How to Conduct a Voltage Drop Test

Hi, everyone. Welcome to the Delphi How-To Series videos. I'm Clay Pennington, Senior Product Specialist here at Delphi Products and Services. This next video we're going to do a voltage drop test. First I'll demonstrate from the power side and then from the ground side. We always put safety first, so be sure you wear your safety glasses. This is a test on the power side, and you're going to need a digital multimeter or a DMM, access to the battery, and access to the fuel pump connector. In this car the battery's in the back seat, so we've accessed the positive lead on the battery. I'm going to move our access cover here to get access to the fuel pump cover. There's usually a number of screws holding the cover. I'm going to have to pry it off. It's usually stuck there after a long time. But if you're lucky enough to have an access cover, it just means you don't have to pull a fuel tank. So there we can see we have access to both fuel pump connectors, one for the power and the ground for the fuel pump itself, along with fuel sending unit, along with the fuel tank pressure sensor. So now that we're ready to do the voltage drop test on the power supply, I have my positive lead hooked up to the positive cable on the battery. In this case, this battery is under the backseat of this car. My negative lead of my meter is hooked up to the power feed wire for the fuel pump. We have our meter set to 20 volts D-C, and I'm using a special connector for back probing this connector when it's still plugged into the pump. With the key turned on and the pump running, we're looking for .2 volts or 0.2 volts or less. Go ahead, Jeff. Turn the key on. There we go, the pump is running. It was at 0.19 volts. That's less than 0.2 volts. We're good with that. So we know the resistance is okay on this power supply. Next, we'll move on to the negative side. Okay, so now we're ready to move on to the ground side. The voltage drop on the ground side is very similar to the positive side. You just switch the meter leads. Now you've got a negative lead to our meter hooked up to the negative post on the battery under the backseat of this car, and the positive lead of our meter is hooked up to the negative side or ground side of this fuel pump connector, which is the black wire on this fuel pump connector. The meter's set at 20 volts D-C. Now, with the key running, we're looking for less than .2 volts. Go ahead, Jeff. Turn the key on. So, key was running. We had 0.18 volts which is well within specifications. We had less than two-tenths of a volt drop, so the ground side circuit on this is great. Now that we've done our voltage drop test and our readings were less than 0.2 volts all our resistance was good. If you have more resistance than that or your voltage drop is greater than that you're going to need to check for high resistance in the power or the ground circuit to make sure your pump runs effectively. That's how you do a voltage drop test. Be sure to check out our other Delphi How-To Series Videos that can help save you time in your shop every day.