How do turn signals work?
Turn signals are one of the most misunderstood sub-systems in vehicles today. They really are simple when you stop and think about how the system performs its job, and why it is wired the way it is. The following procedures will cover most of the basic and common problems found in the turn signal system.
Most commonly, problems occur when the system is wired incorrectly or bulbs are inserted into worn sockets upside down. This can cause malfunctioning of the brake lamp system, turn signals, 4-way emergency flashers, and brake lights (among other things).
The turn signal system is isolated from other lamp circuits, but appears to function with the brake lamp system, which, in part, is true.
The turn signal on a traditional system is a shared element with the brake lamp function at the bulb to the rear of the vehicle. The most common misconception is "I can wire my turn signals and brake lamps together at the light housing and it will be fine." That's typically where the trouble begins.
 Power feed and flasher unit
Power on a traditional turn signal flasher unit is delivered to the system via the flasher unit. A fused, switched lead is run from the fuse buss to the flasher, so that it only receives power in the "Hot in Run" position.
From the flasher, a wire is run to the center wiper (or movable contact) of the turn signal switch inside the steering column.
The flasher operates from internal heat on a tungsten bi-metallic "strip" that makes or breaks the contact at the switch side (or "power output" side) of the contact. The bi-metallic strip functions as it does by two dissimilar metals (Bi-metallic) with dissimilar expansion rates causing it to bend. As the system lamp load is imposed on the flasher unit, the bimetallic strip heats up, and curls upwards, breaking contact to the switch. It then cools down very rapidly, and makes contact again, and the lamp load again heats the strip and it opens up. Thus: flash.
When the load is under-matched to the flash unit (bulb(s) burned out), the flash cycle is slowed or stuck "On”. This is what we perceive as a lamp outage. When this occurs, we pull over and have a new bulb put in.
This occurs because the lamp load on the bimetallic strip inside the flasher unit has been reduced to a point where the time allotted for heat is extended (slowing the flash rate), or never reaches the heat point of the strip (turning on and staying on).
When these flash units fail, it's usually because the load on the strip has exceeded its capability to recover, and the strip just melts or distorts, leaving the switch contact side "open” from power. This is when we normally put in a new flasher. Causes of this type of failure can include age of the flash unit, overtaxing of the lamp load (as in trailer lamps), or a short within the electrical system.
The other side of the coin is too many bulbs within the lamp load circuit, which taxes the flasher unit. This causes the bimetallic tungsten strip to heat and cool at a very rapid rate.
The more current draw through the flasher unit, the faster it opens and closes the contact.
The typical causes of this type of failure are:
- Exceeding the lamp count.
- Exceeding the recommended wattage of the bulbs.
- Installing the wrong bulb in the system (i.e.: a single element bulb, instead of a dual).
- Shorting of the lamp system (common culprit: trailer plugs).
- The addition of extra lamps (such as a trailer) in the circuit, without upgrading the flash unit.
To prevent this malady from occurring, always install the recommended wattage or bulb number when replacing the bulbs.
The net result usually is a very rapid flash rate. And, in some cases, failure of the flasher unit, or premature failures on subsequent units.
 The turn signal switch
This little switch has been responsible for more "hair pulling” incidents than most other parts of the vehicle. When you understand how it works, and why it works the way it does, it’s really quite simple.
At the bottom of the steering column on traditional turn signal systems, there is a plug. From that plug all the system signals (power) go in and come out. And, an added system, a wire from the brake lamp switch.
At this plug you will find flash unit power into the switch handle, and two sets of wires for the left and right turn signal lamp socket power (front left, rear left, front right, and rear right). These wires bring power to the lamps for turn signals, and brake lamps to the rear. Also at that plug you will find a wire for the brake lamp system from the brake lamp switch.
The switch itself, located below the horn ring on most traditional systems, is like any other, except that it is two switches in one, the left side and the right side. It also provides the isolation required between the brake lamp system and the turn signal system.
To understand how the switch works, we can view one side of it as an example. If you remove the steering wheel (and usually the horn assembly), you can see the turn signal switches. They are usually on a plastic base plate, driven by a plastic cam, and set off by two spring steel contacts that shift contact points when the base plate is activated with the turn signal arm.
Let's look at the left side only for now.
As you activate the left turn signal, the cam rotates, and the spring steel contact flexes, removing contact from the normally closed contact to the normally open contact.
This switch is wired traditionally as such:
- The center wiper (movable contact or cam contact) is lamp output.
- The normally closed contact is brake lamp power from the brake lamp switch.
- The normally open contact is power from the flash unit.
When no activation is present at the turn signal, the brake lamps are configured to light when the pedal is depressed through the normally closed contact of the switch and the center wiper (lamp output).
Now, assuming you are at a stop light, turning left, here's what happens:
The contact switches power when activated by the cam assembly, and opens (or turns off) the left brake lamp from the normally closed contact. And, it now has contact between the center wiper (lamp output) and the normally open contact (the flash unit).
If the brake lamp and the turn signal were both on at the same time, you would never see the flashing of the signal. Since the same filament is shared by both systems, even though both would be operable, when the flash unit was off, the brake power would still be on, making no noticeable change in the bulb.
If it were wired this way and both brake lamps were common to each other on the power leg, and no brake pedal was pushed (brake power off), then rear lamps would flash together.
