This vehicle was a diesel and was a horse of a different color because it used a separate Engine Control Module that was specifically used to run the Diesel engine. The crank sensor fed a direct RPM signal to the ECM and there was a cam sensor signal that was also supplied to the ECM coming from the engine driven injection pump. The ECM in turn would output an RPM signal to the PCM so that it could properly control the transmission and the alternator. So I now had to run some tests on the ECM to determine what was going on.
I scanned the ECM and found a code P1693 which indicated a fault in the PCM so I kind of ignored this code because the ECM was playing the “Blame Game” (Figure 5), I next looked at some ECM PIDs and found that it was receiving the RPM signal from the crank sensor and an RPM signal from the cam sensor (Figure 6). The ECM inputs did not seem to be an issue so now I was leaning towards an output signal issue with the circuit going back to the PCM but at this point I needed access to the ECM so I could put my scope on the signal lines.
I had the shop pull the fuel filter housing on the left side of the engine block to gain access to the ECM that was mounted to the side of the block (Figure 7). I placed my scope lead on the RPM output signal to the PCM on channel 1 and placed my other scope lead on the crank sensor RPM Input signal to the ECM on channel 2. I started the vehicle and you could see that the crank sensor RPM signal to the ECM was fine with good triggering transitions from 0-5 volts but the RPM output signal to the PCM was flat lined at 5 volts (Figure 8). The PCM signal line was not shorted because it was elevated at about 5 volts.
I next decided to think out of the box by opening the RPM signal line at the PCM and sending the RPM signal from the Crank sensor into the PCM to simulate that the engine was running to test PCM system strategy. My results were as expected because the alternator started to charge immediately. I went back to view the PCM data and you could now see that the desired voltage was 13.8 Volts and the charging voltage was also 13.8 volts. It would have been nice to just leave it at that just to get the truck out the shop door but the issue now was the PCM was seeing the vehicle running at 2528 RPMs when actually the truck was idling at 700 RPMs (Figure 9).