Power and ground testing

July 24, 2019
One step that is always a prerequisite to module replacement is a test of that module's power and ground circuits. Are you performing this test correctly?

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.

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.

Without getting into an all-out electrical theory lesson here, I think it's important to understand that the reason electrical circuits develop issues is a result of a change in circuit resistance caused by whatever fault we eventually find. For example, a loose connection can lead to an increase in resistance. A corroded connection certainly adds additional resistance. And all are unplanned for - they are the "unwanted" sources of resistance, the "thieves" that rob voltage potential from the primary load - in this case, a blower motor. And what happens to current when you add resistance?

It goes down.

Let me try to clarify that a bit. It's a fundamental electrical principle that all available voltage potential will be used to overcome the resistance in an electrical circuit. And while everything in that circuit has some resistance, the load (the device doing the work) is the biggest, as well as the primary, source of resistance we need to focus on.

A thief can take on many forms — corrosion and burnt or loose connector pins are just a few examples.

The second part of that principle is that every individual source of resistance in the circuit is going to consume its fair share of that voltage potential. Because the other elements in the circuit; the fuses, the wiring, the switch contacts and the like, have extremely small amounts of resistance they will take very little from the total. The load should get the majority share.

However, when there is a thief in the mix — that "unwanted" resistance I mentioned - it can rob the primary load of its full voltage potential. This is the principle of voltage drop and it provides us with a troubleshooting method we can use to uncover the "thieves."

With the connector unplugged, the blower motor circuit is obviously "open." If it's open, there isn't any current flow, is there? And if you don't have current flow, there isn't going to be any voltage drop to measure. That was mistake #2.

Mistake #3 was grounding his test light at the instrument panel. Even if he were performing the test correctly, he is only testing a portion of the circuit. I know you know we have to get the power feed from the battery but remember it is just as critical to get those little electrons all the way back to the battery. Always reference your test equipment at the battery, even if you have to make a 20' test lead to do it.

I explained to my friend then what I'm sharing with you now. Reconnecting the blower motor and turning on the key, I operated the blower motor throughout its different speed settings and found the motor was working, but slower than it should be in every speed. The next step I asked him to do was to carefully backprobe the blower motor connector and measure the voltage available at the power feed with his voltmeter and not his test light.

Be careful when placing a backprobe in a connector. Don't damage the weather seal in the process. Try to follow the wire through the seal instead.

He measured a little over 6.0 volts. Bingo!

The thief shows himself

If my friend had used his test light it would have glowed a lot less brightly. But do you understand why?

It goes back to what I tried to state earlier. ALL available voltage will be consumed by the various sources of resistance in the circuit. ALL will take their fair share away from the primary source of resistance, the load.

Typically, this will only amount to a few tenths of a volt and the engineers factor that in when designing these circuits. What they can't factor in is a damaged connector pin, a corroded terminal end or even a loose battery ground cable. But all of these conditions can add resistance and act as "thieves," stealing potential away from the primary load.

When my friend measured on the power side of the blower motor, we both expected to see a number close to the voltage we would measure at the battery with the key on. When we only saw 6.0 volts, we knew right away that somewhere between the blower motor and the battery's positive post, a thief lay in the darkness, siphoning off the missing voltage potential and keeping it for himself.

This test kit from AES includes a variety of adapters you can use to nearly elminate the need to pierce or probe a connection. It also makes attaching a substitute load easier and avoids damage to the connector pins.

We know he's there, now we have to flush him out. How? By tracing the electrical path back toward the battery, starting at the easiest points first - connections, switches, splices. When we see the meter return to a value more or less equal to the battery's measured voltage, we'll know we've passed him. Then we start back the other way, kind of like "hot boxing" a runner in baseball, until we isolate the little bastard.

In my friend's case, it was easy to find. The first place we stopped on the way back to the battery was the main harness connector where it passed through the firewall. Measuring voltage on the interior side of the connector got us the same number we had at the blower motor connector. That proves that the fault isn't in the cabin, it's on the engine compartment side of this circuit path.

Moving to the engine side of the connector got us a totally different measurement. Here, the voltmeter reported a voltage reading damn near equal to the reading we had at the battery, less a few tenths. That's normal, as I said before, since everything in the path has some resistance.

Opening the connector, with some difficulty I might add, we found the power feed connector had overheated and burned, creating the additional resistance that was stealing voltage potential from the motor. Replacing the connector and damaged wiring connector pins restored the voltage potential to the blower motor.

Voltage drop testing in simple terms

The basic testing method for measuring voltage drop is simple. Measure what's going in with the load on and compare it to system voltage under the same conditions. Make sure you measure as close to the load as you can get, with your negative meter lead referenced to battery ground to measure the entire circuit path. Then move your meter lead to the ground side and measure the voltage there. You should read a few tenths of a volt and no more.

Let's recap that. If you do measure voltage, let’s say 6.0 volts, when you move your meter lead to the ground side of the load, what is that telling you?

