In the last issue of GEARS, we explored testing techniques for failing digital sensors, and covered a sound diagnostic plan of attack. In this issue, we’ll cover another facet of testing: crankshaft and camshaft correlation.
Obviously, a failed crankshaft position sensor can prevent the engine from starting, but the same condition can be caused by other problems. Lack of spark or fuel are common issues we deal with on a daily basis. But valve timing, and the computer’s perception of those timing events, can also prevent the engine from starting.
This condition could be caused by an improper relationship between the crankshaft (CKP) sensor and the camshaft (CMP) sensor. The word “correlation” is often used in diagnostic trouble code definitions when these issues are present. Unfortunately, correlation issues don’t always set codes, or codes that do set may be very vague.
Let’s jump right into diagnosing an engine that won’t start because of a correlation issue. We’ll look at 2005 Nissan Sentra S with a 1.8-liter engine. The customer explained the condition like this: “Sometimes it starts and dies, but most of the time it just doesn’t start.”
The car was towed into the shop. The customer’s explanation was right: The engine cranked but wouldn’t start. Occasionally it would sputter to life for a few seconds.
The first logical step is to connect a scan tool and look for anything obvious. The only code in memory is a P0340 — CMP sensor circuit bank 1.The definition, with the word “circuit,” sounds like the code was set by the comprehensive component monitor and relates to an electrical failure, such as a sensor or wiring problem.
Service information for this vehicle defines the code as camshaft (CMP) sensor phase. A phase, or correlation issue, is much different than a circuit fault. So what’s the problem: a circuit fault or a phase issue? Time to connect and see.
As we discussed in the last issue, the first step is to consult the wiring diagrams and connect an oscilloscope. In this case, pin 14 on the computer connector (called PHASE) is the camshaft (CMP) signal: You’ll want to connect channel A (blue) of your oscilloscope to this wire. Pin 13 (called POS) is the crankshaft (CKP) signal: You’ll want to connect channel B (red) to this wire.
We cranked the engine and captured a waveform (figure 1).
As you can see, the digital sensor signals are switching on and off cleanly, which is what you want to see. The appropriate voltages for the crankshaft and camshaft sensors on this vehicle are 0 to 12 volts, or 0 volts to system voltage to be exact.
In this case, the signals are making it to 12 volts, but the peaks appear “dirty.” This can be due to a couple issues. First, the vehicle is cranking and voltage is fluctuating. You’ll see this more often on a 0-to-12-volt signal than on a 0-to-5-volt signal.
Second, the vehicle is connected to a battery charger. Battery chargers can cause stray voltage fluctuations that interfere with the electrical signal. No matter: both sensors are generating the appropriate signals and the on-and-off signals are clean.
At this point, we know the problem probably isn’t a circuit fault. We’re getting signals to the computer, so the ground, reference, and signal wires are working correctly. We also know that the sensors are functioning, so it doesn’t have a circuit fault. So the next step is to move on to the “phase” issue.
To check phase, we need a known good waveform to compare to our capture. Luckily, the Nissan P0340 trouble chart provides a known good relationship. This is rarely the case, but for this car, we have the information.
The key to taking a known good waveform and comparing it to a potentially bad waveform is to look for something that’s easy on the eyes. A quick observation of the camshaft signal reveals that the sequence of pulses mirrors the firing order of 1-3- 4-2. Further investigation reveals that the falling edge of each cylinder’s first pulse falls in the middle of the missing crankshaft sensor pulse; check the red line (figure 2).
Note: Every vehicle is different; in this case, we used a point that was easy to see. Draw your line wherever it works for you. The point is to reference the two sensors’ timing relation to one another.
Now that we have a known good waveform, we can compare it to the waveform we captured from our subject vehicle (figure 3). The green line indicates a similar correlation point on the captured waveform.
It’s obvious that the two signals aren’t synchronized correctly. The green line doesn’t fall in the middle of the large crankshaft gap as it did on the known good waveform. The camshaft signal is actually to the right of where it’s supposed to be. The camshaft signal is shifted to the right, which means that the camshaft signal is retarded.
To analyze these signals even further, we could count the crankshaft pulses and determine how many degrees the valve timing is off. This vehicle displays 72 pulses of the crankshaft signal in a 720º event.
Imagine the empty spaces in the crankshaft signal to be filled with corresponding pulses: 720º divided by 72 pulses means that each crankshaft pulse is equal to 10º. If we eyeball the reference lines, it looks like the camshaft signal is about 15º to 20º retarded.
Wait! We’re not done: If we count the teeth on the camshaft gear, we’d find 42 teeth. 360º of camshaft rotation divided by 42 equals about 8.6º per camshaft tooth (figure 4).
If we apply this to our previous results, the timing chain has most likely stretched and skipped teeth. The estimation? 8.6º times 2 teeth = 17.2º. That’s close enough to make the call on a timing chain replacement.
Another important point is that this vehicle has two problems with the same root cause. First, camshaft timing is off. But it isn’t off enough that the vehicle shouldn’t run. It should still be able to run; it should just run poorly.
Second, the crankshaft-to-camshaft relationship is off enough that the computer’s confused. It isn’t firing the ignition coils at the appropriate time or injecting fuel correctly. The combination of valve timing and improper fuel and ignition operation are what’s preventing the engine from starting.
Crankshaft and camshaft timing have always been important for proper engine operation, but it’s even more critical in today’s vehicles.
Using the crankshaft-to-camshaft correlation scope technique is very valuable when diagnosing engine issues that aren’t as easy to pick out, such as no spark or no fuel. And it can be a very clean way to confirm a mechanical issue that might otherwise require engine disassembly to confirm.