magnet with a coil of wire wrapped around it would care about polarity. But if you think of the positive or negative ramp of the sine wave as the reluctor wheel passes by, it becomes easy to understand why this simple mistake could confuse a PCM. Magnets are better today than in the past (rare earth vs. powdered), but as Henry Ford was once quoted “I like tires that are round, black and cheap, and I don’t really care that much about the round and black part.”
Manufactured magnets can weaken with age. When that occurs they will have lower amplitude (voltage) and cause hard start / no start conditions.
As with other electrical windings on a vehicle, a short circuit or open circuit can occur with these sensors. Many times they are thermal related. Other less likely culprits with VR sensors are damage to the reluctor wheel and crazy things like a rebuilt engine coming with a missing or incorrect reluctor wheel.
Sometimes, though, the damage is not visible. Because the Hall Effect sensor uses some solid state electronic circuits, it would make for good common sense to not trust one that has been dropped. Many, many years ago there were chronic pattern of internal failures with early GM 90 degree V6 engine Hall Effect sensors.
A light tap test was the order of the day to detect an intermittent sensor. Cave men types who wield wrenches tapped too hard. That Hall Effect failure is no longer epidemic, but I suspect there are a few “cave men” tap testing types still wielding wrenches (not reading Motor Age of course) and working on cars that wind up in your shop. My experience has always been where there’s a trend and a tech tip, there are a few abuses of the tech tip and no matter how long ago the trend was, the abuses still continue.
Optical sensors hate contaminates as much as your CD player hates a dirty CD and are more fragile than VRs. Optical sensors in Nissan distributors back in the 1990s were known for this problem. MRs are less prone to proximity issues but rank with Hall and Optical sensors in fragility and thermal failures.
Diagnostic Tips
For VR sensors, disconnecting and running an ohms test against the published spec will detect an open circuit problem. If the spec is nowhere to be found, remember that shorts and opens are relative to what you are measuring.
Regarding temperature, it goes without saying to think like MacGyver and pop the sensor in the freezer, then test again after getting it warm with a hair dryer. A heat gun is fine if you have good control of the heat. Because amplitude is extremely important at lower speeds, back probe the connection across the VR sensor’s two terminals. Set your voltmeter to AC volts and record the average level at normal cranking speeds. Consult the service manual but 2 to 3 volts of AC is typical at cranking speeds. At higher engine speeds, 30 or more volts is not uncommon. Obviously, a scope will tell you peak-to-peak AC voltage and let you look at the health of the pattern. Does it drop out? Are there sudden variations beyond the norm for that vehicle’s typical pattern? Keep in mind that although VR sensors create their own voltage, the wiring and module on the other end of their circuits presents an electrical load that will drop the amplitude of the output signal so always test with the sensor plugged into the harness.
Like anything else electrical, voltage drops can be a problem when checking a stubborn intermittent with a CKP or CMP. Simply disconnect the sensor and ignition module/PCM so you can connect a power source on one end of the circuit and substitute load on the other end of the circuit. Pick a load suitable for the gauge of the wiring to the sensor (something that won’t draw more than 10 amps should be fine) and perform a voltage drop test to detect wiring/connection issues that might cause a problem with the signal, ground or sensor power feed circuits. I’ve uncovered more than a few splice and terminal retention problems this way.
With Hall Effect, Optical and MR sensors, you sometimes can bump the engine around with the starter to check for a simple low/high state change on the sensor output. Once the engine is cranking however, a DVOM will have to be a model that contains a frequency function to be of any help to you in supplementing what the scan tool says for CKP and CMP speeds. Some scan tool data streams are simply not fast enough to catch anything less than a total sensor failure so a lab scope is the best tool for the job.
Some Hall Effect sensors can be unbolted from the engine to allow a metal feeler gauge or knife blade to be moved in and out of the window between the magnet and the Hall wafer to test for an on/off signal. All of these solid state sensors are subject to cracked solder and other heat and/or vibration related woes so apply heat/cold and tap test with a long screwdriver (gently) while watching the signal on a scope.
Normally there will be a DTC for the sensor not reporting in. If your drivability problem is erratic in nature, scope all CKPs, the CMP and RPM Ref/Tach waveforms to determine which sensor is breaking down or in rare cases, being interferred with by EMI affecting the wiring to the sensor. Sensor DTCs, misfires, surges and igntion cut-outs are all symptoms of a sensor that is erratic. If a CKP sensor is flatlined, naturally the symptom should be a no start unless there are dual CKPs. No CMP signal can, in some cases, cause no starts but normally the result is a low power or poor fuel economy complaint and a DTC from running in non-Sequential Fuel Injection (SFI) mode.
Sometimes scoping both CKP and CMP can help you detect a slipped timing chain or belt. Having access to a database with known good patterns (or creating your own database) can aid with this advanced use of a lab scope. This same technique also is useful in verifying variable valve timing problems. DTCs, rough idle and low power are all symptoms you might encounter. Keep in mind if the reluctor slips, or in the case of some Chrysler products the flyheel slips, the CKP signal will be out of sync with the actual position of the crankshaft.
Finally, there are housekeeping rules regarding these sensors and their replacements as well as replacement/reprogramming of PCMs. To calculate the normal mechanical variations (normal manufacturing tolerances) between the crankshaft or camshaft and their reluctors, the PCM on some engines must undergo a relearn procedure. A DTC will persist until the procedure is performed with a scan tool. Another CKP-related OBDII housekeeping rule is for Ford misfire monitors. This monitor won’t run after a Keep Alive Memory (KAM) reset.
To get the monitor to run, simply drive the vehicle and perform a few decels without braking. Defueled decels are the key to getting Ford crankshaft mechanical variations learned so the PCM can accurately monitor CKP speed variatons that might indicate misfire conditions.
As a matter of of fact, the scrutiny today’s PCMs put on crank and cam position sensors is mind boggling as you read the atom splitting details from various OEM websites. It’s no wonder these two sensors can be considered the MVPs of the engine management team.
