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Last night, the passenger-side tail-light on my step-daughter's 2004 Mitsubishi Lancer ES went out. After determining that the bulb was OK, we looked in the fuse box, and saw that one of the two 7.5 amp fuses for the tail lights was blown. A replacement fuse immediately popped.

Today, we attempted to diagnose it with a VOM, and got some puzzling results.

With the battery disconnected, both fuses pulled and the head-lamp switch in the off position:

  • I read 0 Ohms across the terminals of the tail-light bulb socket (with bulb removed.) This immediately made me think "the positive side is shorting to ground somewhere between this bulb socket, and the head-lamp switch on the steering column".

  • However, I also read 0 Ohms across the terminals of the (separate) turn signal bulbs, which are working just fine! They use a different fuse, and turn signals/hazards were not engaged at the time, so that 0 Ohm reading really confused me.

  • At that point my step-daughter drove off to run an errand, and when she came back the other tail-light was out. By this point that second 7.5 amp fuse was blown. After disconnecting the battery again, I checked ohmage between the positive terminals of both fuse sockets, and measured 0. I did the same thing for the negative terminals of both fuse sockets, and also measured 0. This lead me to believe that the 2 tail-light fuses are wired in parallel together, which IIRC means they effectively act together as a 15 amp fuse? ... That also makes no sense, because if they were co-redundant, I'd have expected both tail-lights to continue shining so long as at least one of those fuses was still good.

Should I be approaching this problem in some other way? (And, for bonus points, can anyone explain the oddities described above?)

  • The parking lamp/headlight switch applies power or ground so the circuit would be open if you were testing with the switch off. Same with the turn signals. The tail lights are on separate circuits. They don't share a ground. Does the car have an aftermarket trailer harness? If not your going to have to remove panels and visually inspect the harness for rub through. Does the car have an aftermarket radio? If so inspect the wiring behind the radio for short to ground. – Ben Feb 6 '16 at 0:06
  • aftermarket trailer harness: no. aftermarket radio: yes. Clarification: when I said "tail lights," I meant the lamps in the back that light up at the same time as the headlights. (Just in case that was ambiguous) – Ryan V. Bissell Feb 6 '16 at 0:28
  • And, as to your assertion that "so the circuit would be open if you were testing with the switch off. Same with the turn signals." Yes, that is precisely why I was surprised to read 0 ohms through the turn signal light socket (with no bulb inserted) – Ryan V. Bissell Feb 6 '16 at 0:30
  • Your going to have to visually inspect the wiring, the only common point the two circuits have is in the front control module. And the fact that they run through the same harness. I'd share the wiring diagram publicly but I'm not sure on the legality and I don't see a way to private message? – Ben Feb 6 '16 at 0:37
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    I was just thinking whether you could be getting a false reading from some bulb failure electronics. It would be interesting to know it there is a voltage on the lamp connectors even when the light is switched off. – HandyHowie Feb 8 '16 at 10:51
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The parking lamp/headlight switch applies power or ground so the circuit would be open if you were testing with the switch off.

Same with the turn signals.

The tail lights are on separate circuits. They don't share a ground.

Does the car have an aftermarket trailer harness? If not your going to have to remove panels and visually inspect the harness for rub through.

Does the car have an aftermarket radio? If so inspect the wiring behind the radio for short to ground.

Fair use... right?

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  • You say "the parking lamp/headlight switch applies power or ground so the circuit would be open if you were testing with the switch off." Yes, that is precisely why I was surprised to find the turn-signal circuit not open. ... I was not surprised to find the tail-light circuit not open, since after all it is the one that is blowing a fuse. – Ryan V. Bissell Feb 6 '16 at 1:21
  • As to your diagram: it explains why the fuses 20 & 21 appear to be wired in parallel: on the positive side, they are. As for the fuses' negative sides... it might be possible that I tested continuity between those terminals when the light switch was closed. In any case, I will dig around inside the dash and report back / accept this answer. – Ryan V. Bissell Feb 6 '16 at 1:31
  • What does the radio have to do with this? – HandyHowie Feb 6 '16 at 8:50
  • @HandyHowie It has an after market radio installed. behind the center dash is a common power splice, His initial complaint was that the right rear tail light fuse was blowing. A short to ground in this location would be the main suspect. Due to the installation of an after market radio. – Ben Feb 6 '16 at 14:01
  • @Ben I didn't see any chafing on the wire harness that runs directly behind the radio, but I wrapped it in electrical tape just for good measure. Dash is reassembled, and fuses are holding. ... This might have been an intermittent thing, though, so if the problem returns I will next inspect that center-of-dash joint connector, which takes more labor to get to. Accepted, thanks! – Ryan V. Bissell Feb 6 '16 at 20:13

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