In the above proper configuration, both systems would gain isolation from each other through the switch.
The exact same wiring is true for the right side, with one exception: the brake lamp power is picked up from the left contact (daisy chained).
In review, the system works in the following manner.
When no turn signal is selected, both brake lamps will receive power when the pedal is pushed. When the left turn signal is selected, the brake lamp for the left side is disabled by the turn signal switch, and the bulb now receives power from the flash unit. At the same time, no change has been made to the right side brake lamp. The result: left lamp flashing, right lamp steady with brakes on. Right lamp is out when brakes are off.
Now, switch to the right side. The reverse becomes true: the right side brake lamp is disabled, and power to the lamp is now provided by the flash unit.
Brakes on or off: the left side remains on (no change to the left side) steady with brake power, and off without it.
This is how the two systems are isolated. This is the most problematic issue for a novice to understand, primarily because nobody ever took the time to explain it to them in any detail.
 Turn signal system troubleshooting
 Single lamp failures
This is one of the most common failures found on vehicle lamp systems. The first thing to do is check the bulb. If the bulb is good, check for power at the socket.
 How to check for power at the socket:
- Get your DVOM out, set it for volts scale, V X 50 or higher, and turn the key on then the offending signal.
- Carefully insert the probe of your meter into the socket, to the contact pad (S) on the bottom of the socket, and the other to a good ground.
- If you read 0, then 12 volts then 0 again (flash signal), the power to the lamps and the system are working fine. You have a bonding issue.
- Shut off power to the system, and key, and return to the offending socket.
New cars today have plastic bumpers and anti-collision systems. Proper ground bonding is becoming scarce compared to bygone days. Also, lighting systems have had to rely on wired or remote location grounds. It is quite common to lose ground to a single socket and disable it.
Set your meter for OHMS Scale, R X 1, calibrated 000, and measure between the socket and a known good ground. Your reading here must be 000, if infinite, or random, you need to perform good bonding repairs to the socket.
Using a good contact cleaner, carefully clean the socket, pads and lamp brass base and its pads. Next, recheck the socket for bonding, if your readings are still infinite or random, replace the ground wire to a known good ground area. Use a star washer and burnish all the paint and dirt from the bonding area.
That will get a single lamp back to operation.
 Dead system
If none of the turn signal functions work at all, follow the steps below.
- Pull the fuse, and with your DVOM, measure it on ohms scale, R X 1 , calibrated 000.
- If it reads infinite, it is open. Replace the fuse and test the signal system again. If it reads 000, it is good, put it back in the buss box.
- Next, set your meter to DCV, V X 50 or auto range, turn the key on, and remove the flasher. Place the black probe on ground, the the other into either of the two flasher terminals. One will read infinite, the other should read 12 volts. If you have that, power is good to the flash unit.
- Next, set your meter to ohms, RX1 scale and measure across the flasher. It should read 000. If it does not, it is open, replace the flasher. If it does read 000, it is good (at least for one flash). If that is what happens, replace it with a known good or new one.
- Next, Replace the flasher. Turn either signal on, turn on the key, and isolate the wire from the flasher to the steering column plug. Measure it with your DVOM set for DCV, V X 50 or auto range, and ground. It should read an intermittent 12 volts, or a steady 12 volts if nothing in the system is working. If you have neither, check the wire between the flasher and the column plug for cut condition, or a burned socket.
- If you have either a cut condition, or a burned socket, check the other side of the column plug. It should read the same as the first reading. If you don't have that, pull the plug, inspect the pins for clean mating surfaces, and burned or bent pins or wires. Repair or replace as required.
- If you don't have either a cut condition, or a burned socket, and still dead, check any lead front, or rear of the selected side. It should have 12 volts, either steady or intermittent on it. If not, your trouble is in the column harness between the plug and the switch. Or, the switch itself is bad. Repair or replace as required.
 Front signals work, but rear signals are dead (or vice-versa)
- Isolate the two wires on the effected end from the column plug. Using your voltmeter, set to the above parameters, select a side, and measure the wire from the plug. It should show 12 volts (solid or intermittent). If not, you have a switch or harness failure. Inspect, and repair or replace as required.
- If you have 12 volts, trace the harness rearward (or frontward) until it comes to a plug. Inspect the harness for cut or burned conditions as you go. Repair or replace as needed. If good, inspect the plug(s) for clean proper mating pins, and burned wires.
- If the plugs look good, go to the lamp buckets, and measure between the socket ground and a known good ground. If it shows 12 volts, you may have an open ground wire. Replace or install a ground from each socket to a hard frame ground, following proper bonding procedures (star washers and burnishing).
 Left or right side turn signals dead
Follow the same procedure as outlined for front or rear dead above. However, apply it to the affected side.
 All rear turn signals flashing
Inspect the turn signal switch for a short between the brake lamp circuit and the signal circuit. Check the column plug and harness for shorts. Ohm the harness out with no power on, but either side selected. You should have no reading from the selected side to the brake lamp circuit, and continuity in the "Off" position to the brake lamps. If not, inspect the switch and harness, and repair or replace as needed.
 Tools, materials, and equipment
- Digital Volt-Ohm Meter
- Jumper wire set
- An accurate diagram
- Contact cleaner
- Acid brush
- Standard hand tools
- Dental pick
- Emery cloth
- Miscellaneous hardware