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Old 04-22-2008, 04:10 PM   #1
BT WRX
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Default STi trunk light not working even though it's getting 12V?

The trunk light on my husband's 05 sti quit working about a year ago. He's not really sure when it stopped working, but I believe it was around the time he installed the oem trailer hitch wiring. All the lighting/electrical in the vehicle works except the trunk light.

The first thing checked was the bulb of course, that was fine. I even tried a few other bulbs. I tested the plug and socket with my dmm and they're both getting +12V and have a good ground. The trunk latch/ground switch works fine. We even took a working trunk light from a wrx and put it in his car and it still doesn't work despite getting power.

Any ideas?
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Old 04-22-2008, 10:06 PM   #2
ringe
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I would test it with a test light rather than a DMM. A testlight will tell you if the source and ground circuits are really up to the task of lighting that bulb. My guess is he spliced into the ground with a scotch lock (jesus save us from those!) and ruined it.
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Old 04-23-2008, 12:14 AM   #3
BT WRX
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I don't own a test light, but some of the guys I work with have do. He made his connections with cheap wire taps at the wire harness going to the tail lights on the passenger side of the trunk. We disconnected the trailer harness, but that had no effect.

Also, I just made a seperate ground and it still didn't work so it's not a ground issue.

Last edited by BT WRX; 04-23-2008 at 01:30 AM. Reason: Added info
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Old 04-23-2008, 01:31 AM   #4
ringe
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Quote:
Originally Posted by BT WRX View Post
I don't own a test light, but some of the guys I work with have do. He made his connections with cheap wire taps at the wire harness going to the tail lights on the passenger side of the trunk. We disconnected the trailer harness, but that had no effect.
I suspect the wire tap might have severed all but one strand of the ground wire. Or it could be that I am entirely wrong, lol. I would just suspect your husband's work first, and maybe take some time to redo it with either solder and shrink wrap or some good butt connectors with built-in shrink wrap. That's what General Motors uses for warranty wiring harness repairs anyway, so I trust them. Wire taps however I hate with a passion; I would recommend redoing his splices with what I suggested first in order to eliminate his work as a possible cause, and at any rate to make sure his wiring harness isn't going to be a future cause of trouble.
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Old 04-23-2008, 01:50 AM   #5
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I temporarily rewiring the ground, it's not that. I looked over the open sections of insulation where the wire taps were and it doesn't appear that they did much if any damage to the strands of copper. I did this by flashlight though. The wires he tapped provide power and ground to the taillights and they work just fine why would only the trunk light not work?

Only thing left I can think of is to find another +12V source.
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Old 04-23-2008, 11:21 AM   #6
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Since you stated that you have already tried tieing the light to a known good ground then the only thing left is the power side of the circuit. I assume the testing you did for voltage was with the light out of the socket and the circuit had no load on it. I think the problem you are having is due to a faulty connection somewhere and using a high impedence meter to test with can make you think there is voltage really available when no load is present on the lead.

Leave the bulb in the circuit and then check the voltage getting to it while the light is supposed to be on. Check the hot wire near the socket for voltage. You may want to use a pin to pierce the insulation so you can measure the voltage. Use some silicone to seal the pin hole when you are done. If you don't see any voltage then you need to move further back and check the nearest connection point for a problem.
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Old 04-23-2008, 07:26 PM   #7
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Quote:
Originally Posted by Cougar4 View Post
I think the problem you are having is due to a faulty connection somewhere and using a high impedence meter to test with can make you think there is voltage really available when no load is present on the lead.
This is why I suggested using a test light; the bulb won't illuminate brightly unless the circuit is truly sound, in most cases. This also eliminates the need to peirce insulation or find another way to probe the connector with a bulb plugged into it.
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Old 04-23-2008, 11:25 PM   #8
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Quote:
Originally Posted by ringe View Post
This is why I suggested using a test light; the bulb won't illuminate brightly unless the circuit is truly sound, in most cases. This also eliminates the need to peirce insulation or find another way to probe the connector with a bulb plugged into it.
I agree with your thought there Ringe. Since the OP appeared to own a meter and not a test light probe I made those suggestions.
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Old 04-24-2008, 12:16 AM   #9
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Quote:
Originally Posted by Cougar4 View Post
I agree with your thought there Ringe. Since the OP appeared to own a meter and not a test light probe I made those suggestions.
Sorry if that post sounded argumentative, didn't mean to be. Your advice is great stuff as well; I just prefer using a test light to diagnose circuits. YMMV.
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Old 04-25-2008, 09:41 PM   #10
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If 12v is getting to the light socket, and you have a good ground, then it is possible that the bulb is not mechanically seating properly in the socket. Check to see if a contact is bent or if a spring is broken or something.
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