Motor Age Garage: All In a Day's Work

Jan. 1, 2020
The Jeep Wrangler belongs to a fellow employee and just needed a routine oil change. A simple task, right? No matter what the customer complaint is, I always do a complete visual inspection so I can advise my customer on his or her car's current cond
The Jeep Wrangler belongs to a fellow employee and just needed a routine oil change. A simple task, right? No matter what the customer complaint is, I always do a complete visual inspection so I can advise my customer on his or her car's current condition. My personal belief is that this should be an integral part of any tech's code of ethics. It would be a breach of confidence to allow any vehicle out of the shop with an identifiable defect that could affect its safe operation, not to mention opening the shop and the tech to civil liability if that defect results in an accident. As a plus, it provides an opportunity to generate additional service revenue and create customers' faith in your shop's desire to look out for their best interest. This Jeep may have belonged to another employee, but that doesn't minimize my responsibility as a professional technician.

Giving it the 'Once Over'

Florida doesn't have a mandated vehicle inspection program like many states, including my home state of Virginia. You can imagine what kinds of problems I routinely find as a result.

Many shops have some type of visual vehicle inspection form for their techs to use, and if yours doesn't, I strongly encourage you to create one and use it on every vehicle that passes through your bays. At the very least, inspect any components or systems that could affect vehicle safety. Make sure you routinely check tire pressure and condition, operation of the exterior lights and all fluid levels (including the washer fluid). Report your findings and any corrections you made to your customer at the same time you review the repair needs for the complaint they originally came in for. Do so from the sincere standpoint that you have their best interest and safety in mind, and your add-on sales will increase, as will your customer satisfaction ratings. (Note: Some states' consumer protection laws forbid the removal of the wheels or accessing underhood components unless the original repair requires it or you have the customer's permission to do so.)

A thorough check of the Jeep turned up a few potentially serious problems. One was low air pressure in the right rear tire. A quick look revealed a split valve stem that, left alone, easily could cause a blowout. Another was front brake linings worn down to 3/32 of an inch — not at minimums yet, but replacing the pads now would not be a bad idea. The last item I found, and the main topic for today, was a malfunction in the turn signal/brake light system. I explained what I found to the Wrangler's owner, and got the OK to take care of all three issues.

One Side Slow, One Side Fast

As I do on every car when I first bring it in, I checked the operation of the wiper/washer system and started checking the lighting system. The left turn signal indicator blinked at a normal speed, but the right side blinked much faster. Typically, this is a sign that a bulb was blown on the "fast" side.

That wasn't the case. I found the right rear bulb on, but dim. Since this bulb is also used by the brake lights, I checked that next. No doubt about it, it was trying to work but couldn't. The question now was "why not?"

A fairly new addition to my toolbox is the PowerProbe 3. More than a simple test light, it has a built-in voltmeter and the capability to add power or ground to any circuit I'm testing with the press of a toggle switch. Connected to the vehicle's battery, it also is capable of performing basic voltage drop tests, and I had a suspicion that that's where I would find the problem.

With the lens removed for access and the brake applied, it was easy to see that the backup lamp also was dimly lit, one more indication of a bad circuit ground. The voltage reaching the brake light bulb was trying to find its way back to the battery through the other bulb instead of a more direct route to ground.

Where is Home?

This was a simple circuit, according to the schematic. Power applied through the brake light switch then passes through the multifunction switch on the steering column. From there, it travels to the bulb, then back to the battery by grounding to the body at the socket and through the taillight mount. A quick probe of the socket terminals told me system voltage was present, and I had not expected to see anything less, since all the other rear lights were lit.

The bulb itself is grounded to the bulb socket, and this spot was the one closest to the circuit's load. To test the groundside for any voltage drop, this is the place I wanted to be. Touching the probe tip to the socket shell with the bulb "on" gave me a reading of 6.4 volts, definite proof that I had a ground problem. A quick toggle to supply a good ground to the socket made the bulb nice and bright and just verified what I already knew.

What it Means

Voltage drop testing is a dynamic test of the circuit, meaning that the circuit is on and operating when tested. It can spot problems that regular resistance testing with an ohmmeter might miss. It is based on a simple rule: The addition of resistance in a series circuit will cause total voltage to be split proportionally between those resistances. If my brake light circuit was operating properly, all of the system voltage coming in would be used and my reading on the groundside of the bulb should be 0.0 volts. An acceptable range for the real world, though, is no more than 0.10 volts for basic 12-volt circuits. For low voltage circuits, like the Electronic Control Module (ECM) and other modules, an acceptable voltage drop is 0.05 volts. Anything more is a sign that some form of unwanted resistance is robbing my primary load of the voltage it needs to work properly.

For your voltage drop test to be accurate, you must test the entire groundside from load all the way back to the battery's negative post. To test with a DVOM (digital volt ohmmeter), make a lead extension long enough to reach from the battery to anywhere on the car. Attach your meter's ground lead to this extension if the battery is out of reach. Then use your positive meter lead to check voltage drop as close to the load's ground connection as possible. If your reading is high, like mine, start working your way back along the ground path to the battery until your reading returns to normal. The fault then is isolated between that point and the point where your reading was last high.

Finding the Block

I started working back by following the ground path from the bulb socket to its point of contact with the body. The mounting plate is held in place with three bolts, all with plastic insulating washers under their heads. As soon as I began removing the first bolt, the bulb began to flicker from dim to bright, and I knew I was close. Leaving the other two bolts tight, I placed the probe into the body's mating threads and measured 0.0 volts. That told me the ground path through the body and back to the battery was OK.

Corrosion on the threads of all three mounting bolts, right where they met the body, was evident upon a close inspection. That was the "unwanted" resistance I was looking for. A chase of the bolt threads and a quick tap through the mating body threads was all that was needed to restore the electrical contact I had been missing. But even so, I figured a little dielectric grease on the threads couldn't hurt to keep it that way. A quick verification after everything was reassembled, and this Jeep's brake lights once again were nice and bright.

Pete Meier is an ASE certified Master Technician, member of iATN, and full-time tech in Tampa, Fla. His experience reaches back more than 30 years, and his contributions to Motor Age reflect a wide variety of experience with almost every make and model. You can contact Pete directly at www.autoservicetech.com.

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