I am wiring up 2x 55W Halogen lights and 2x 100W Halogen lights on my '01 Subaru Outback. I want all 4 lights to come on when I turn the switch on. I created the wiring diagram which I followed during the install (see attached pic) but ran into an issue:

As the system stands (following the diagram), with the engine off, only the middle two, 100W lights turn on.

I measured voltage at one of the 55W lights with the switch turned on, and measured only 0.5 VDC (working 100W lights had 11.8 VDC), which means that my understanding of how the relay works is flawed. It seems that I cannot use the circuit across the relay coil to power the smaller 55W lights. The 85-86 circuit is used for activating the relay, and cannot be used for other lights.

I noticed that adding a wire to connect the wires coming from relay terminals 87 and 85 produces the desired result (all lights come on), but those wires quickly get hot and the relay starts buzzing after about 1 minute of the lights being on. By my calculations, this would result in 310W/12V = 26A through the circuit, which is a lot for the smaller 14 AWG wires. I would have expected the 20A fuse to blow, but it did not. The relay is a 30A relay and neither the 15A nor the 20A fuses blew, so I do not think the relay was drawing too much current... any idea what was causing the buzzing?

In summary:

  1. Why can I not use the relay (as drawn) to also drive the smaller 55W lights?
  2. What could be making the relay buzz when a wire is added between terminals 87 and 85?
  3. What can I change in my current setup to turn on all 4 lights without stressing circuit components?

Wiring Diagram

Thanks all!

  • \$\begingroup\$ The 12VDC battery is missing the connection to ground on the negative side... You also shouldn't be running 110 watts through the rocker switch, you need another relay for that. \$\endgroup\$
    – Ron Beyer
    Commented May 12, 2020 at 16:38
  • \$\begingroup\$ I did not draw the battery's Ground connection, but it is connected. The rocker switch was included with the set of 2x 100W lights designed to be run directly from the battery, so I think it should be safe to use with the smaller 55W lights. \$\endgroup\$
    – easterbran
    Commented May 12, 2020 at 19:58

1 Answer 1


You are attempting to power the 55W lights through the relay coil. Relay terminal 85 should be grounded, so the relay coil will get the full 12 volts.

The 55 W lights should be connected to terminal 87, in parallel with the 100 W lights.

  • 1
    \$\begingroup\$ The 55W lights could also be powered by terminal 86, so they'd be in parallel with the relay coil (which I suspect is what the OP really intended ?). \$\endgroup\$
    – brhans
    Commented May 12, 2020 at 17:27
  • \$\begingroup\$ @brhans: Yes - but then the switch would have to carry (and switch) the 9 Amps for the 55 W lights - don't know the switch rating... \$\endgroup\$ Commented May 12, 2020 at 18:24
  • \$\begingroup\$ Switch should be good for at least 17A, it was included with the 2x 100W lights, so that's an option. Thanks! \$\endgroup\$
    – easterbran
    Commented May 12, 2020 at 20:03
  • \$\begingroup\$ @Peter Bennett Thank you! I grounded terminal 85 and connected the 55 W lights to terminal 87 in parallel with the 100 W lights and they all come on now! No relay buzzing either, not sure what that was about. The 14 AWG wires get a little warm, but nothing excessive. \$\endgroup\$
    – easterbran
    Commented May 13, 2020 at 0:17
  • \$\begingroup\$ When you connected terminal 85 to 87, when the relay operated, the relay contacts shorted the coil by a slightly roundabout route - the relay then released, removing the short, allowing coil to close the contacts again, which shorted the coil etc. \$\endgroup\$ Commented May 13, 2020 at 0:25

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