Simple Tests Can Help Diagnose Defective PCM on GEO Tracker
By Dave Martin
Vehicles:1991 through 1995 Geo Tracker.
Engine: 1.6L L4 TBI.
Symptom/problem: No start. No codes. There is spark but no injector pulse. There is no power to the injector during cranking. The PCM is not providing power due to a defective output circuit.
After verifying that there is consistent spark from the coil, disconnect the injector connector and check for power on the red wire during cranking. The PCM provides power on this wire during cranking and running. If none is present, provide power to the red wire through a 5 amp in-line fuse connected to B+. Attempt to start the engine. If it starts and runs, verify that the red wire to the PCM is good and that the resistance of the injector is between 1 and 2 ohms. If it is, replace the PCM.
Because of the design of the circuit, the diagnostics for a defective PCM can be reduced to these simple tests. Reducing a diagnostic routine to a simple procedure can only be done after the circuit is completely understood and all possible problems are reviewed.
Background: The 1.6 L engine found in the Geo Tracker uses a throttle body injection system where the PCM not only provides the injector ground side pulse, it also provides power to the injector. This power is run through a resistor circuit inside the PCM to reduce maximum current flow through the injector windings and is only present during cranking or running.
The PCM provides power to the injector when an rpm pulse is received at the PCM. This rpm pulse is generated by a camshaft position sensor (CMP) in the distributor and is sent to the PCM. When the PCM receives the signal, it looks at other values such as rpm, TPS, MAP, CTS and EGR to calculate the correct timing advance.
The PCM has direct control over the ignitor, which in turn controls the coil. If the ignition system has spark during cranking, then, of course, it is receiving this CMP pulse, and therefore should provide power to the injector.
By providing a separate fused power to the injector, the injector ground side operation of the PCM can be tested. If the engine starts and runs normally, the PCM is defective.
The resistance of the injector needs to be checked because a shorted injector could have damaged the PCM drive circuits and would have an effect on the new PCM.
Low resistance injectors in most vehicles are current limited by the PCM using the peak/hold method. When the injector is initially turned on, a current flow of about 4 amps is passed through the injector. This opens the injector up and generally takes about 1 millisecond. After the peak flow of amperage, this circuit shuts off and the hold circuit is engaged to keep the injector open for the duration of the pulse width. The PCM in the Geo Tracker does not use this method. Current flow is limited by a resistor built into the PCM and is in the circuit at all times. This control circuit is what burns out.
Some technicians suggest installing a power relay to the injector rather than replacing the PCM. This will not work because there is no current limiting feature in the circuit and will cause the ground side of the injector drive circuit in the PCM to fail in a short period of time. The correct repair is to replace the PCM.
Dave Martin is an Identifix GM specialist. To pay his way through college, Martin started a business he named Auto Repair at Your Home. It was so successful he ran his own shop for 10 years. He is ASE master and L1 certified. Martin helped rewrite the L1 test.