All relays have some inductance, meaning that when power is removed from the relay coil circuit, and the magnetic field collapses, a reverse voltage spike is sent out to the trigger circuit. On a sensitive circuit, a diode across the terminals to damp the spike is a good idea, it can potentially shorten relay life unless used in conjunction with a zener diode.

relay spike info page

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The high beam circuit on the S4 is sensitive to reverse spikes!





I haven't looked at a wiring diagram, but I believe the high beam tell-tale on the dash is directly connected to the high beam wiring. With one relay connected, or lowish battery voltages, the reverse spike isn't high enough to cause a problem. With both relays connected, and full engine voltage, the instrument cluster may intermittantly "reboot" when the high beams are turned off. A double flash or more would consistantly do it. I believe a high voltage spike is going back up the wiring and into the cluster. Oops :(

I fixed it using a 1N4004 diode soldered across the high beam bulb socket so the polarity would not get mixed up down the road. One diode fixed the problem, but I put one on each relay to be safe.




The striped end _must_ be connected to the positive terminal, purple in my case.


My cluster seems to have survived, and other relays may cause the problem, but I wanted to forewarn of this quirk.