It's All or Nothing on This Diesel
Many seasoned technicians have run across a problem vehicle that could not be explained. I took a call one day on a Dodge diesel truck that would idle or go to wide-open throttle - one or the other.
It idled perfectly and had good power; dare I say, too much power for normal driving. The gauges were inoperative and many of the warning lights on the dash were lit up. There were two codes in the Cummins engine control module (ECM). It showed code P0516 (battery temperature sensor voltage too low) and code P1652 (J1850 circuit shorted to ground). The J1850 circuit is the programmable communication interface (PCI) bus communication circuit. There was also a code P0122 stored in the powertrain control module (PCM) for throttle position sensor (TPS), indicating the voltage was too low.
One of the first things I like to check when I suspect a bad computer is the 5-volt reference wire. Often I see a shorted sensor or wiring pull the 5-volt reference voltage down, which can cause the computer to function incorrectly (or in some cases, not at all) and this can explain inoperative gauges. In this case, however, the 5-volt reference was normal.
Next, I asked the technician to check the voltage from the accelerator pedal position sensor (APPS) on the scan tool. The APPS was reading normal, yet the engine rpm did not respond properly to movement of the pedal. The TPS voltage in the PCM read 0 volts at all times, which was verified using a digital volt ohmmeter.
On this vehicle, the ECM receives the signal from the APPS. The ECM then sends a corresponding voltage to the PCM for air conditioning and transmission controls. We often refer to this circuit as the "mimic" circuit because the ECM mimics the APPS. I asked the technician to check this mimic circuit between the ECM and the PCM. I thought the ECM might be bad, but wanted to find some way to prove it.
We needed to check the mimic circuit wire for shorts to ground. The easiest way to do this is to unplug the connector from the ECM and check for voltage on the wire at the ECM end of the harness. The PCM sends out 5 volts to the ECM. If the wire is OK, it should have 5 volts. If it does not, then the wire will have to be checked for a short to ground. As it turned out, the technician found it did not have 5 volts when unplugged from the ECM, so he followed the wiring harness toward the PCM. In the rear of the engine, he found a braided ground strap rubbing against the wiring harness. It had rubbed through and was shorting out the mimic circuit and the PCI bus wire.
Theoretically, the mimic circuit is an output of the ECM and is just used as an input to the PCM to control the A/C and the automatic transmission. Was this causing the ECM to not control the fuel properly? When the wiring was repaired behind the engine, everything worked normally. The technician and I were ready to condemn the ECM on this vehicle. Due to the high cost of the ECM, we really wanted to know for sure, so I had the technician look at the one circuit that I knew was not working properly.
If I had not directed the technician to look at this circuit, we would have installed an ECM on this vehicle, only to find the problem was still there. Our carline specialists can direct technicians what to check and where, but ultimately, we rely on them to be observant enough to spot those oddball wiring harness issues that may be hidden behind the engine or some other obscured area.
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