short in turn signal / backup circuit...suggestions?

Kurt K

Kurt K

SCCoA Member
Here's what I know. The 10 amp fuse for the turn signals and backup light blows as soon as I put the fuse in while the key is on or if I install the fuse and then turn the key on. The hazards still work, since they are powered from a different circuit. I've replaced the multifunction switch and the flasher relay with known good ones. My next thought is the neutral safety switch, but I'm open to suggestions. Thoughts?
 
The fact that the flashers work tells me the bulbs/sockets and wiring from the switch/flasher can r probly OK. powered from diff circut, but same wires from those points out, and blows before directional is turned on right? Car is a manual, the wiring harness to the reverse lites runs under the car to a switch on the trans. That's where I'd look next if it is also on that fuse, could be road debris got it:rolleyes:, or may be melted to exhaust:eek: If the car starts I would think the neutral safety is OK too, it just passes power to starter relay so if you start your good there:D It could also be inside the steering column harness before the multi function sw, which is why changing that didn't help. Could also be in ignition sw, Ford are notorious for troubles there.
I don't know if that helps, but some places to look. These cars are just about as flat out evil electrically as any I have ever seen:cool:
Good luck
Adam
 
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Well, the car has an AOD, but your thoughts are still valid. The problem was intermittent for a little while, before getting to the point it is now, which is the continuous short. I actually tried to buy an ignition switch from Ford, but apparently it is obsolete.
 
Kurt K said:
Well, the car has an AOD, but your thoughts are still valid. The problem was intermittent for a little while, before getting to the point it is now, which is the continuous short. I actually tried to buy an ignition switch from Ford, but apparently it is obsolete.
Click to expand...

That's news to most everyone else ... although, right now RockAuto doesn't show the Motorcraft unit at http://www.rockauto.com/catalog/raframecatalog.php?carcode=1198672&parttype=4516 .

For the original Motorcraft, FordParts DOES list it as current production: http://www.fordparts.com/Commerce/P...ch=true&year=1991&make=Ford&model=Thunderbird

RwP
 
RalphP said:
That's news to most everyone else ... although, right now RockAuto doesn't show the Motorcraft unit at http://www.rockauto.com/catalog/raframecatalog.php?carcode=1198672&parttype=4516 .

For the original Motorcraft, FordParts DOES list it as current production: http://www.fordparts.com/Commerce/P...ch=true&year=1991&make=Ford&model=Thunderbird

RwP
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Well, I said apparently because I didn't completely trust what that dealer was telling me and I did not have time to check alternate sources yet.
 
I think the dealer may have fibbed to you... I picked up an ignition switch from Advance Auto last Friday (they even had it in stock :eek:)
 
Ahh, you mentioned neutral safety sw and I ASSUMED 5 spd, see what happens:p
I am not sure where the auto picks up the neutral/park position to alow starting, or the reverse lites
Adam
 
Kurt,

Try removing the flasher module and seeing if the fuse blows. If it still does, you can rule out the MFS side of the circuit from the fuse, leaving the backup switch, etc.

Do you have the wiring diagrams?

I doubt it is the ignition switch this time. The fuse in question is downstream from the switch.

Also, the backup switch itself is not likely to cause the problem, since it is only a switch. If you can rule out the MFS and everything it feeds, the problem would have to be: day/night mirror, keyless entry module, or wiring between these and the fuse (also the backup lamps).
 
DrFishbone said:
Kurt,

Try removing the flasher module and seeing if the fuse blows. If it still does, you can rule out the MFS side of the circuit from the fuse, leaving the backup switch, etc.

Do you have the wiring diagrams?

I doubt it is the ignition switch this time. The fuse in question is downstream from the switch.

Also, the backup switch itself is not likely to cause the problem, since it is only a switch. If you can rule out the MFS and everything it feeds, the problem would have to be: day/night mirror, keyless entry module, or wiring between these and the fuse (also the backup lamps).
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Definitely not the multi-function switch itself. I never really thought it was, but I bought a new one just in case....and now I have one to add to my spare parts collection. :)

I don't have a day/night mirror, so that is out.

I do have a EVTM and the actual wiring schematics, which are huge and were a challenge to trace out all things that are fed from that 10 amp fuse. I'm really hoping I don't have to trace wires.
 
Kurt K said:
Definitely not the multi-function switch itself. I never really thought it was....
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True - but removing the flasher will not just prove that the MFS switch is good, but all the wiring from the flasher module, to the light bulbs in the back - if the fuse still blows. If removing the flasher module keeps the fuse from blowing, then you'll know to look at the wiring between the flasher module, MFS, and lights. :)

Assuming no way to check continuity (for shorts), the next thing I would do is unplug the reverse switch and see if the fuse blows. At least this way, you can rule out large sections of the car's wiring before chasing wires/devices.

I don't know of any particular problem-prone areas in the wiring harness for either of these, but I do know that under the carpet, there are places where it is possible to have wires damaged on some of the large harnesses - especially if there is damage on the underside of the car from rust, improperly placed jacks, running over chunks of concrete :)o), etc.
 
Isolate the problem by removing electrical components or unplugging harnesses at certain locations - the key to electrical diagnostics. At least you have an EVTM for reference.

Unplug something like the Transmission sensor and check both sides of the wiring for continuity to ground with a multimeter to see which side of the circuit your short is on and go from there.


- Dan
 
OK, made some progress tonight. I removed the flasher relay and all of the connectors from the MFS. Then I installed a new fuse. I dimmed the garage lights and with the key on, I slowly started plugging things in and looking for the flash of the fuse blowing.

1st, the flasher relay....no flash from the fuse (I removed the fuse and verified that it had not blown).
2nd, I started connecting the plugs to the MFS. Each time, the fuse stayed in tact.
By the time everything was connected, my turn signals started working:eek:

So I think I can assume that the wiring issue is somewhere under my dash. I just wish the problem was a little more obvious.
 
Kurt K said:
So I think I can assume that the wiring issue is somewhere under my dash. I just wish the problem was a little more obvious.
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Kurt,

Suggest looking near the stainless braided hose you installed for the mechanical oil pressure gauge.

David
 
David Neibert said:
Kurt,

Suggest looking near the stainless braided hose you installed for the mechanical oil pressure gauge.

David
Click to expand...
I can look there, but I wrapped the stainless hose with rubber so that it wouldn't rub through anything. I need to move the car off the lift so I can take a better look under the dash...maybe tomorrow night after I try to fix my passenger side exhaust leak.
 
Kurt K said:
I can look there, but I wrapped the stainless hose with rubber so that it wouldn't rub through anything. I need to move the car off the lift so I can take a better look under the dash...maybe tomorrow night after I try to fix my passenger side exhaust leak.
Click to expand...

Good move on the rubber wrap deal.

David
 
If your back up lights are on the same circuit, did you trace the tail light wiring around the trunk lid to make sure it's not grounding out to the lid?
 
Micahdogg said:
If your back up lights are on the same circuit, did you trace the tail light wiring around the trunk lid to make sure it's not grounding out to the lid?
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That spot where the wire loom breaks all the time (on the trunklid hinge) might be a good place to look - also there are alot of sharp edges on the trunklid inner-panel where wires could be worn through. The only thing is that if the car is not in reverse (and the reverse switch is good), then there should not be power back there on that circuit anyway.

Really, unless Kurt happened to have it in reverse when testing or the backup switch is bad AND there is a short back there somewhere, I doubt the problem is any further back than the backup switch.

Sounds like your getting closer though Kurt. :cool:
 
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