No Start Problem on a VT365

 Hello, all. I recently worked on a customer 4300 box truck that wouldn’t start. When the vehicle refused to start for the customer, he replaced the batteries. The truck then wouldn’t crank and he couldn’t figure it out, so he had it towed to us.

Before I go on I will take a shortcut and decode the acronyms used in the text:

ECM: engine control module

IDM: injector drive module

ICP: injection control module

IAT: intake air temperature

EBP: exhaust back pressure

Vref: the 5 volt source voltage to most sensors on a vehicle

  

                One of the other guys got it to crank; when the customer replaced the batteries, he hooked the ECM and IDM ground wires to Batt+. I was sent over to get the thing running, and the first thing I did was to check the stupid stuff-fuel, oil level, anything disconnected, etc. and everything looked ok. The ECM and IDM fuses in the battery box were good.  I hooked up the scanner to check for codes and check the ICP pressure while cranking. It had several codes for sensor voltage- it looked like everything on engine Vref was dead. Of course, this meant that the ICP sensor wasn’t showing any information, either. The only sensor that appeared to be working was the IAT sensor, but in looking at a wiring diagram it appeared that sensor has a different voltage source than the rest.

                I checked for reference voltage at the ICP sensor, and it had less than 1 volt. I found the same thing at the EBP sensor on the same circuit. Since the intake air sensor was right next to the ICP sensor, I ran a jumper to a backprobe to cheat power to the ICP. I wanted to see if just missing engine Vref was the problem or if I had something more. When I put 5 volts to the reference voltage circuit the other sensor connectors also showed the 5 volt Vref but as soon as I plugged in the ICP sensor the voltage dropped to about half a volt, so I thought there must be a problem in either the Vref ground or the signal return. I went to the wiring diagram and ohmed from the ECM to the ICP sensor. As I expected there was continuity, but in ohming each conductor to ground it appeared either that the Vref and signal return were shorted together, or to ground, or both. It looked like the customer had melted something when he hooked up the module grounds to power, so now I would need to overlay the harness for engine Vref and return from the ICP sensor.  

                Before I did that I wanted to see if I could find out where in the circuit the problem was so I didn’t need to run any more overlay than necessary. The only sensors I had left plugged in from earlier were the engine oil pressure and the engine coolant temp, so I unplugged them one at a time and checked circuit function after. As soon as I unplugged the ECT sensor the Vref appeared on the circuit and everything ohmed good! I plugged the rest of the sensors back in and checked datastream on the laptop. All sensors now were feeding back to the ECM, so I cranked the engine to check injection control pressure. Three times cranking showed the pressure built to only about 325 psi, which is not enough to start the engine. One of the guys who works on the Ford 6.0 has a known good ICP regulator so that could be easily checked by swapping out. To do so I had to remove the doghouse and use a custom (bent and cut down) wrench to change it, but after the change the engine started immediately and ran well.

                When the ECT sensor and the ICP regulator were changed the engine survived a period of idling followed by a test drive, and released to the customer.

                I assume from the above scenario that the customer hooking up the ECM ground to power somehow burned the ECT sensor to short; the engine wouldn’t have run with all the sensors not working. The failed ICP regulator may have been the original reason for the no start, then the customer ran the batteries down and changed them. I do know that from now on I will unplug all sensors from the circuit before checking it for problems. This is not part of the procedure in the manufacturer troubleshooting process, and I have never seen a sensor shorted like that, but if it happened once it can happen again. If I hadn’t happened to unplug the ECT sensor before starting to run overlays I would not have found the real problem and would have done a bunch of work for nothing.