I have run into a problem wiring a set of driving lights on my 2009 Subaru Outback. Google led me to this forum, and I can see that there are more than enough knowledgeable people here to set me straight. I closely read two threads about putting a diode in parallel with a relay in order to keep back EMF from causing mischief with the car's circuitry, but neither one addressed my particular issue.
I have installed driving lights in all but one of my vehicles over the last 31 years, always successfully. What has changed is car electronics.
I always use a Bosch 30 amp relay. There is no problem with the power circuit. Terminal #30 of the relay is connected to the "+" terminal of the battery with a fused 12 ga. wire. Terminal #87 of the relay is connected to the driving lights, which ground through the bumper.
The issue is with the trigger circuit, terminals #85 and #86 of the relay, which connect to the relay coil. On every installation until now, I simply connected terminal #86 to the wire supplying current to the driver's side high beam and terminal #85 to ground through a toggle switch in the passenger compartment. When the toggle was up, the driving lights came on and off with the high beams. I put a Radio Shack diode parallel to terminals #85 and #86 to protect the car's circuitry. Perfect!
It's more complicated this time around. The connector for the high beam has two wires going to it, one green and the other white. The bulb is well insulated from the headlight shell, and the path to ground has to be through one of the wires. I disconnected the connector from the high beam bulb and tested for the presence of voltage to ground with a simple circuit tester. I clamped the lead of the tester to ground and probed the connector and here is what I found:
With the low beams selected, *both* the green wire and the white wire have voltage to ground; and
With the high beams selected, *only* the green wire has voltage to ground.
That is, the green wire is always "hot" even though it's connected to the high beam. What I am assuming is going on is that this is a variation of a switched ground. Instead of cutting off the path to ground, the car puts a voltage on the white wire equal to the voltage on the green wire when the low beams are selected and, as a result, the high beams remain off.
If this is the case, I could make my driving lights come on and off with the high beams by connecting terminal #86 of the relay to the green wire and terminal #85 to the white wire (through my toggle switch). We come now to my questions:
Assume I tie the green (always hot) wire to terminal #86 and the white wire (sometimes ground) to terminal #85 and place a diode in parallel with the two terminals, oriented so as to protect the green wire and whatever is connected to it.
1. When the coil of the relay is energized and then de-energized, will there be a voltage spike (from back EMF) in one direction or two? That is, will a voltage spike appear on terminal #86, terminal #85, or both?
2. Will the diode act to shunt the voltage spike from the back EMF down the white wire?
This was never an issue before because one side of the circuit, the ground side, needed no protection since it consisted only of a wire straight to ground through a heavy duty toggle switch. But in my Outback, both the green and white wires are apparently tied to microprocessor controlled switches, which a voltage spike from back EMF might fry.
To rephrase my questions, if I place a properly oriented diode in parallel with terminals #86 and #85 of my relay, wired as described above, will the white wire be affected by back EMF when the relay coil is de-energized?
I would be grateful for any help you can give me.
VMM
I have installed driving lights in all but one of my vehicles over the last 31 years, always successfully. What has changed is car electronics.
I always use a Bosch 30 amp relay. There is no problem with the power circuit. Terminal #30 of the relay is connected to the "+" terminal of the battery with a fused 12 ga. wire. Terminal #87 of the relay is connected to the driving lights, which ground through the bumper.
The issue is with the trigger circuit, terminals #85 and #86 of the relay, which connect to the relay coil. On every installation until now, I simply connected terminal #86 to the wire supplying current to the driver's side high beam and terminal #85 to ground through a toggle switch in the passenger compartment. When the toggle was up, the driving lights came on and off with the high beams. I put a Radio Shack diode parallel to terminals #85 and #86 to protect the car's circuitry. Perfect!
It's more complicated this time around. The connector for the high beam has two wires going to it, one green and the other white. The bulb is well insulated from the headlight shell, and the path to ground has to be through one of the wires. I disconnected the connector from the high beam bulb and tested for the presence of voltage to ground with a simple circuit tester. I clamped the lead of the tester to ground and probed the connector and here is what I found:
With the low beams selected, *both* the green wire and the white wire have voltage to ground; and
With the high beams selected, *only* the green wire has voltage to ground.
That is, the green wire is always "hot" even though it's connected to the high beam. What I am assuming is going on is that this is a variation of a switched ground. Instead of cutting off the path to ground, the car puts a voltage on the white wire equal to the voltage on the green wire when the low beams are selected and, as a result, the high beams remain off.
If this is the case, I could make my driving lights come on and off with the high beams by connecting terminal #86 of the relay to the green wire and terminal #85 to the white wire (through my toggle switch). We come now to my questions:
Assume I tie the green (always hot) wire to terminal #86 and the white wire (sometimes ground) to terminal #85 and place a diode in parallel with the two terminals, oriented so as to protect the green wire and whatever is connected to it.
1. When the coil of the relay is energized and then de-energized, will there be a voltage spike (from back EMF) in one direction or two? That is, will a voltage spike appear on terminal #86, terminal #85, or both?
2. Will the diode act to shunt the voltage spike from the back EMF down the white wire?
This was never an issue before because one side of the circuit, the ground side, needed no protection since it consisted only of a wire straight to ground through a heavy duty toggle switch. But in my Outback, both the green and white wires are apparently tied to microprocessor controlled switches, which a voltage spike from back EMF might fry.
To rephrase my questions, if I place a properly oriented diode in parallel with terminals #86 and #85 of my relay, wired as described above, will the white wire be affected by back EMF when the relay coil is de-energized?
I would be grateful for any help you can give me.
VMM