This throws more guys and gals off than any other test measurement I can think of. Threw me for years, too, so you're not alone if you're shaking your head and trying to grasp how you can measure voltage when both of your meter leads are attached to grounds.

You can't see the headlight I'm using but you can see it lit as I measure the voltage drop on one of the grounds of this ECM. Using a substitute load makes the process of verifying power and grounds dynamically much easier.

I've described how an electrical "thief" can rob voltage potential away from the primary load. And I think this is where the confusion begins to set in. We tend to think linear, that the voltage is moving down the wiring from the battery, to the load and on to ground. That must mean that in order for any "thief" to steal from the load, it HAS to happen upstream - between the battery and the load.


When you measure voltage potential on the ground side of a working load, the reading is telling you that there is a "thief" on the path from the load back to the battery's ground post. And even though you'll see the correct voltage reading going IN to the load, you won't see the correct reading coming out!

The "thief" is waiting on the other side and DEMANDING its share. That's what you're measuring on the ground side. The share of the total voltage potential the "unwanted" resistance is taking for itself. It's not linear, it's more like everyone dipping into a communal watering hole. Think of the water as the total voltage potential available to the circuit and around it stands every possible source of resistance.

The little guys — the wiring, the connectors, the fuse, the splice — they all get shoved off to the side and they can only get a little water out of the watering hole.

The primary load is the lion, and when he's there everyone else backs off. He gets the, I'm sorry, "lion's share" of the water supply.

But along comes a hyena. Bigger than the others, not as big as the lion, but he's getting a fair share of that water. And that leaves less for the lion.

"Unwanted" resistance can occur on either side of the load but ground side issues are, by far, the most common you'll run into. Don't let that meter reading mess you up. Remember that ANY reading you get on the ground side of a load that is over a few tenths is a big RED FLAG telling you where the "thief" is. And you'll find him in a similar way we found him on the GMC - by working your way toward the battery negative post until you see your meter reading normal again, and then backtracking to isolate his exact location.

Applying voltage drop testing to ECUs

ECUs, whether for engine management or rolling up the driver's window, are primary electrical loads first. In order for them to work they have to have a clean power supply and a clean path to ground.

And while you can perform the voltage drop test on them the same as you would any other load, using a substitute load (I use an old sealed beam headlight) makes testing easier. For instance, I don't have to wonder what operation or process the module has to conduct in order to complete the connection through its own internal load, or (in the case of multiple feeds and/or grounds) what ground pin goes to what power feed internally. Using a substitute load also allows me to apply a bit more current load on the electrical pathway and that can help reveal problems I might not otherwise so easily see.

As I just mentioned, there may be more than one power feed and more than one ground path to test. The power feeds may also be "hot" under different conditions. The connector pinouts in your service information system are a great resource in identifying which pins do what, as are the system wiring diagrams. As part of your pre-test preparation, take a few moments to review them carefully.

(Diagram courtesy of ProDemand) Be sure to carefully review the wiring diagrams associated with the ECU you're testing to insure you identify all the power and ground connections.

Also keep in mind that we'll be tapping into the ECU's harness connector(s) to attach the substitute load. I have a couple of breakout lead kits that I use so that I can properly match my terminal connector to the ECU's connector, avoiding the possibility of damage in the process.

With our substitute load ready, the proper attachment leads chosen, and the pins to be tested identified, we can move on to disconnecting the ECU from the harness. Be sure to follow the OEM process here — some specify that the battery needs to be disconnected prior to disconnecting the ECU to prevent voltage spikes from damaging the circuit board. It's also a good idea to make sure you ground yourself and get rid of any static electricity you're holding on to.

Personally, I prefer to test the ground paths first. The majority of issues with unwanted resistance are on the ground side. To begin, I connect one side of my substitute load to a power feed pin that I've identified as "hot at all times" (if possible). Then I connect the other side of the load to my first ground pin. The headlight should illuminate if my connections are correct and there are no issues with the ECU's circuit.

Then I grab my voltmeter and measure the voltage drop on the power feed side, right at the ECU connector pin if I can. This just eliminates any potential bad reading I may get from the substitute load's connection. If the power feed tests within specs, I move on to all the ground pins, one at a time, taking each measurement as I work my way through them. If you do find a voltage drop problem, be sure to follow through and correct it before you move on to the other pins.

Once the ground side is verified, I move back to the power feeds - again, testing each one, one at a time, and correcting any issues as I go.

And one side note here. If you did find a voltage drop issue during any of these tests, retest the ECU's operation to see if the original concern has been corrected before you install that expensive, and potentially unneeded, replacement.

Whether it's an ECU or a tail light bulb, a primary load can only work properly if it has a good, clean power feed and good, clean ground. And the only way to know for sure is to perform a voltage drop test to dynamically verify the circuit's integrity. If you didn't know how before, you know how now!

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