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Diesel charging system diagnostics

Saturday, December 1, 2018 - 08:00
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I had a shop call me for advice on a 2002 Dodge Ram Truck with a 5.9L diesel engine that had a charging system issue (Figure 1). I probably field at least 50-70 calls a day and there are many times that I will provide my shops with free tech advice over the phone. This issue seemed simple to me because Chrysler, for many years, has used a strategy to control their alternators externally. In the early days, they mounted voltage regulators on the firewall but once onboard computers came into play the regulator worked its way into the circuit board of the Engine Control Module. The alternator field circuit was usually fed an ignition feed and controlled by a ground feed on the opposing side of the field circuit.

Figure 1
Figure 2

The owner of the vehicle was a do-it-yourselfer and had changed the alternator with an aftermarket one but it did not resolve his problem. The truck was then shipped to the shop where they checked all the wiring and decided to opt for an OEM alternator to see if that would resolve the charging system issue (Figure 2). When this didn't work, the shop gave me the call.

Before I make the drive

I ran through all the steps with the shop technician and educated him on how the system should work and he mentioned that there was no power feed coming out of the PCM to feed the one side of the alternator field circuit. I explained to him that it was possible that the PCM was bad or that the PCM was sensing a partial short to ground in the feed line and was possibly turning off an internal driver to protect itself.

The next day the shop tech called me back to explain that he temporarily ran dedicated wires for both the power feed and triggering lines of the field control circuits between the alternator and PCM. The alternator still would not charge but if he supplied the 12 volts into the power feed circuit the alternator was charging fine so he decided that he was going to order up a rebuilt PCM unit from the dealer. I told him that once he purchased the PCM and installed it, to call me so that I could swing by and program it for him.

Later in the week I received the call that all was ready to go so I ventured out to his shop at the end of my day to program the PCM. Once I arrived there I set up my old Chrysler DRB III scan tool as a pass thru device with an interface cable to my laptop. I configured the PCM with the vehicle VIN number and then downloaded the necessary software into the PCM. The truck started up but guess what? We were back to square one. We both sat there scratching our heads and then I offered him to let me perform a full diagnostic on the vehicle in order to retrace ALL the steps that had been taken and he agreed.

Diving in a little deeper

I hooked up my scan tool and pulled codes from the PCM. One code was P1765 that indicated a loss of ignition feed to the transmission relay and the other code was a P0622 indicating that the generator field was not switching properly (Figure 3). These codes both had a common power feed issue because the PCM was still not sending the 12-volt output required to satisfy their circuits. There had to be a reason why the PCM was not doing its job and I decided to look at some data PIDs hoping to find a piece of the puzzle that could guide me in the right direction.

Figure 3
Figure 4

When I was viewing the data I saw something that raised my eyebrows. There was no RPM signal getting to the PCM and the desired charging voltage was 0 volts (Figure 4). The PCM was not performing its task of charging the alternator because as far as it was concerned the vehicle was not running. So then how was this vehicle running without an RPM signal? Why didn’t the PCM set a code for loss of RPM signal input? I now have to pull some diagrams to do some onsite strategy research because at this point I had no answers.

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