Back Up Lights, one works, one doesn't ????
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Back Up Lights, one works, one doesn't ????
OK, this is what I'm working with, '75 F-250 4x4, 360FE, C6 Automatic. I replaced the bed on the truck and re-wired the taillights using the stock wiring harness of the replacement bed. All my turn signals, brake lights, running lights work on the truck. However, I do not have backup lights, nor did I have backup lights on the old bed that I replaced.
I suspect the neutral safety switch (NSS) is at fault. But first, I wanted to test my backup lights to see if they are working. I unlugged the rear lighting wiring harness that is located in the engine compartment by the ingition module. I applied a +12VDC battery lead to the plug that contained that bk/red back-up light wire. Result: One taillight worked, the other one did not. I took apart both taillights, cleaned contacts, di-electric grease, and did some diagnosis. Here is what I found out:
Working Taillight (With power applied):
1) +12VDC between the bottom and side contacts in the plastic bulb housing.
2) There is continuity between the BOTTOM contact in the plastic bulb housing and the harness grounding screw.
- I believe the current flowpath to be side contact => bulb => bottom contact => grounding screw.
Non-operating Tailight (With power applied):
1) +12VDC between the bottom and side contacts in the plastic bulb housing.
2) +12VDC between the bottom contacts in the plastic bulb housing and the harness grounding screw.
3) +12VDC between theharness grounding screw and a separate grounding point elsewhere on the frame.
4) There is continuity between the SIDE contact in the plastic bulb housing and the harness grounding screw.
- I believe the current flowpath to be bottom contact (shared with the grounding screw) => bulb => side contact => ???? (some weird feedback)
I tested the bulb with battery voltage and it works with voltage in either direction (+12VDC applied to either the side, or the bottom contacts). I think my problem is that (somehow???) the wires to the bulb housing were switched. Somehow, the grounding lead has continuity with the same lead as the power supply (the bottom contact in the plastic housing). I plan on just cutting the wires at the bulb housing and swapping the wire lead inputs to the housing. I wanted to see if anyone else has this experience before I go hacking away at a factory wiring harness. Maybe I'm missing something?? Thanks, Mike
I suspect the neutral safety switch (NSS) is at fault. But first, I wanted to test my backup lights to see if they are working. I unlugged the rear lighting wiring harness that is located in the engine compartment by the ingition module. I applied a +12VDC battery lead to the plug that contained that bk/red back-up light wire. Result: One taillight worked, the other one did not. I took apart both taillights, cleaned contacts, di-electric grease, and did some diagnosis. Here is what I found out:
Working Taillight (With power applied):
1) +12VDC between the bottom and side contacts in the plastic bulb housing.
2) There is continuity between the BOTTOM contact in the plastic bulb housing and the harness grounding screw.
- I believe the current flowpath to be side contact => bulb => bottom contact => grounding screw.
Non-operating Tailight (With power applied):
1) +12VDC between the bottom and side contacts in the plastic bulb housing.
2) +12VDC between the bottom contacts in the plastic bulb housing and the harness grounding screw.
3) +12VDC between theharness grounding screw and a separate grounding point elsewhere on the frame.
4) There is continuity between the SIDE contact in the plastic bulb housing and the harness grounding screw.
- I believe the current flowpath to be bottom contact (shared with the grounding screw) => bulb => side contact => ???? (some weird feedback)
I tested the bulb with battery voltage and it works with voltage in either direction (+12VDC applied to either the side, or the bottom contacts). I think my problem is that (somehow???) the wires to the bulb housing were switched. Somehow, the grounding lead has continuity with the same lead as the power supply (the bottom contact in the plastic housing). I plan on just cutting the wires at the bulb housing and swapping the wire lead inputs to the housing. I wanted to see if anyone else has this experience before I go hacking away at a factory wiring harness. Maybe I'm missing something?? Thanks, Mike
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There is a rule when troubleshooting bulb circuits that if not followed, will lead you down a rabbit trail. Always check for voltage at the bulb, and anywhere else in the circuit with the bulb in the socket.
I know this is not convenient , since the bulb is in the way of the contacts, but without a load on the circuit, your readings could be anything.
I know this is not convenient , since the bulb is in the way of the contacts, but without a load on the circuit, your readings could be anything.
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Thanks for the response. I didn't get around to posting it till now, but I found my problem last night. My voltages were all good up to the bulb housing. I ended up cutting the plastic housing out of the wiring loom. I tested the bulb at the wires, and presto, bright as anything.
Ended up that the contact on the side of the housing was severely corroded where it interfaced with the wire (couldn't see it initially because it was in the plastic shroud). I found it when I did a continuity test between the contact and where I cut the wire off. I cut out the rest of the just wire, polished the metal contact with the wire wheel on my Dremel, and soldered a new wire to the contact. Light works fine now!! Thanks for the help - Mike
Ended up that the contact on the side of the housing was severely corroded where it interfaced with the wire (couldn't see it initially because it was in the plastic shroud). I found it when I did a continuity test between the contact and where I cut the wire off. I cut out the rest of the just wire, polished the metal contact with the wire wheel on my Dremel, and soldered a new wire to the contact. Light works fine now!! Thanks for the help - Mike
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