How to correctly perform automotive power and ground tests

Oct. 29, 2018
Ever increasingly complex electronics are changing the aftermarket repair landscape. If you aren’t comfortable with electrical troubleshooting techniques, you need to get comfortable – and soon.

I struggled with the concept of voltage drop testing until a contest held by my last employer embarrassed me enough to go home and do what was necessary to learn the technique. As a result, I found myself solving more electrical concerns for our customers and that didn’t go unnoticed by my boss. It didn’t take long for him to refer other techs in the shop to me when they were stumped by some electrical gremlin. While I certainly didn’t mind, I also didn’t want to do their work for them. After all, we were all on flat rate.

And didn’t a wise man once say something about teaching a man to fish?

One instance that really stands out in my mind for some reason was a GMC blower motor that didn’t want to run at full speed. The tech working on the concern had ordered and installed a replacement blower motor only to have the new one act the same way. That’s when he came to me for help.

This is why many OEMs do not want you to backprobe. If you damage the weather seal, corrosion will not be far behind.

“Did you test power and ground to the blower motor?” I asked. He assured me he had but I was curious on exactly how he had performed the tests. So I asked him to show me.

Grabbing his test light, he disconnected the connector from the blower motor, turned the ignition key “on” and switched the blower motor control to “high.” He grounded his test light to a nearby screw head under the dash and placed the probe end of the test light in the connector socket bringing power to the motor. It lit up brightly. I didn’t really need to see him do anything more. I knew he had failed to test the power circuit correctly and I was willing to bet he hadn’t tested the ground side any more effectively.

Do you know why?

This particular job happened a long time ago, yet I still meet and hear stories from technicians who are making this same mistake every day. One foundational fact about testing for voltage drop is that the circuit MUST be “on” with current flowing. By disconnecting the connector from the blower motor, the tech was doing nothing more than measuring OCV (Open Circuit Voltage).

I went back to my bay and grabbed my PowerProbe, a tool I used frequently for electrical testing on most circuits. Another foundational rule is to always reference your meter to the battery so you can be assured of testing the entire circuit path. The other side of that rule is to get your measurement lead as close to the circuit load (in this case, the blower motor) as possible for the same reason. It is important to remember that you are only measuring the voltage potential (or lack thereof) between the two leads of your tool.

I plugged the blower motor connector back in and wasn’t surprised to hear it spinning much more slowly than it should. I gently probed the power feed wire right at the connector and measured less than 6V. That’s a far cry from the 12V+ I should be seeing! I explained to the technician I was helping that the reading was telling us there was an unwanted resistance somewhere between the connector and the battery positive cable connection.

Piercing a wire for test purposes also has its disadvantages. The biggest is forgetting to seal the hole you made with your tool.

Reviewing the schematic he had printed out, I explained that we needed to backtrack on the power side of the circuit, heading back to the battery. Once our meter reading returned to normal (on the power side using the PowerProbe, that would mean seeing the full system voltage) and then heading back toward the load until the reading went bad again, eventually closing the gap on the fault kind of like two baseball players with a runner trapped between them. The easiest way to do that would be to break the wiring path into easy segments and the first to present itself on the truck was the connector at the firewall, bringing the power in from the underhood junction box to the interior cab.

Measuring the same wire on the cab side of the firewall gave us the same reading we had at the blower motor, so we knew the problem had to be on the underhood side of the wiring. Moving to the other side of the same connector, the PowerProbe showed 12.3V – or normal. Disconnecting the plug (with some effort, I might add) revealed the problem. A loose connection created extra resistance in the circuit. That extra resistance also resulted in a hot spot that burned the contact, adding to the problem, and melted the connector, making it difficult to separate. A replacement connector was ordered and installed, the old blower motor was reinstalled and the concern was corrected.

Before you replace that ECU

An ECU, not matter what system or systems it’s in charge of, is still an electrical “load” first. And like any other, it has to have a clean supply of voltage and a clean path to ground to function. Often, an ECU has more than one of each and ALL need to be tested and verified before the ECU itself is condemned. And while you can do it the same way as we did the blower motor, it’s not as efficient.

An easier way is to disconnect the ECU, identify the harness side connector pins supplying the grounds and power feeds (both “hot at all times” and those that may require a certain key position) and then install a substitute load for testing.

The first step I like to take after identifying what does what is to use my PowerProbe (or test light) to identify which power feeds are “hot at all times.” With one identified, I use my connector test kit to select a lead that will fit the connector properly and install it on the power feed I’ve located. I then add a second test lead to the first ground connection I want to test. To these two leads, I attach my substitute load – typically a light bulb of one kind or another.

Damaged or poorly fitting connections in the harness plugs is an increasingly common cause of electrical issues. Mike Miller offers tips on finding these problems in his own article in this issue of Motor Age.

I used to use a headlight bulb exclusively, but after I shared that in one of my Trainer series videos (The Trainer #74), some readers shared their concern of exposing the ECU wiring to excessively high current draw. And while I never had a problem with that, it did get me to thinking. The whole purpose of testing this way is to uncover the presence of unwanted resistance elsewhere in the circuit. If the applied available voltage will be consumed proportionally to all the resistances in the circuit path, doesn’t it make sense to use a substitute load that will mimic the normal current load of the ECU? So now I look at the fuses protecting the ECU I’m testing and choose a substitute load accordingly. The 55-watt headlight I use draws roughly 5 amps and fits most of my needs just fine.

With the substitute load connected and lit, I measure voltage drop on both the power side and the ground side but not at the load connections. To avoid reading any drop that I added in making those connections, I measure at the point where my leads are plugged into the ECU harness connector. The meter connections are simple – DVOM set to the Millivolt scale, negative meter lead on the positive battery post and the positive meter lead on the power side connection as described. If the drop is over 0.50V, the meter will typically read “OL” or provide some other over limit warning. Then I move the negative meter lead to the ground post at the battery and the positive meter lead to the ground connection and repeat the test. If either fails, I know what side of the circuit I need to start backtracking on. If the first ground point passes, I move the connector and test lead to the next ground pin and test it, moving through all the grounds until I’ve checked them all.

(Images courtesy of Mitchell ProDemand) Pinpointing the power and ground feeds for an ECU may require you to look at more than one page. Be sure you identify and test them all.

If all the grounds pass, I pick one and use it as the ground point for my substitute load and begin checking any remaining power feeds going to the ECU. The schematic will tell me if the feeds are “hot at all times” or what position the key must be on to provide the complete the circuit.

Ever increasingly complex electronics are changing the aftermarket repair landscape. If you aren’t comfortable with electrical troubleshooting techniques, you need to get comfortable – and soon. Hope this helped you get on your way!

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