Road Runner Nest

Hi guys,

I'm having problems with my tail lights. The fuse for the rear driving light pops when I turn the lights on. If I remove the fuse, the stop light and turn signal funktion works fine, its only the driving light that makes trouble!

Should I look for the short in the wires from the dash to the rear or could it be the light switch or??

I hope you guys can help me out!

Capsule

I had a fuse popping issue relating to my rear lights, and it ended up being bare wires INSIDE the grommets where the wires go through the inside rear of the trunk. They had chafed and one was rubbing past the grommet. I couldn't see it until I pulled the wires back into the trunk a wee bit and EUREKA! Hope this helps.

Thanks aerodynamic,

I did actually find one place where the wire core was rubbing on the metal clip in the trunk. I fixed it but there's still a short somewhere!

I think I will try and pull new wires to the rear, it's not that many wires and the carpet and seats are out= not too much hazzle to do the job!

Capsule

disconnect the harmess by the drivers side kick panel for the rear harness.

next- remove all the bulbs- tail, side and licence plate.

now with a ohm meter- check the black wire at the kick panel location that goes to the back and other lead of the ohm meter to a clean ground- should be open-this would be a normal reading.

now with a short it will show resistance. get a extra buddy to watch the ohm meter while you go to the rear and wigle the harness around- when the reading changes- you have found the area of the problem. if nothing then check along the drivers side to the back of the rear seats- the wiring could be pinched along there.

John Mac

OK,
so I changed the wires for the rear lights hoping this would solve my popping fuse problem. -well, doh! still pops the fuse.

When I look at the fuse block, it says "Miscellaneous" on the fuse that pops. I'm thinking that this means lots of small circuits are using this particular fuse and that it might not nessesarily be in the rear light harness the short is located!?

In other words - it could be anywhere in the car!?

How about the light switch in the dash, is this a common place for a short cut?

I hope someone's got an idea or two?

Capsule

The switch in the dash could have a bad wire. I remember when I pulled mine that the wires weren't that good. It didnt pop breakers but I know it would be a matter of time. I found an NOS switch. But I think the lights have their own cavaty in the box itself. It sounds like someone wired the lights to "misc." Do you have a schematic? More than likely, you'll have to start chasing wires.

That fuse is probley the acc. fuse- many things can be there- there is two yellow connectors in the harnes that have 3 rounds bullet type connectors. uses are---low fuel relay, air cond. feed for the clutch, cruise control, headlamp delay relayand rear defrogger.

this should be on the right bank of fuses- second from the top!!

John Mac

The fuse that blows is located on the bottom left row.

Does anyone know if my tail lights should even be on this fuse? -if not, which one?

Capsule

OK- THAT IS A 20 AMP FUSE- FOR THE CIG.LIGHTER, RED CONNECTOR FOR ANY BATTERY ACC.- IE: TIME DELAY LAMP, MAP LIGHT, GLOVE BOX LIGHT

ALSO GOES TO THE B2 TERMINAL FOR THE HEADLIGHT SWITCH- SHOULD BE THE PARKING LAMP POWER.

NOW DID YOU DISCONNECT THE CONNECTOR AT THE DRIVERS KICK PANEL THAT FEEDS THE REAR OF THE CAR???? DID THE FUSE BLOW THEN????

JOHN

Thanks 71_DEPUTY,

no I didn't try the power on without the rear light harness connected, will do that next!

Capsule

Assuming that the 71 wiring is pretty close to the 72 wiring diagram I have, with the rear body harness unplugged(left kick panel) and still blows fuses when parking lamps are switched on, you will need to disconnect the forward body wiring at the bulkhead connector (firewall engine compartment). It’s the middle plug (containing pin 10-black/yellow wire), that will take the front parking/side marker lamps out of the picture. Should the problem remain, then the short is in your dash harness between the headlight switch and the bulkhead connector (black/yellow wire) or between the headlight switch and the rear body connector (black wire). The fact that the fuse blows only when the park lights are turned on eliminates all the above mentioned electrical devices that share that fuse. More likely to be a problem with a parking lamp or side marker bulb socket.

electrical trouble shooting guide

if a fuse blows load is increased over the fuse element

a good way to resolve this issue is a multimeter test

check resistance over a known good conduit in the circuit without power applied.

make sure the resistance measured from the wire to the fuse in the block is equal to a known good draw source.

secondly is to get accurate amperage reading off a known good conduit.

state if draw is 10amps(arbitrary amp amount) and your draw is 20amps(arbitrary amount)
then you have a amperage overload condition being met

remember to observe polarity in the amp test and to place the meter in series with the device

overload can be caused by too many devices on a single circuit

the incorrect gauge wire being used in the circuit conduit
(usually to small a gauge wire not usually recognized by automotive repair in the 60s-70s repair shops)
and short to ground condition.

how to fault isolate once condition has been found

with multiple devices

remove one device from circuit at a time

test for voltage present on both ground points and power conduits

you can use a test light for this

older vehicles are notorious for bad ground condtions.

old wiring frayed or brittle wiring needs to be replaced.

test continuity in the circuits.

Final resistance checks.

if you find you have too many devices overloading the fuse.

Check your fuse block rating

you may have to upgrade the fuse or find another power source for that

device. This occurs a lot of times when people take older vehicles and

add newer devices such as car audio systems without proper electrical upgrades

if you have any questions feel free to message me on yahoo josephgiannini or aim joegiannini3

Okey,

so I tried to disconnect the rear light harness at the drivers door, and it still pops the fuse!

I tried to unplug the front light harness by the bulkhead, then there was no power in either the dash or the rear lights.

I just found out that the colour wiring diagram that I got from ClassicCarWiring doen´t match my car that well but luckily I do have the Body and Chassis Service manuals.

Next step will be to try and get the light switch out. How do I get the rally cluster out of the dash, it seems that the heating controls prvents the cluster from coming out!?

Capsule

power from the battery routes thru that same connector.

best way is to remove the black/yellow tracer wire out from the connector and then plug it back in and recheck to see if it blows or not.

if it doesn't - the problem is in the dash somewhere.

Overlooked that minor detail, the battery feed to the dash harness is in that center connector. If the fuse doesn’t blow after removing the black/yellow wire from the connector would actually indicate a problem in the forward lighting harness and not the dash harness however.