Injector control circuits
good morning everyone. So briefly here is what is going on. truck starts and runs good for a few minutes, and then it begins to misfire on four of the cylinders ( group 1 ) and at that point it sets the 5 circuit codes for that group. randomly it stalls setting the 5 circuit codes for group 2 . I know this is a rather common issue on these trucks. in the past i have seen injector 7 and injector 2 electrical connectors go bad causing these problems . i have also seen injectors cause these problems and i have seen engine wiring harness rubbed through causing this problem. so i started my testing and while the engine was firing on all cylinders i wiggled injector 7 and 2 electrical connector , that did not duplicate problem . i wiggled the entire engine harness and still did not duplicate problem. i unplugged injector 7 and 2 electrical connector and inspected them looking for the chaffing dust that is a telltale sign of bad connection and connection looked clean. i then removed connector from the fuel control module and ohm all injector circuits and they were all the same ( i do not remember what the exact reading was right now but they were in spec.) so at this point still i question the integrity of the circuity i took my test light and with the connectors unplugged at the fuel control module i feed voltage into the power side of the injectors and with 4 channels hooked up to my scope i measured the voltage that came out on the control side of the injectors , this covered all the wires ,connectors and injector coils , and i wiggle tested the harness while watching the scope . i tested both groups this way and found no problems . so then i watched powers and grounds to the fuel control module through a failure and they all remained good . i watched the power and control circuits to the injectors with the scope and the 48volts that is supplied to the injectors would drop down to 10 volts and then it would begin misfiring. it seems to me we have a fuel control module that is going bad . i just have never seen them fail quite like this before just wondering if anybody has any thoughts on whether they have seen a fuel control module fail this way before
Yes, they fail like this all the time! Good luck finding a FICM right now. Bosch has stopped making them and they've been on galactic backorder from GM for some time. We have had very poor results from the aftermarket rebuilds. There are several shops in my area that have LLY Duramax's sitting around as lot ornaments waiting on FICMs.
I would move my leads to the powers and grounds for the module and see what results you get. Possibly a poor connectio is causing the module to fail after running for some time.
Hi Clement, Test the main power to the banks of injectors. The IDM has built in safety circuits to allow the engine to run using every other injector on a bank. As I recall these were larger gauge white wires with a red stripe, in the IDM connector(s). Reference a correct wire diagram to confirm. Test each one by back probing the wires with T-pins. When the engine runs normal. both will show…
Thanks every one for your replies. through testing we were pretty confident it needed a new fuel control module. And yes we have been finding out these modules are a bit tough to get and quite expensive. So i was just making sure i was not missing something. i have in the pass replaced these modules for leaking fuel into the module . but then usually the module was dead , crank no start. but…
Did you check injector current draw for the power supply at the time of fault as in a high current draw and then shutting down a bank also I once had an ECM on a machine that was really expensive and there was only one in the manufacturer's Parts system so I got a local electronics shop to check out the e c m it took 3 hours he had to increase his microscope power to 40x to find a bad solder…
Yes I did actually do current ramp with the lab scope on the main power wires and there was never any circuits pulling more current which also leads me to believe the fuel control module is unable to produce sufficient voltage.
We have concluded that the fuel control module is faulty on this truck. We have been having trouble find a replacement. Customer choose to have module sent to a rebuilder and at this point we do not have it Back yet . So I will try to post a fix when we get it back.
Thanks again every one for your reply to this message. customer chose to have this fuel control module rebuilt. we just got it back and the rebuilders report was they found and fixed the problem. so yesterday i reinstalled and the truck is running good . Customer came and picked truck up last night
Glad you got it sorted out. GM had these by the pallet load once upon a time, but like everything else, stocks are depleted or thrown out after a vehicle goes over the magic “10 year old mark”. I have worked at dealers that literally threw away good old stock instead of selling it to wholesale warehouses. They would simply “write it off” as a loss.