Auto Wiring Harness Test from ATS

Testing auto wiring harness in your vehicle has never been easy using cheap tools and common sense. Save yourself the cost of expensive repairs by following these proven auto wiring test used by a lot of successful mechanics!

auto wiring harness

Shown above is another illustration how to test your auto wiring harness in a practical manner. Using "jumper wires" and cheap voltmeter, you can get a quick look if there is a shorted circuit in your engine wirings. Using your diagnostic link connector (DLC) of your vehicle, this auto wiring test can be done in minutes. Why go to an expensive garage if you can do this yourself!

When testing for shorted circuits in your wiring harness, you need to probe the voltmeter negative lead to pin 5 terminal and the positive terminal to pin 16. When you do this, you get the voltage drop between the ground distribution system which feeds all the ground connections to the ecm and other modules of your vehicle.

I use a short jumper wires that I both feed to the DLC terminals to secure wiring connections when performing this test. Hooking the voltmeter tester between pin 5 and pin 16 terminals should give you the following reading:

1.) With engine on run position but not starting, voltage reading should be slightly higher or equal to battery voltage. It is preferred that the battery is fully charged before doing this test.

2.) No voltage reading should be lower than 10 volts which indicates that a shorted circuit exist especially if the battery is fully charged. To test for shorted circuits, you need to unplug the defective component and watch if the voltage reading returns to the normal battery voltage (12 v). Practical test involves removing each fuse one at a time to identify which circuit is involved. Another example is to use the wiring diagram and test all the fuse volages on both sides. Before the fuse and after it (should be the same). Same can be done on the output voltage of the ignition switch or relay. The secret here is knowing the path and flow of the circuit which will be shown in the wiring diagram.

ecm wiring harness test

3.) If wiring is suspected as the culprit, a wiring diagram will be required so you can do a physical inspection of the affected circuit including performing a wire wiggle test. When you disturbed the suspected wiring, the voltmeter reading will show the changes in the voltage readings.

Note: Use an electrical insulator spray whenever you disturbed a wiring connection. Likewise, you can prevent moisture from wires that were pierced by spraying a silicone seal.

Auto Wiring Harness Tips:


1.) When disconnecting the ecm or any modules, the battery has to be disconnected to avoid damaging the ecm or it's terminals. To resume voltage test, reconnect the battery connection. All voltage test must be done live which means the ignition is turned on but engine not running.

2.) To get proper voltage reading, all battery connections must be connected. To determine the voltage drop of a component, you need to measure across the voltage wire supply of the component and the ground supply. Note also that the voltage value could be battery voltage or controlled voltage from the ECM and 5 volts is the common value. Never attempt to feed 12 volt supply to the controlled voltage or it will damage the ECM.

3.) Always use a proper wiring diagram before poking any wires at the ECM or any electronic device. You can buy a cheap digital voltmeter (under $20 at Amazon) to this job for you and using a test light or analog voltmeter is strictly forbidden or damage can happen to the ECM.

4.) If you suspect there is a definite shorted circuit in the wiring harness, turn the key off and disconnect the main battery negative cable connected to the frame and check it for corrosion. A lot of shorted circuits are attributed to this connection if not properly maintained.

If you can test the integrity of your auto wiring harness, you just eliminated half of the mechanics job. And once you are able to fix it yourself, it will boost your confidence knowing such jobs are expensive when brought to a garage.

Auto Wiring Test Tips:

1.) If you suspect an auto wiring harness failure, do a visual inspection first in the engine compartment. With the voltmeter leads connected to the DLC pin terminals, you can watch the voltage reading while an assistant wiggle the suspected wiring harness for you. A shorted wiring harness will cause the voltmeter reading to change readings if the shorted circuit is disturbed when the ignition key is turn on. And with engine running, you can observe the changes in voltmeter voltage readings if you rev up the engine speed. On some intermittent auto wiring problems, the shorted wiring harness won't show up until the engine is warmed up.

2.) A quick way to check a PCM failure is to verify it with a pocket size scanner. Priced under $50 it will tell if a sensor code fault exist narrowing your search to the specific component. You can also do it manually if you don't have a scanner by checking the reference voltage as described in Part 1 article of auto wiring test.

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