How To Test The Engine Computer

To troubleshoot the engine computer or Ecm in your car, proceed to the appropriate series of ECM tests, arranged by manufacturer. The following tests must be performed in sequence to test a defective engine computer . In addition to an obd scan tool , jumper wires are required for some tests. A remote starter switch is also helpful in performing many tests, otherwise you will need an obd scan tool. In the following procedures, all tests conducted with an obd scanner, or the voltmeter section of a voltlohmmeter (VOM), are made with the ignition switch turned to the "on" or "start" position. With the ignition switch in either of these positions, the voltage from the car battery should be present in the circuit wiring and all parts of the Ecm system. Obd tests must be made when the sys­tem circuits are "alive," or completed, and bat­tery voltage is available at the Ecm test connec­tions to be valid. All tests conducted with an obd scan tool, or the ohmmeter section of an obd1, are made with the ignition switch turned to the "off" position and no volt­age is present in any part of the Engine computer beyond the ignition switch. Obd diagnostic tests must be made while the Ecm circuits are "dead" and no battery voltage is available at the electronic control unit.

Obd Ecm Reset Procedures

Whether you're trying to repair a domestic or import engine computer, the basic procedures for troubleshoot­ing an Ecm are generally similar:
1. You first check the battery voltage to see that there is sufficient charge in the battery to power the Ecm. Most elec­tronic control units need a minimum of 9 volts.
2. Next you turn the key on and check to see if voltage is reaching the positive side of the ignition coil. Your Obd scan tool should read the ecm codes.

3. If the electrical system passed the spark i test described earlier, then you can assume the coil, distributor pickup, and
control module computer are working. The only exception here would be a system that misfires or dies intermittently. This problem indicates an intermittent short or open somewhere in the module or pickup, or in the wiring between them. If the engine computer failed to pro­duce a spark, you have to figure out whether it is the coil, the pickup, the module, or the wiring somewhere in be­tween the Ecm.

4. To rule out the coil, you turn the engine off and check both the primary and secondary resistance of the computer. Always refer to the exact specifications listed by the manufacturer, but generally speaking, the primary resistance be­tween the Ecm positive and negative ter­minals will be between 0.5 and 3 ohms. A very high reading indicates an open in the coil primary windings, so replace it if that's what you find. Secondary re­sistance between the coil negative ter­minal and high-voltage lead is usually somewhere between 8000 to 12,000 ohms. A very low reading would indi­cate an Ecm ground whereas a very high reading would indicate an open. If the Engine computer primary and secondary resistance readings are within the man­ufacturer's specs, you can rule it out as the source of trouble.
5. The next item to check is the pickup coil in the distributor. To test it, unplug the pickup connector and measure its resistance with your obd test tool. Again, refer to the manufacturer's specs. The general range of values here is 150 to 1200 ohms. A very high reading would indicate an open in the coil or its leads, while a very low reading would indicate an Ecm short. You should also check to see that the pickup coil is not grounded by checking resistance between both pickup coil leads and the distributor housing. Your obd meter should read infin­ity if the engine is not grounded. If the computer passes these tests, you can rule it out as a possible cause of engine failure code problems. Note: On some Chrysler vehicles, the distance between the pickup coil and the arms on the distributor shaft arma­ture can have a bearing on ignition per­formance. This is referred to as the air gap setting, and it must be within specs for the application.
On ecm computers that use a "Hall Effect" pickup, you should check to see that the shutter on the un­derside of the computer is grounded. If it isn't, the engine won't generate a signal to the electronic control unit.

6. Now you're down to three things that could be responsible for your no-start or intermittent ecm malfunction condition: the engine computer or the wiring or a bad ecm. To check the wiring, you'll need to refer to your shop manual again to determine which connector leads do what. Basi­cally, you rule out wiring problems by checking the wires between the ecm and pickup for continuity, then check­ing the engine power connector for bat­tery voltage when the key is on. Many ecm's are also grounded through this computer connector. If your obd test shows no opens or shorts in the wiring between the module and pickup, and the module is still sending fault codes then chances are you have a bad ecm.