ECM RESET: GM VEHICLES

By Roy, BAT Auto Technical

An ECM (computer) reset occurs when either circuit power, circuit ground or reference signals to the computer are lost or overloaded

If the vehicle is having intermittent driveability problems such as surging, bucking and/or stalling and the CEL (check engine lamp) is seen either solid or flashing with no trouble codes being stored then, there is a good chance that a reset is occurring. This can sometimes be seen with a scan tool as “NO DATA” or, the scanner seems to “freeze up” while the reset is occurring.

Any time you have an intermittent driveability problem with no CEL or stored trouble codes the power, ground and reference circuits should be checked by making sure all connections are clean and secure. The ECM can’t store codes if it doesn’t have power or ground circuits. I’ve found that the ground circuit that is at the thermostat housing on some GM vehicles is a potential ECM ground problem so, I add a ground wire from the battery negative post to the ground at the thermostat housing. I’ve also found ground circuits that were attached to a head bolt, and the bolt was broken. So if you have a ground circuit at a head bolt be sure and check the bolt.

Another type of reset is an internal ECM circuit failure. These usually occur after the vehicle has been driven for a period of time and are affected by temperature and vibration. To check for this condition, remove the ECM from it’s mounting but do not disconnect it from the vehicle harness. Place it as close to the heater outlet as possible and set the heater to full heat and highest fan speed. After the ECM becomes warm tap firmly with the tips of your fingers on the case of the ECM. Nothing else, just your fingers! If the engine sputters or dies or the CEL comes on or flashes while your doing this “tap test” the problem lies within the internal circuits of the ECM. This will require an ECM replacement.

Another type of ECM reset is the output reset due to quad driver and/or injector driver resets. I’m working on an article dealing with quad driver circuit faults and hopefully I can post it soon.

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