Knowing how to perform a voltage drop test is an important skill to have. It's a test that isn't difficult to perform and that gives a YES or NO answer (YES, the problem is this circuit or NO, keep looking). Let me give you a real world example of how this test can help you.
You can find this tutorial in Spanish here: Ford F150 No Prende Por Caída de Voltaje (Estudio De Caso) (at: autotecnico-online.com).
It was one of those mornings (that I'm sure can you relate to) of perfect weather. Not too cool, not too warm and the most bluest of blue skies. After having spent the humid and hot Texas summer roasting inside the shop, life finally felt worth living. Before I could really take it all in, the first diagnostic of the day rolled in. A 2000 Ford F150 Pick up.
The customer's complaint: as soon as the headlights were turned on, the engine would stall. If he only turned the headlight switch to park-lamps, he was OK. Any day driving, as long as it was with the headlights off, he was OK.
He had already spent money trying to get the problem solved elsewhere to no avail. In desperation to continue going back and forth to work he attached a jumper wire to the right side headlight low beam circuit. He would manually connect it to the battery positive terminal for driving at night. Can you imagine what a pain in the neck that must of been to do!
Getting back to the problem at hand, I had seen and repaired this kind of thing before. A ground circuit somewhere in or on the vehicle that wasn't doing its job. If only I could remember what I had done on that previous vehicle (that had the exact same symptoms) to solve the problem. The only thing that came into my mind was that the ECM grounds probably were bad. But, I was sure that that hadn't been the problematic ground circuit on the previous vehicle.
The Start Of The Troubleshooting Process
Anyway, after the boss had assured the customer that we would do our best to figure it out and the truck was in my bay, I started my diagnosis testing the basics. Visually inspecting battery terminals, voltage drop testing the engine and body ground circuits. I even fast tested the alternator with a multimeter. Nothing.
Still not remembering what I had done the time before to solve a case like this, I decided to voltage drop test the ECM ground circuits. The ECM is located on the right side of the engine compartment behind the battery. (As you can see in the photo, the battery has to be removed to gain access to the ECM's connector.)
This is how the voltage drop test was done:
- I removed the battery to gain access to the ECM's connector.
- Whipped out my multimeter and attached a wire piercing probe to the RED lead (of the multimeter).
- I attached the wire piercing probe to one of the ECM ground wires.
- I reinstalled the battery.
- I put the multimeter in Volts mode, lowest setting.
- Started the engine up with the headlights OFF and then I touched the center of the negative battery terminal with the black multimeter test lead. I read the multimeter's volt reading. And....
Interpreting The Results Of The Voltage Drop Test
The reading was good. 100 milliVolts. But remember, the owner said that it stalled with the Headlights Switch in the ON position. So then I:
- Turned the headlights ON. The engine immediately stalled.
- I went back to my multimeter, leaving the Key in the RUN position and the headlights ON.
- I then touched the center of the negative battery terminal with the black multimeter test lead again.
- And I got the exact same reading as before. 100 milliVolts.
I tested the other ECM ground circuits (always tie up all loose ends in your diagnostic process). Now there wasn't any doubt that the ECM grounds were not the problem on this Ford F150 Pick Up.
The above voltage drop test was done pretty fast (this is a quick test with surefire results). Then it hit me!!! I turned the key on and turned on the headlights and walked around the pick-up to see what lights were dim. The park lamps in front were OK but the tail light park lamps where very dim. I knew it then that I was on the right track.
I removed the left tail lamp assembly to expose the wiring to the bulb. I pierced the Ground wire with my wire piercing probe connected to the multimeter and voltage drop tested the circuit. The voltmeter reading came out to a whopping 7.5 Volts!!!
I pulled my Power Probe out of its hiding place in the tool box. The Power Probe enables you to either power a circuit or ground it with a flick of a switch (I LOVE this tool).
With the key on, engine off and the Headlight Switch in the ON position I connected the Power Probe to the bulb's Ground circuit. I then proceeded to Ground it and voila, all of the Park Lamps shone brighter (normal radiance). Now I called out to my fellow tech to start the truck (while I was still grounding the circuit and with the headlights still on) and the engine started and kept on running! I released the switch on the Power Probe (to release the ground) and immediately the engine stalled.