“How to systematically diagnose a failed PCM low side driver”

The automotive industry like any other is feeling the strain of supply and demand for parts. Along with skilled labor and Automotive technicians who that can read, understand and comprehend a schematic, interpret scan tool data. While grasping how each system in a vehicle interacts with the other are a rare breed. As a advanced level and OEM master certified automotive technician in two different brands. This means l posses a diverse skill set that has led me away from the typical 2 vehicle bay. Into the world of mobile diagnostics a new frontier that I’m enjoying exploring with the mindset of diagnose never guess.

Collecting Data

    Upon arriving at a shop that was having trouble with a 2015 GMC Sierra . I conversed with the lead mechanic and shop owner. In order to get a idea of any other variables that could possibly affect vehicle current state. Then off to the patient l went to investigate. My diagnostic approach always begins with a stable power supply connected to the vehicle. Visual inspection, diagnostic tests, data analysis and then recommendations to remedy the concern. Following a simple diagnostic approach for performing tests for any automotive concern reduces room for error and creates a better chance for a correct diagnosis. 

A pre-scan dtc report reflected no communication U0100 from PCM by all modules on hi speed data bus.

Diagnostic Test

Minutes later I had the correct schematic pulled up for PCM module power/ground (fig 1) and communication by checking the PCM X1 connector making sure no male terminals were bent, broken, or missing. I proceed to conduct voltage tests in a left to right fashion in relation to battery ground up to circuit 5291 terminal 62. All tests are performed with the key on the engine not running with the PCM connectors fully seated. First up was ign circuit 439 terminal 51 and voltage results are 13.43v due to connected power supply (figure 2). B+ circuit 440 terminal 52 is 13.43v. Circuit 5991 terminal 72 shows battery voltage 13.43v (figure 3) also seen again while ignition on terminals 73,  67 and 62 shows 0v. 

Surprisingly within about 10 minutes I had already run across the circuit(wire) or module fault that needed further testing to clarify my findings. 

The schematic shown is of a normally off key remove state, indicated by the open relay and low side driver symbols. Since my tests up to this point are perform on this circuit in its energize” key on” with the engine off state. I expected to see near 0v for circuit 5991 terminal 72 (figure 4). This is a low side driver control circuit where the PCM relay control operation is completed by a transistor.

Diagnose and never guessing

My next course of action is to check the integrity of the other 3 Ign circuits and fuses (5290, 5292 and 5291). By removing the actual engine controls relay in the under hood fuse block.

To verify indeed power supply battery voltage of 13.4v is present at terminal 30. Moving on to check the entire relay output side is perform by turning vehicle back completely off and applying a auxiliary ground to circuit 5991. Then the relay is reinserted and audible click is heard while battery voltage of 13.43v is measured at terminals 73, 67 and 62 by back probing method. This confirmed even more that PCM low side driver (transistor) was indeed inoperable but the circuit was fine. Further more tests were perform to verify data bus communication was present at the PCM using a multimeter and lab scope along with load testing the PCM ground circuits 451 and other power supply ignition circuits which all had less than a 100mv drop.