#12 fuse shorting out

[jon]

#12 fuse is shorting out.  Fuel gauge, tachometer and temperature gauge do not work.  Does anyone know what the likely culprit would be?

['76mintgrün'02]

I believe #12 also sends juice to the distributor. 

 

Does your car run? 

 

What year is your car? 

Late models have a resistor wire feeding the coil.  I have seen photos of melted ones.  That could cause a short. 

 

You might try pulling the fuse box out of its hole and unplugging #12 wires one at a time, to try to isolate the problem.

[jon]

The care is a 1976 BMW 2002.  I will check the resistor wire feeding the coil for a short.  

[Simeon]

1 hour ago, '76mintgrün'02 said:

I believe #12 also sends juice to the distributor. 

 

Does your car run? 

 

What year is your car? 

Late models have a resistor wire feeding the coil.  I have seen photos of melted ones.  That could cause a short. 

 

You might try pulling the fuse box out of its hole and unplugging #12 wires one at a time, to try to isolate the problem.

 

Ignition is generally unfused but could be fed from the supply side of the fuse. 

[Mike Self]

Besides the instruments and tach, fuse 12 on your '76 also serves the the oil pressure sender and the handbrake on warning light (also the low brake fluid warning, since they share the same dashboard bulb).  First thing I'd check is the handbrake warning light switch, under the rubber handbrake handle boot.  The bracket is flimsy and easily bent.  One of the wires connecting the switch could be grounding and causing a dead short, thus blowing the fuse.  Second thing to check is the oil pressure warning light switch.  Again, if the wire comes off the switch, it'll short the circuit.  If neither of them are the culprit, then you'll need to disconnect each item one at a time until the fuse quits blowing.  But first...

 

Two other things to check:  look under the dash near the driver's side cowl (vicinity of the turn signal flasher box and wiper relay) to see if there's a stray wire that doesn't go anywhere. (it was for the electric fuel pump on tii's and may still be present on 76 wiring harnesses)   Make sure it's not grounding.  

 

And...if fuse 12 on squarelights is the same circuit as fuse 11 on 12 fuse roundies, the seat belt warning light goes through this circuit--and it isn't on the factory wiring diagram.  Look for the seat belt sensors in the seats and in the seat belt buckles and make sure the wires aren't shorting out.  Or, disconnect the battery, then carefully remove the fuse box and compare the wires on fuse 12 with those in the factory wiring diagram.  If you see one that isn't on the diagram, it's for the seat belt warning system.  Disconnect the wire and then try a new fuse.  I had this happen on my '73 and it took me several days to discover this little anomaly.

 

Let us know whatcha find...

 

mike 

[PaulTWinterton]

Geez Mike,  I think you nailed it with the handbrake switch.   I was thinking temp sensor, as that connector could possibly pop off and touch ground, but that h/b switch is probably old and failed. +1 on that easy fix.   Here's hoping. 

[ray_]

The temps sensor line is merely a ground signal iirc.

 

Cheers,

 

 

[jon]

When I tried a new fuse, the circuit quit after a while when one lead of the fuse had corroded and blew the fuse only when I twisted the corrosion off the lead.  Does this mean the short is intermittent, not a dead short?

 

I zeroed in on Mike's input about the flimsy handbrake switch.  The lead wire sits loose in the plastic hole.  Both sides of the metal clip are positioned between the lead and the metal frame.  Should one side of the metal clip be on the outside of the lead holding it against the plastic hole?  Anyone have a photo? 

[02Les]

[Mike Self]

Jon, the brown wire at the handbrake switch is the ground wire, the other lead is hot, so make sure the hot lead isn't touching anything metal that can ground it.  

 

If it's not that, I would suspect the seat belt warning light circuit as described in a previous post.  If you suspect that, PM me as I did a column on discovering the same problem.

 

mike

[jon]

All fuse 12 devices NOW WORK.  The "Automatic Choke" (first item listed on the label) was the culprit.  I didn't realize the choke was controlled by fuse 12.  Ziggy in Doylestown found the problem and replaced the choke.

 

Thanks everyone for your inputs.