How to Test an ECU with a Multimeter

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Do you experience cold morning car running problems until it warms up? Well, the problem could be with your car’s ECU. But how do you diagnose it using a multimeter?

What is It?

ECU refers to the Engine Control Unit. It is commonly known as the engine control module. ECU controls a series of actuators on an internal combustion engine to enhance optimal engine performance. ECU performs its duty by reading data from multiple sensors within the engine, interprets the data using performance maps, and adjusting the engine actuators. ECU dynamically manages a couple of engine activities which include:

  • Control ignition timing. ECUs are fitted with variable valve timing, which takes care of the engine cycle at which the valves open. The system automatically opens the valves at a higher speed than at a lower speed. This function ensures fuel economy by increasing airflow into the cylinder, thus increasing power.
  • Control of idle speed. The ECU monitors the idle speed control in modern engine systems. The crankshaft position sensor primarily plays a role in engine functions for valve timing and fuel injection. Idle speed is controlled by a programmable throttle control system, which ensures cruise control functions and limitation of speed monitoring ECU section for reliability.
  • Control air-fuel mixture. Modern ECUs use fuel injection to transfer fuel to the cylinders. The system determines the fuel amount to inject based on the number of sensor readings. The air sensors notify the ECU if the engine is running with too much fuel or too little oxygen, or vice versa.
  • Controls electronic valve. Some modern engines have been made without camshaft, though with full electronic control of the fuel intake and valve control. These engines can only be operated with a static start to enhance efficiency and reduce pollution.

What is a Multimeter?

A multimeter is commonly known as a multitester or volt-ohm-milliameter (VOM). A multitester is a measuring gadget combining various measurement functions in one unit electronically. Some of these measuring functions may include; resistance, current, and voltage. There are two types of multimeters:

  • Analog multimeters

Analog multimeters use microammeter fitted with a moving pointer to display the readings on a screen. Digital multimeters that are commonly acronomized as DMM have multiple numeric displays modified to form a bar graph to represent the measured data.

Despite digital multimeters being a far more improved version of the analog multimeters, they come at a slightly lower cost and with high precision. Analog multimeters are also preferable in some circumstances.

How DO ECUs Work?

Modern Engine Control Units are fitted with microprocessors to enable them to process the inputs from the engine sensors efficiently. ECUs are made up of both hardware and software components. The hardware comprises electronic fittings on a ceramic substrate and circuit board.

The major component on the circuit board is a microcontroller chip that stores software. This component enables re-programming through the uploading of updated code. The updating of chips is referred to as an electronic engine management system.

A modified version of the Engine Management System gets inputs from other sources and facilitates control of other parts of the engine. They may also include some features like transmission control, anti-theft control, anti-skid brake control, and cruise control, among others.

Using Your Multimeter

A multimeter loads the circuit under test to some extent, with full-scale deflection of 50 microamps (amps) or “ohms per volt.” Here are four simple steps you need to consider while diagnosing the ECU system of your car:

  1. Get your multimeter and set it up to test for the circuit using the best range available multimeter gadgets. It is also advisable to use the current breaker with one of the leads as a precaution measure for your multimeter.
  2. ​Conduct an overall visual inspection. This is to ensure that every component is well connected and to spot any obvious problem. It is always advisable, to begin with, the easier tests before jumping to the harder ones.
  3. Test every component using your multimeter. Test the fuse and relay first and then perform the amperage draw. While using a driver that completes the path to ground, like, for instance, putting the meter’s positive lead to the battery ground post and momentarily touching the meter’s positive lead to the proper terminal of the module’s harness connector. If you happen to find no circuit reading, ensure there is power to the component. Turn the key on, and if the driver supplies power, carefully move the meter’s negative lead over to the positive battery post.(CAUTION! apply the current briefly to avoid burning out the component being tested)
  4. Record the reading. A current reading in excess of 1.2 amps is an indication of a faulty circuit or component.

Nevertheless, you may be testing the current where the ECU driver has failed already. The failure might be the ground circuit or primary power within the ECU. If you happen to get no results at all using the above steps, you might test the circuit with the harness disconnected. In this case, you are required to supply power and ground to the component being tested.

Extra Info…

While performing the ECU test, the ignition key should always be off. This is preferably the best and most effective way to verify the whole circuit with its connections. In case the problem still persists, you will be required to perform a voltage drop test on the circuit before abandoning your computerized ECU system.

There are some published service procedures to be followed while working on ECU related components. These procedures are geared towards avoiding accidental voltage spikes that can cause the future failure of the circuit board. Some common mistakes that the technicians make are unplugging ECUs while they are powered up. Some technicians also fail to discharge the static electricity from their bodies while handling sensitive electronic modules.

ECUs are often fitted with a data logger to record all sensor data for future analysis. The recorded data is useful in facilitating the identification of undesired behaviors and misfires. To enhance effective communication with the driver, the ECU can be connected to the dashboard to display the current data. The data link interface is located underneath the steering column.

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