New starter, no start

scope & scan Nissan POS CKP CMP repair shop training technician training automotive aftermarket

The first time I ran into this doozy of a problem, I was lucky in that a friend and fellow mobile diagnostics professional knew exactly what the problem was. John Thornton out of Chicago saved my bacon on the first one. I have since seen this defect half a dozen times. So as all good deeds should go, I am passing the knowledge on to my valued readers in the hopes of saving their bacon one day.

The story usually goes like this: A Nissan Maxima comes in with the condition of the starter not cranking the engine. The starter is deemed defective and is replaced. Now the new starter will crank the engine over but the engine will not start.

The ignition firing sounds sporadic and out of time. Occasionally, the engine will start just as you release the key from the cranking position. Or, a transmission or engine was recently swapped into the vehicle and the exact same starting problem described above occurs.

First, this engine uses two crankshaft positions sensors (CKP) and a cam sensor (CMP). One CKP is called the reference (REF) sensor and is mounted just below the crank pulley on the front of the engine. It detects piston Top Dead Center (TDC) at each 120-degree mark (720 degrees of crankshaft rotation divided by six cylinders equals each piston at TDC compression once every 120 degrees). The second CKP sensor is called the position (POS) sensor and is mounted near the flywheel. This rear mounted POS sensor senses trigger teeth mounted on the engine side the flywheel. Each tooth represents 1 degree of crankshaft revolution. The POS signal is used by the PCM to determine ignition dwell time.

It is this POS CKP sensor signal that can be affected by these two different installed defect scenarios. What gives? Electrical noise from the starter motor crosses into the POS sensor signal causing the ignition to misfire. In Figure 1, we can see the POS signal in red, the CMP signal in blue, the TDC signal in green and ignition current in yellow. Notice all the electrical noise in all three engine position signals with excessive noise in the POS signal causing the ignition to fire infrequently, out of sync and with an incorrect dwell time.

The most frequent cause of this defect is a recently installed and incorrect starter motor. Apparently the 1995 Nissan Maxima originally had an eight-tooth starter drive gear. Later this was changed to a 10-tooth drive gear. Some later Maxima models had an 11-tooth gear. It seems it is very easy to get the wrong starter for your particular application. The explanation of why the amount of gear teeth results in creating excessive noise from the motor windings is way above my pay grade. We have just seen that it does.

An old and defective starter also can create the same type of excess starter motor noise. The starter does not always have to be new, though this is more typically the case. The reason the vehicle occasionally will start when the ignition key is released from the start position is that the momentum of the engine causes it to continue to revolve even after the starter is no longer driving the crankshaft. This period of rotation sometimes allows enough good clean POS signals to occur to fire the cylinders and start the engine. In Figure 2, we have the same three engine position sensor shown but we have replace Channel 1 (yellow) with a high current probe on the starter positive cable. Notice the right side of that waveform capture when the starter is no longer rotating: the POS, TDC and CMP signals all clean up and provide functioning normal signals.

There have been some cases of a recently installed engine or transmission that created an imperfect ground connection between the engine block and the bell housing. Apparently any oxidation or corrosion on these mating surfaces will create a bad connection which results in the same noise being introduced into the POS signal. I have found that adding an extra ground wire between these two assemblies does not work, even if using jumper cables. You must separate the engine from the transmission and clean these surfaces with some type of abrasive cleaning pad to correct this problem.

When faced with this type of defect scenario first inspect the vehicle for a recently installed starter motor or transmission or engine assembly. Be sure to clean and voltage drop check all starter and ground connections also. Then, if need be, use your oscilloscope to check for excess electrical noise in the three position sensors.

Jim Garrido of "Have Scanner Will Travel" is an on-site mobile diagnostics expert for hire. Jim services independent repair shops in central North Carolina. He also teaches diagnostic classes regionally for CARQUEST Technical Institute.