Gonzo's Toolbox: Understanding Electrical Circuits Will Expedite Diagnostic Repairs
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Gonzo’s Toolbox: Understanding Electrical Circuits Will Expedite Diagnostic Repairs

A work order comes to you with this complaint: “My turn signal indicators light up when I step on the brake pedal or when I’m driving at night.” Do you have a good idea what the problem is? Or, are you completely lost and debating sending this job to another shop because it’s not something you think you can handle?

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A work order comes to you with this complaint: “My turn signal indicators light up when I step on the brake pedal or when I’m driving at night.” Do you have a good idea what the problem is? Or, are you completely lost and debating sending this job to another shop because it’s not something you think you can handle?

Let’s see if I can simplify the problem for you and retain this customer for your shop. Before making the repairs, it’s important to first understand how things are supposed to work.

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So, what are you looking at? Essentially, you’re witnessing an electrical current trying to finish its journey from positive to negative. This can only happen if the vehicle is equipped with a dual-filament bulb in the rear of the car, where the turn signal and brake are ­combined into one bulb ­filament.

In order for this to happen, there has to be a good bulb in the circuit. (If there is a broken ­filament in the bulb, you won’t see this phenomenon.) In regard to the original complaint, this cannot happen any other way than what was ­described in the work order. There is a similar occurrence with the front bulbs, but, in that case, only the indicator is on and the brakes do not ­affect it.

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Very simply put, it’s a bad ground signal at or near the bulb or bulb housing. A lot of manufacturers have gone to these one-piece bulb housings in the rear sections and they can be very problematic and lead to many false diagnoses if they are not properly checked. The housings can melt the connection from the bulb to the socket or, in many cases, melt the connection between the harness and the housing. Always disconnect the housing from the harness and examine both the connector and the housing for deformation, melted leads or discoloration.

HOW TO SPOT THE PROBLEM
The first thing to do in any diagnostic procedure is to duplicate the problem. Since this can only ­happen with a dual-filament bulb where the brake/turn lights are combined, you can safely ­assume that the problem is coming from the rear of the vehicle. As you turn on the signal, observe the condition on the cluster as you apply the brakes. Unless both rear bulbs have lost their ground signal simultaneously, you should easily spot which side is the problem area.

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Keep in mind that as you flip on the turn signal (with combined turn/brake light systems), you are essentially turning “off” one of the brake lights, thus blocking the electrical signal to that individual bulb filament. One side will work correctly while holding the brake on, and when flipping the turn signal in the opposite direction, the feedback will show up on the dash with one turn signal ­indicator lighting up at the same time that the other one is flashing.

CONFIRM THE DIAGNOSIS
The only tool I use for this procedure is a good old-fashioned test light. Before getting out of the car, turn on the hazards and the parking lights. (This is another good pre-test.) Now, go to the rear of the ­vehicle and you’ll notice one side of the car is not ­illuminated. This should coincide with your early test results.

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Next, remove the housing and find a good ground for your test light. Then, test the three leads to the bulb. With the hazards and the parking lights on, two of them should be working — one will be flashing and one will be on solid. If you’re in the right area, you’ll find out there’s going to be another lead showing positive current.

Since your test light is grounded, you’re actually seeing a completed circuit when you touch the test light to the (bad) ground lead. This all starts from the true positive lead, then through the bulb filament, then through the ground lead and, ­finally, through your test light. (A good ground would not light up the test light.)

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In a lot of cases, the bulb filaments might even start to show signs of a dim output, because you have provided a small trace of a negative feed into the circuit by way of your test light. At this point, I’ll use a jumper wire attached to a good ground and touch it to the “real” ground wire for the circuit. If there are no other breaks in the connections through the housing or the wiring, the bulb should start working normally.

SPECIAL NOTES
Keep your test light probe lead as sharp as possible. Also, when probing a lead, do not use the probe in the actual socket; ­instead go from the backside of the connector. This way, you avoid spreading the connector open and creating a new problem. 

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When I’m in an area where I can’t get to the rear of the ­connector, or it’s sealed, I use a sewing needle-size safety pin or a quilter’s pin (it has a “T” handle top that’s easy to hold on to) to stab the wire. It leaves a very small mark that ­almost completely disappears when removed. I also make up my own jumper wires from a roll of flex wire (not standard wire); I try to make them all about 2-½ feet in length. This seems to be the best length for maneuvering around and from side-to-side in a car.

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It’s an easy repair and an even easier diagnosis once you understand the fundamentals of how these electrical circuits operate. With a bit of practice, you’ll find electrical repair is easier than you thought and you’ll be able to retain a lot more work in your shop, as well as keep more customers­ coming to your door.

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