Turn signal and grounding problems on BMW motorcycles
I wrote this piece for the /5 group to address a problem that one of the riders had with his turn signals. They were acting very strangely. This isn’t intended to be a “recipe” article but an explanation of why some of the anomalies occur. The inconsistent turn signal flashing description is typical of a grounding problem. Grounding is simply a term for the electrical system on one “side” of a circuit. It is made up of the same stuff as the “other” side of a circuit, namely resistance. We tend to think of resistance as a fixed thing. That is so often not the case, and it is very easy to get confused.
I will use simple terminology, not the stuff that engineers like. I have little choice but to use my “farmer” type explanations. I hope that this helps my readers.
We will slowly build a simple turn signal circuit. Let us start with a simple bulb as an example. The bulb is 12 v and rated at 12 watts. The power law tells us that 12 watts at 12 volts will have a current flow of 1 ampere or amp. Volts (12) times amps (1) is the power in watts. 12 X 1=12. If, in fact, we discovered that the circuit actually had 2 amps flowing, then we would be consuming 24 watts of power.
The resistance of the bulb would be defined by Ohm’s law. The current (I) equals the voltage (E) divided by the resistance (R), or I = E/R. In this case, the resistance would be one Ohm.
Here is where it starts to get interesting. You would be surprised if you put your multimeter on the bulb to measure the resistance. It wouldn’t measure anything close to one Ohm. How can that be?
We incorrectly assume that the resistance is fixed. It often isn’t. The multimeter doesn’t use 12 volts to test resistance. It uses much less. At a lower voltage, the resistance is quite different.
For our next example, we are going to use a more accurate type of multimeter called an oscilloscope. That is nothing more than a voltmeter that uses a TV screen to show us what happens to the voltage and current.
We like to think that since electricity is fast (moving at the speed of light), as soon as we throw the switch on our simple circuit with only one bulb, the current would instantly come up to one amp. The TV screen would show zero volts and then rise to 12 volts. We would have a “square” jump at the bottom and square again at the top of the 12 volts.
We would quickly find that something else happens. It would start up quickly (not quite square, but close), and when arriving at the top, it would take forever to get to 12 volts finally. It is very rounded off at the top. It takes 1/4 of a second to get there. Observation of the bulb also confirms that the bulb doesn’t get to full brightness instantly, but there are more factors to explain that one.
The bulb starts off at a very low resistance, and as the current goes through the “business part” of the bulb, the resistance actually climbs up a lot. The resistance is affected by the heat in the wire. The resistance isn’t linear with respect to heat.
Now we add a “flasher” to turn the bulb on and off. It is nothing more than a type of switch. The flasher is (the older original type) a metallic strip of metal that heats up and turns on or off a switch. The higher the current, the faster it heats up and then turns off. It cools down and makes contact again. The “cycle” then repeats itself.
So now we have a bulb that is nonlinear in operation. We have attached it to a switch that we want to be nonlinear. Now we have a more complicated situation. We then add another bulb for the other end of the motorcycle. The total current drawn by the two bulbs is what now controls the flasher. A flasher that is correct for one amount of current will be quite different for another amount.
Our next addition to the circuit is two more bulbs for the other side of the motorcycle to make up a “right and left” indicator for traffic to see. This requires a “turn signal” switch and associated wiring. Our flasher will “see” either the right side circuit or the left side circuit, but not both at the same time. Next, we add another bulb for the driver to see as an indicator. The system is starting to become complicated. We haven’t even started talking about a 4-way flasher or cancel the system.
To reduce production costs, we can cheat a bit. We have to run copper (read expensive) wires “out” to the 4 turn signals. We don’t need to run a return circuit back if we just “ground” them. Just let the metal of the motorcycle carry the current back. This is where we start getting into trouble.
At each connection to the ground (metal of the frame), we have two metals in contact that may not be of the same material. This is an invitation for trouble. With moisture and odd chemicals getting into the contact, we can end up with an unpredictable connection. In extreme cases, it can be a very simple radio frequency transmitter. It is more common for the connection to be nonlinear. That means that while the resistance of the connection should be zero, it will be some real amount and not fixed. It will change, sort of like the bulb, but not in a predictable way. It will seem to have a mind of its own.
Think of all of the electrical connections on our old machines. Each is potentially a problem, not just the return (ground) side. That is why a special grease can do wonders for keeping the moisture and oxygen out that start the unwanted chemical processes.
With so many connections changing resistances, it is quite challenging for a person to diagnose them by using a multimeter.
The most common solution to turn signal defects is to run a separate ground wire from the turn signal housing back to a ground.
Updated 17 July 2022