Diagnosing a Brain Dead Ford Ranger
On Friday I was asked to program a replacement PCM on a 2008 Ford Ranger. The vehicle had been diagnosed at the dealer as a bad PCM. It was a no-start with the PATS (immobilizer) lamp flashing. The instrument cluster had hyphens all the way across the odometer. When I attempted to program the PCM, there was no communication. I switched to read codes and found codes listed for no communication between the Instrument Cluster and several other modules including the PCM. I was unable to spend time on it so I scheduled time on Saturday to check it out.
Forward to Saturday AM. After verifying the same communication issues with the original and the replacement PCM, I decided to do some troubleshooting. I first disconnected the battery and checked the integrity of the CAN network. It had 117.8 Ohms. Something was missing here. From the block diagram I could see that the Instrument Cluster and the PCM each had the terminating resistor for opposite ends of the network. Since it was simple to disconnect the PCM, I started there. The network stayed steady at 117.8 Ohms. So the Problem appeared to be in the wiring between the PCM and the DLC. Next test was reading continuity between the terminals at the PCM and the DLC. Wires came in on pins 11 and 23 on the center connector. What? There was continuity. What is going on here. I looked at and worked with the connectors that plug into the PCM. The network ran through the center connector. I examined the pins at the PCM. I did not have my drag test pins so I was not able to test the barrel end. In working with the connectors I noticed that the detents that hold the release bars were all worn or broken. The connectors moved out slightly after pushing them in place. I surmised that when the last PCM was installed the detents were broken. After time, the connectors walked out enough to loose the network connection. After getting the connectors to stay in place I read the CAN resistance again. 60.3 Ohms. Hopped in the truck, turned the key and it started. The mileage read correctly. No more codes.
The best practice for repairing this is to replace the connectors or the entire loom. This was not an option I could sell. So what I did was use some Q-Bond to hold the bars in place. If needed it could be chipped off to access the PCM but it would hold the bars in place. Again we see that having the techs follow a flow chart or them using the trouble code as a diagnosis is not the best practice. The end user went to the dealer for top notch diagnostics and was led down the wrong path. In one hour the problem was found and corrected without selling any parts. This was done successfully.
Nice work! It's always nice to diagnose a network issue with an ohmmeter
Glad you found it! I had an old POS "suped-up" S-10 in here one time, and the trick to the no-start was pulling sideways on one of the PCM connectors (to tweak the circuit board). We cobbled something together to hold it cocked, and told them they needed a PCM. They are still driving it cobbled (of course).
Good work on the diagnosis and fix Michael. The one thing that sticks out though, is the frequency in which shops or individuals make unsubstantiated statements that a problem was diagnosed or miss-diagnosed "at the dealer". As both an independent shop technician, followed by many years as a dealership technician, I've seen these claims errantly made in both camps. Some dealerships will make
Hi Martin, Thank you for your constructive criticism. It is good to see other perspectives and learn from them. The owner of this vehicle had purchased it from a private party. In looking at the PCM, it has fairly fresh Silver paint on it. A replacement had been done previous to the sale. They had no idea of any earlier repair. The shop that replaced the PCM did not recognize the issue with
No worries Michael. We are all here to serve together and hopefully raise the bar to a level where truly professional technicians receive the respect and remuneration that they are worth. I worked with some of the best and some of the worst, in my first 12 years as an independent and in the following 23 years as a dealership technician. Unfortunately hacks abound everywhere, despite the