Duramax Synchronization

I recently took a call on a 2005 GMC C4500 Medium Duty truck with a 6.6L Duramax engine that would not start since a remanufactured engine was installed. The starter would crank the engine over but the engine would not run. The Engine Control Module (ECM) was setting code P0340: No Camshaft Position Sensor (CMP) signal for more than two seconds. The CMP and Crankshaft Position Sensors (CKP) were new. Using an oscilloscope, understanding the operation of the CMP and CKP sensors and their reluctor’s characteristics, and having a vehicle to compare test results with enabled us to diagnose the problem.

The 2005 Duramax ECM needs both the CMP and CKP sensor inputs for the engine to start so the code was cleared to see if it would reset. When the technician tried to start the engine, code P0340 reset. CMP sensor power, ground and signal circuits were tested and all were confirmed to have no faults. I suggested using an oscilloscope, if the shop had one, to check CMP and CKP sensor signal voltage patterns to confirm the sensors were functioning properly. The tech said he had one, but did not use it very much and was hesitant to perform a diagnosis with it. I suggested this was a case in which the oscilloscope could be really helpful. He wanted to improve his oscilloscope skills and build more confidence with it, so he decided to use it.

The CMP and CKP sensors produce square-wave signals when their respective reluctors pass by the sensors. The CMP sensor reluctor has two large notches and one small notch. The CKP reluctor has 57 evenly spaced small notches and one large notch equal in size to three of the small notches. The tech tested the CMP and CKP sensor signals at the ECM, superimposing the patterns on the screen. I was expecting him to see a CMP sensor signal with a flat line or maybe a pattern that had an improper voltage level that the ECM could not recognize. He stated the patterns looked good. I said it would be good to be able to look at the pattern from a vehicle with the same engine that was running normally. It was our lucky day—this was a fleet vehicle and an identical truck that ran was in the shop that he could test.

The tech tested the CMP and CKP patterns on the truck that ran and said that they had the same voltage levels and number of pulses as the truck that would not start. We seemed to be striking out. I asked him to send me the patterns so I could take a look at them. The CMP signal was not a flat line and the voltage levels were normal. Previously, I had seen issues with GM vehicles setting code P0340 because the CMP sensor pulses were too far out of time with the CKP signal to be recognized by the ECM. The technician’s patterns (Figure 1 and Figure 2) revealed that the short duration Yellow CMP sensor pulse occurred before the large gap in the Green CKP sensor signal on the engine that ran, but it occurred after the large gap in the CKP sensor signal on the engine that would not start. Something in the engine was not synchronized properly.

The technician removed the CKP sensor and placed the #1 piston on Top Dead Center (TDC). Then he turned the crankshaft by hand, counting the small reluctor notches as they passed by the CKP sensor opening until the large notch appeared in the CKP sensor opening. He performed the same test on the other engine. More notches passed by the opening on the engine that ran than on the engine that would not start. The CKP reluctor was incorrectly timed.

The patterns and test results were sent to the engine remanufacturer. They agreed with our diagnosis. The technician did not want to do anything to affect the engine warranty so the remanufactured engine was replaced with a different one. It started right up and ran normally.