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A valuable lesson in testing voltage drop

Tuesday, January 30, 2018 - 09:00
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During my visual inspection, I also noticed the odometer was displaying only dashes. Performing the vehicle network test using the Ford IDS resulted in eight DTCs stored in the three modules that reported — but showed no communication with the PCM or the IC (Instrument Cluster). I informed the owner that it is impossible to program a module that cannot communicate on the network. I also educated him about how programming a module does not “allow” it to talk on a network (as he had thought). Then I began my own testing. 

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Where to go? 

There are several tests published for when one or more modules are unable to communicate with a scanner. I reviewed them to refresh my memory of procedures that should be followed, and hoped they gave me some insight as to what might make it impossible to access those modules with the scan tool. 

In most cases, there are redundant steps to follow, which could waste a lot of time, that if one reads through the whole test before beginning to work on the vehicle, one could skip those and still not misdiagnose the problem. For example, why check the power at the fuses and then again at the module if the fuses are “good?” It would be safe to assume that the fuses are “good” if proper voltage is read at the module if voltage were tested there first. Of course, if the voltage supplied to the module wasn’t what it should be, one could then work towards the battery; look at what voltage was read at the fuse, at the battery connections to the fuse block, at the ignition switch, etc. 

Ford’s PTS site properly identifies the vehicle based on the VIN (including the original color and powertrain options).

I thought now would be a good time to take a quick look at the communication networks and the PCM wiring diagrams. If you’re reliant on one service information provider, then you might get a bit frustrated if you are the one trying to look for the correct network communication wiring diagram for this truck. I have a few providers, precisely for when I run into information that doesn’t match the vehicle I’m working on. That’s exactly what happened when I went to use my first choice. 

It’s instinctive to just look for a “Pickup” or “F-150,”, since “Lightning” wasn’t one of the choices available at the time I was entering the vehicle information. I saw two choices for a 2004 Ford, but the “Pickup F-150” choice showed me an incorrect diagram (and I went back thinking I erred when building the vehicle). The second time I made the other choice, “Pickup Heritage F-150.” What would you have done if your information provider’s data is incorrect? What would you do if neither choice matched the vehicle? Add the complication of possibly having the wrong wiring in the vehicle because the truck cab was replaced and now you’ve got a REAL mess! 

I chose an alternate source of service information, the OEM’s site to compare with my aftermarket information. Ford’s Professional Technician Society (PTS) site, also known as Ford’s Installer Support, showed the same wiring diagrams as Mitchell’s (Shopkey). I knew my choice was correct when I saw “Heritage F-150” is what Ford’s service information indicated this vehicle is — after I identified it by the VIN on their website. It would have saved me time to look there first in this case.   

Once I had accurate-for-the-vehicle service information, I was able to perform circuit testing. It is nearly impossible to do so without an accurate wiring diagram! The easiest test to perform verified the integrity of the Standard Corporate Protocol (SCP) network wiring by using continuity. Continuity to both SCP wires was verified between the PCM connector and the Data Link Connector (DLC) under the dash by back-probing both while the ignition key was off. Neither wire was shorted to the other, nor was either shorted to voltage or to ground, and both showed continuity between the connectors. This meant the communication network to the PCM was intact. 

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