Auto Wiring Test For Shorted Circuits!

This practical auto wiring test can save you a lot of money in vehicle cost and aggravation. There is no need to bring your vehicle to a garage or use expensive tools to diagnose auto wiring shorted circuits.

auto wiring test

Shown above is a simplified solution in finding wiring shorted circuits. The correct term is "shorted to power", meaning the ground circuit is able to access the power supply on it's own. Testing for shorts is easy by using your engine diagnostic link connector (DLC) plugin. This test is done "live" by using the MINMAX feature of the digital voltmeter or DVOM. This voltmeter can be bought online thru Amazon and the last time I looked at the price was $19.83 (INNOVA 3320).

It's normal for your vehicle to fail sometimes if you have an auto wiring shorted circuits. This is inherent because of the manner your vehicle wiring harness is manufactured. Example, if the wiring harness is too close to the exhaust manifold, it will create a short soon after a few miles of driving. The same thing happens if somebody worked on your vehicle whether in the dealer or not. Sometimes the mechanic forgets to secure the wiring harness after replacing a defective component and it will soon develop a shorted circuit if the wire rubs the vehicle metal frame. The mechanic could also forget to seal the wiring connector wires when working on it resulting in corrosion which leads to shorted circuits.

When caught by vehicle manufacturer before your warranty expires, they normally issue a RECALL or service bulletin to fix the problem. But when the problem occurs when the vehicle is out of warranty, this becomes your problem! So what do you do? Bring it to the garage? Of course not if you know how to test it properly using cheap tools that you can afford, right?

Wiring shorts caused numerous engine problems like no starts, battery drain when parked overnight and a lot of intermittent problems which may or may not give a code. Weird lights coming in your dash panel is one of the common symptoms of having an auto wiring shorted circuits.

Auto Wiring Harness Test:

To start this auto wiring test, put the positive lead of the voltmeter to the DLC pin 4 terminal. You might need a short small wire to insert to the pin terminal if the lead wont connect. (You will see this jumper wire example in Part 2 of this article). Then put the negative lead of the voltmeter to the battery negative post and turn your ignition key to the on position without starting it. If you get more than 0.250 volt, then you have a short.

To verify which wire circuit is affected, start removing the fuse at the fuse box one at a time until the voltmeter reads zero. Then when you find the affected circuit, you can trace the shorted wire by wiggling the wiring harness until you see the voltmeter gives you a voltage drop reading which is higher than 0.250 volt.. If you also have the wiring diagram of the circuit from ATS, you can basically check the other components which failed beside the wiring harness. Remember, a zero volt or below 0.250 volt reading means no shorted circuit.

To check if the shorts occurred in the ecm circuit itself, move the positive lead to the pin 5 terminal. Using the same process, you test the shorted circuit this time by unplugging the ecm or other components related to the ecm. (see my wiring tip notes below before unplugging the ecm). Other modules like the body control module could be affected too but it can be verified using a wiring diagram. Now when you move the voltmeter positive lead to pin 16, it should read battery voltage but not lower than 10.5 volts which is the average voltage reading if the battery is good.

If the battery is fully charged, you might get around 12.5 volt range as long as the engine is not running. Once you get the engine running, you will get 14 volts or more which shows your alternator is working. Please remember these tests are the basic way to start troubleshooting wiring short circuits and can be expanded depending on what you find initially.

Auto Wiring Test Tips:

1.) If you suspect the ECM has failed because of a failed shorted wiring in the circuit. Always check the reference voltage. The best convenient way to access this reference volt supply is your TPS sensor located on top of your engine. It has 3 wires and one of them is the reference wire. With key on engine off (KOEO), test those 3 wires and if none of them shows 5 volts, this shows your ECM has failed. (12 volts is worst because you definitely has a short).

Before condemning the 5 volt reference though, disconnect all sensors that has 5 volt reference volt supply including those output devices. The 5 volt reference supply could be shorted by a defective sensor. A quick way to test it is to unplug the TPS sensor and if your 5 volt reference goes back to normal, the sensor itself is no good. Using a wiring diagram, you can actually check the vref 5 volt signal right at the ecm terminal. To see if problem is the wiring or sensors, disconnect the vref at ecm (some mechanics cut it). If no signal at ecm itself, ecm is suspect. If it does, the sensors or wirings are the primary cause.

2.) This voltage drop test is done using a cheap voltmeter and measured across the battery negative terminal and the DLC pin 4 and pin 5 terminals. If you want to see if the DLC device itself is shorted, move the voltmeter pins at either pin 4 or pin 5 as your negative lead and pin 16 as your positive lead of the voltmeter. This is explained better in Part 2 article.

3.) The voltage drop from the battery to pin 4 is expected to be lower when measured direct at battery negative post 5. Why? because in pin 4, it connects directly to the battery ground terminal post/wirings. However if you measure it between battery negative terminal post and pin 5, it passes thru other ground wiring harness circuits like ground distribution box.



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