in Technical Articles

Variable valve timing gives engine designers the best of both worlds—power with economy.

It is common knowledge that it’s easier to fix a broken vehicle if you have a clear understanding of how it works when it’s not broken. When you have the time, it pays to connect your diagnostic equipment to known-good vehicles! This is especially true when an oscilloscope is involved. If you only pull out your oscilloscope when you are faced with really tough problems, you’ll need to spend time familiarizing yourself with how your scope works before you can get down to business. But if you put the time into building your scope proficiency, you’ll also become more familiar with how certain waveforms should look when a vehicle is running properly. So when the time comes, you’ll have a much easier time spotting abnormal waveforms produced by a vehicle with a problem.

To illustrate this point, we’ll begin with a vehicle that has no known problems and no stored DTCs. It’s working the way it’s supposed to. Our first case study concerns a 2001 Nissan Sentra GXE, equipped with a 1.8L engine (engine code QG18DE) and variable valve timing (also called Nissan Variable Cam Timing or N-VCT). Nissan first introduced this system in 1987 on the VG30DE, which was installed in the 300ZX in this country.

Variable valve timing allows engine designers to incorporate the best characteristics of different camshaft profiles into the same engine. In the past, designers needed to make compromises to achieve often contradictory emissions, power and economy goals. With variable valve timing, when the valves open, the relationship between intake and exhaust valve opening (overlap) can be changed by the PCM, depending upon engine speed and load inputs. At least on this Nissan Sentra system, valve lift and duration are not altered and the valve timing of the intake camshaft is controlled. The exhaust cam timing can’t be changed by the PCM.

These systems can malfunction in a number of different ways. If the intake cam timing fails to change at higher engine RPM, the customer may notice a loss of engine performance. If the electric solenoid (cam phaser) on the end of the camshaft fails, or if the phaser’s oil passages become clogged, the phaser may become stuck in the advanced position. The engine may run normally at higher RPM, but have a rough idle or other driveability issues at lower RPM.

We’ll use this Sentra case study to demonstrate a known-good relationship between the crank sensor and cam sensor signals, both of which are used by the PCM to control the QG18DE engine’s variable valve timing system.

The crankshaft position sensor (POS) is located at the rear of the cylinder block, facing the gear teeth of the signal plate at the end of the crankshaft. The sensor consists of a permanent magnet and Hall effect integrated circuit (IC). When the engine is running, the changing gap between the high and low gear teeth sections causes the magnetic field near the sensor to fluctuate. The Hall effect sensor converts the changing magnetic field into a digital voltage output to the PCM. The PCM uses the changes in the POS sensor signal to detect variations in engine speed.

On vehicles not equipped with the Nissan antitheft system (NAS), the POS sensor signal is accessed at PCM pin #75. One crank revolution could potentially produce 36 evenly spaced pulses. However, the signal plate has two missing teeth, so during one crank revolution the POS sensor is exposed to one missing tooth, then 17 teeth, then another missing tooth, then 17 teeth.

The camshaft position sensor (PHASE) senses gaps in the exhaust cam sprocket, which allows the PCM to identify the camshaft position for a particular cylinder. This sensor consists of a permanent magnet, core and coil. When the engine is running, the high and low sections of the sprocket teeth cause the sensor gap to change. The changing gap causes the magnetic field near the sensor to move. This causes the sensor’s generated A/C voltage to vary.

The PHASE sensor signal is accessed at PCM pin #76 and/or pin #66 (without NATS). The exhaust camshaft turns at half crankshaft speed, so two crankshaft revolutions are required to produce four groups of pulses in the firing order sequence. On the scope, look for a single pulse, followed by three pulses, followed by four pulses, then two pulses before the sequence repeats. The numbering and sequencing of the pulses in this series is the same as the engine’s firing order (1, 3, 4, 2). Refer to Figure 1 for an example of known-good POS and PHASE sensor waveforms.

The POS and PHASE sensor waveforms have been enlarged in Figure 2. We’ve overlayed the waveforms and further enlarged them for a better illustration of the relationship between the two in Figure 3.

What did we learn? Each series of pulses in the PHASE sensor waveform begins between the 16th and 17th crank teeth pulses in the POS waveform. The first or only PHASE sensor pulse transitions low five crank degrees before the first POS sensor signal pulse. This is a normal relationship between the two sensor signals, and will be an important piece of information in our upcoming diagnosis. These waveforms were collected on a 2001 Nissan Sentra GXE 1.8L with 35,000 miles.

Our second case study involves a 2000 Nissan Sentra GXE, also equipped with a 1.8L QG18DE engine, but with 98,000 miles on the odometer. The customer’s complaint is that the MIL is illuminated. We’ve checked for diagnostic trouble codes and DTC P0335 (crank sensor) was pulled. The crank (POS) and cam (PHASE) signals will be checked to see if a clue can be found regarding the P0335. The PCM has been removed from its case for easy signal access.

Refer to the series of waveforms in Figure 4 through 8. Are the crank and cam signals in sync? It looks like the cam timing is off. The crank-to-cam relationship is incorrect.

We’ve gradually zoomed in on the sensor waveforms in this series of screen captures, to illustrate the relationship between the two. In the last two screen captures, it’s very apparent that the cam sensor’s waveform is retarded.

Figure 9 shows the retarded cam timing when compared to a known-good vehicle. Further diagnosis will be required to determine the cause of the problem. Is the timing chain stretched? Has the chain jumped a tooth? At 98,000 miles, either of these defects is a distinct possibility.

We’ll be back next time for a further look at Nissan’s Variable Cam Timing.