Scoping Out the TDC/Crank/CYL Sensors
Over the last three months, three different members had problems with their TDC/Crank/CYL Sensors. Three for three! Ouch! Is this some kind of trend? Let's hope not.
This month we'll show how to test these sensors using a scopemuch simpler and more meaningful than the resistance checks shown in the factory service manual. We'll see how to "watch" the sensors in real-time action! If one isn't doing its job, we'll be able to "see" it immediately. And if need be, we'll see how to replace them.
First, what are they and what do they do? We saw a few months ago that the sensors are magnetic inductance sensors and we saw how to check their resistance. These three sensors tell the ECU what the engine is doing in terms of mechanical rotation.
The TDC Sensor is used to determine ignition timing while the engine is cranking-over and starting up. It tells the ECU that a piston is at TDC, but the ECU doesn't know which one yet. The CYL Sensor tells the ECU where the number one piston is. Once the ECU gets the CYL input, it has a reference for the TDC input, so it can determine which cylinder is at TDC and when.
When the engine is running, the Crank Sensor takes over from the TDC sensor and its input is used by the ECU to determine spark timing and fuel injection timing as well as rpm. If the Crank sensor reports an abnormal crank angle, then the ECU will ignore it and use the TDC sensor.
All three sensors take their cues from the cam pulley, where they're mounted. The CYL sensor is mounted on the front of the cam pulley and the TDC/Crank sensor is mounted to the back side (see the illustration below). Pulsers on the pulley activate the sensors.
Rememberthe ECU must have a CYL and a TDC signal to start the engine. It must have a CYL and a Crank signal to run the engine, although it can "run" in "limp home" mode without the Crank signal.
When the number one piston is at TDC (as indicated on the crank pulley marker), and the camshaft marker indicates number one piston's compression stroke, everything is in sync. This relationship was established when the timing belt was installed and tensioned. And this is the point in cam rotation when the CYL sensor will produce a pulse.
PGM-FI infers crank position from there on out. One complete revolution of the camshaft equals two revolutions of the crank shaft. So, when the CYL sensor produces its second pulse, the ECU will infer that the crank has turned two complete revolutions and that the number one piston is again at TDC on its compression stroke.
This is important to keep in mind. Some engines have a crank-mounted sensor. If the T-belt ever slips a tooth or two, the ECU will notice the difference in crank and cam angles immediately. Not so on a Vig! If the Timing Belt ever loses tension and jumps one or two teeth on the pulley gears, the ECU will have no direct way of determining this.
Testing and Verification
Testing is done at the same connector we used when we checked the sensors' resistance. However, we won't have to disconnect the connector if we use a scope. We can back-probe the sensors instead, using the test probe shown below. This probe uses a sewing pin for a needle and has a banana jack at the rear.
If we backprobe the WHT and ORG wires with our scope leads, we should be able to watch the CYL sensor.
You can see we've got our scope set to display from -10V to +10V on a 250-msec time-base. On this scope, that means the total left-to-right waveform is displaying 250-msec. Most automotive DSOs divide the screen into eight equal vertical segments and express the time-base in terms of "Time per Division." If it's set to 25-msec/div, then an eight segment display will take 200-msec to complete. Keep that in mind when setting the time-base on your scope.
We can "see" that every time the pulser moves into proximity with the CYL sensor's inductive pickup, the sensor generates a voltage. As the pulser rotates away from the sensor, the voltage drops... to zero. When the pulser works its way around again, the sensor puts out another pulse. Et cetera. We can actually watch it! Oh, the marvels of the automotive DSO!
If the time between the two events equals one rotation of the camshaft, we can calculate engine rpm from this scope display. The time base shown above is 250-msec. If the first event begins at "zero," the second event begins at... shall we say... 170-msec? (It's tough to estimate on this scope, because there's only one graduationat the half-way point.) If one revolution of the cam takes 170-msec, then 1,000 msec (1-sec) divided by 170-msec equals 5.88 revolutions per second. 5.88 times 60 gives us 352.8 revolutions of the cam per minute. Since there are two revolutions of the crank for each revolution of the cam, 352.8 time 2 gives us 705.6 rpm. Not as easy as tach, but still pretty darned close!
Here we can see the cam pulley and the CYL sensor. (See photo above.) The white arrows are pointing to (1) a raised tooth on the front of the cam pulley (left) and the sensor's pickup coil (right). The sensor is mounted so that the pickup coil is directly over the teeth of the pulley. Every time the raised tooth (the pulser) passes under the pickup coil, the coil will produce the voltage we just saw on the scope.