Turn Signal Backfeed Diagnosis

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.”  What do you do? 
Do you have a good idea what the problem is?  Or are
you completely lost and are thinking of 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 in your shop. 

Before we get into making the repairs let’s start with the
how and why this can happen.  I always feel it’s important to
understand how things are supposed to work before I fix it.
The more you know about how an electrical system works
the more you’ll know how to repair it. 

What am I 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 and brake are combined into one bulb filament.

In order for this to happen there has to be a good bulb in the circuit. (A broken filament in the bulb and you’ll not see this phenomenon.) In regards to the original complaint this cannot happen as it was described in the work order any other way.)  There is a similar occurrence with the front bulbs; In these cases the indicator is only on and not affected by the brakes.

What is it?

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 these can be very problematic and lead to many false diagnosis if not checked properly.  The housings can melt the connection from 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 it?

The first thing to do in any diagnostic procedure is to duplicate the problem.  You can accomplish a lot of your diagnostic work before you drive it into the service bay.  Since this can only happen with a dual filament and the brake/turn lights combined you can safely assume that this is coming from the rear of the vehicle.  As you turn the signal on observe the condition on the cluster as you apply the brake.  Unless both rear bulbs have lost their ground signal simultaneously you should easily spot which one side is the problem area.  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 in the opposite direction the feedback will show up on the dash with one turn signal indicator lighting up at the same time the other one is flashing. 

Confirm the Diagnosis

The only tool I use for this procedure is a good old fashion test light, nothing fancy just a plain old test light.  Before getting out of the car turn on the hazards and the park lights.  (This is another good pre-test also) Now go to the rear of the vehicle and you’ll notice one side of the car is not lit up.  This should coincide with your early test results as well.  Remove the housing and find a good ground for your test light.  Now test the three leads to the bulb.  With the hazards on and the park lights on two of them should be working.  (One flashing and one 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, through the bulb filament, through the ground lead and finally through your test light. (A good ground would not light up the test light.)  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 but go from back side of the connector.  This way you avoid spreading the connector open and creating a new problem. 

When I’m in an area that I can’t get to the rear of the connector or it is sealed I use a sewing needle size safety pin or a quilter’s pin (they have a “T” handle top easy to hold on to.) and use it to stab the wire instead.  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 for maneuvering around and from side to side in a car.

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 keeping more customers coming to your door.

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