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Power and ground testing

Wednesday, July 24, 2019 - 07:00
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If you know anything about me, you know that I am big on turning techs on to the power of voltage drop testing. I won't repeat the whole story here but let's just say that a simple diag kicked my butt because I didn't know what I didn't know.

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And judging by many of the comments I've read on a variety of social media automotive groups, many of you still don't either. And that's OK! Let's see if I can win a few more converts.

Did you check...?

I'll never forget one particular day in the shop. A fellow tech was struggling with an electrical concern when he walked over to my bay to ask for some help.

"Pete, can I get your help? I've got a GMC pick-up with a slow blower motor. I checked power and ground to the motor and it was ok, so I ordered a new blower motor. I just installed it and it's doing the same thing."

I checked power and ground(s).

I don't think I know any tech that doesn't know it's important to verify power and ground(s) when diagnosing an issue with an ECU but the truth is the test is valid for any electrical device you're considering replacing. No electrical load (that is, any electrical device designed to perform some kind of task - like a light bulb, a fuel pump or even an ECU) can function without a good, clean supply of voltage and an equally good, clean return path to ground.

While everything in an electrical circuit has some resistance, it's the LOAD that is the primary source of resistance — unless a thief is present!

Back to the story. I stopped what I was doing and walked over to my friend's stall. I asked him to show me how he had tested power and ground to the blower motor and the first thing he did was unplug the connector at the motor.

Mistake #1

The next thing he did was grab a test light, grounding the light to the instrument panel brace and inserting the other end into the open connector. With only two wires to pick from, he had a 50/50 shot of hitting the power feed on the first try. He reached up and turned the key on and selected "high" on the blower motor speed switch. The test light glowed brightly.

Mistakes #2 and #3

Looking at me, he said, "See? I've got power." He then shoved a T-pin into the ground side pin on the open connector and attached the test light's alligator clip to it. Once again, the light glowed brightly. "See, I've got a good ground, too. What could be the problem?"

Oh my...

Let's start with the mistakes

The first mistake my friend made was disconnecting the load from the circuit and testing an empty connector. And I think most of you recognize why. There is no load on this circuit and all we're measuring is Open Circuit Voltage (OCV). He might as well have been up at the battery itself for all the good the test did him.

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