Electrical System Testing | The Power Probe

The Power Probe

There have been multiple versions of the Power Probe. The original version has a LED that glows red to indicate power or green to indicate ground. It has a switch that allows the user to supply power or ground to the circuit. The Power Probe II has the same features with the addition of an audible high pitched tone when power is indicated and low pitched tone when ground is indicated. It has 2 white LED’s on the end to illuminate the work area. Power Probe Ill has the same features as the other units but also has a voltmeter added. This version makes it convenient for the tech to test a circuit and components by measuring or applying voltage and/or ground.

Use extreme caution when using this tool. Ensure that you know which circuit you are testing before applying power or ground to it. A mistake can lead to major component damage.

Real World Testing- 2003 Caravan No Brake Lights

2003 Dodge Caravan 3.3 Liter | No Brake Lights

This vehicle was scheduled with a customer concern that the brake lights did not illuminate. He had already tested the brake fuse and all the bulbs. Additionally he replaced the switch, but the lights still don’t work. Someone told him the lights were computer controlled so he decided to bring it in to the shop for a diagnosis and probable computer repair.


Wiring Schematic – 2003 Dodge Caravan

I was able to easily verify that the lights did not work when the switch was pressed. Next, I retrieved the wiring schematic for the brake light system which revealed that the power originates with fuse 26 in the Integrated Power Module and goes directly to the brake lamp switch. The power exits the switch and goes directly to the lamps. The brake lamp switch is used as an input to the Front Control Module.

Using my Power Probe III, we will test fuse 26. The test concludes the fuse to be good. Next step, back probe the GRY/RED wire at the brake switch. There should be B+ on this wire at all times. The Power Probe indicates an open circuit. There is neither power nor ground on this wire. While pressing the brake pedal, the tool is now showing 0 volts with continuity to ground. This proves the switch is working and providing a complete circuit to ground through the bulbs. Now lets use the Power Probe Ill to supply power to the circuit and have someone verify if the lights are now illuminated. Confirmation of the lights proves the problem is somewhere between the fuse and the brake switch.

It is relatively simple to lift the fuse box out of its bracket for testing. This way we can verify there is power exiting the fuse box. Next we remove the left front inner fender to trace the wire harness as it exits the bottom of the fuse box and is directed to the interior of the van. Once the inner fender is removed, I noticed there is a green spot in the wire harness.

Upon closer inspection I find there is a lot of corrosion on the GRY/RED wire that is supposed to supply the B+ to the brake light switch. I have also finds that several other wires are”nicked’. After repairing the wires we tape up the harness so the problem won’t occur again. After the repair was completed the I was pleased to see the lights all worked properly.

This post was written by: Martin Hand


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About Martin Hand

ASE Certified L1 Advanced Mastertech. Martin Hand has over 15 years experience in Asian and European Import Auto Repair. Specializing in electrical diagnosis, engine performance, AT/MT transmission repair/rebuild. Martin is also pursuing a degree in Computers Science & Information Systems starting at Portland Community College while he plans to transfer to OIT. Certified in Java application level programming, experienced with other languages such as PHP, Ruby, JavaScript and Swift. Martin has future plans of automotive diagnostic software development.


  • Sarah says:

    I am having problems with my turning signal lights. On my 2003 dodge caravan 3.3 engine lights shut off my signal lights work I turn them on to normal or high they dont work at all we replaces the clicker part in the steering wheel and the box that had the lights butten on the dash also the light bulbs and still won’t work. I was wondering if this means my fcm (front control module) needs replaced to make my signal lights work again!?!?!

    • Martin Hand says:

      Sounds to me like you’ve replaced the parts I would think would cause this problem. At this point you need to get a wiring diagram and try to trace down the problem that way. I couldn’t tell you for sure if a control module has failed or not. I jave seen it before on these vehicles but there is just too many other things it could be.

  • Joe says:

    Can a used or aftermarket tcm be programmed to a 2000 Subaru Legacy gt

  • Yashwanth says:

    The multi-function switch is a few swecihts housed together in one unit. Just because one operation of the unit functions does not mean the others are ok as well. In most cases, the multi-function switch is the cause. \You can use a multi-meter and test voltage input and output to be certain.

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