Brake light fuse failing

[Chadsorangespider]

I am a novice DIYer and just got my fathers 69-70 spider that had been sitting for several years. I was able to get it started after oil change, draining gas, new battery and after driving around the neighborhood came to realize the brake lights were not functioning.

The second from the left fuse goes out as soon as key is turned and after some troubleshooting I have determined that the white wire is causing the fuse to fail.

Any direction on how to track down what would be cause the fuse to fail would be greatly appreciated.

[AlexD]

I'm not familiar with the particulars of the early year Spiders but in looking into the schematics of that model year, I'm going to assume that you also don't have a functioning fuel gauge, tachometer, turn signals, engine temperature gauge, or cooling fan as they appear to share that circuit (Fuse I or 9). Starting with the most simple troubleshooting stuff

1. Make sure you have the right size fuse in the spot - I believe it's an 8A fuse and lightly clean up the fuse connection points to make sure you're getting a good connection to the metal.

2. (This will always be high on your list of solutions when dealing with these cars) will be to check your grounds - when in doubt, a loose/corroded ground is usually the culprit of wonky electrical issues. When I first got my car I read about everyone cleaning and checking their grounds and did the same - I cleaned all the ground pods and connection points to the car body (which serves as a central grounding point for these cars) and things just started to magically work, light, spin, etc. Start from the point where the negative battery terminal attaches to the body and work your way all around - scuffing up the paint where each ground pod meets the body and throwing a star washer on there to ensure "bite" into the metal. Does wonders.

3. Join Mirafiori.com and download all the reference material about your car - particularly the wiring diagrams as you will become intimately familiar with them and they will help focus your troubleshooting. Note that after 40-50 years, enough hands have been in these cars where some of the wiring might not match what you see in the diagrams so some problem-solving may be necessary but for the most part, this will be your roadmap. Not sure I'm attaching it correctly here (or if it will work) but here's the wiring diagram for the '69 Spider from the site. Looks like the white wire you referenced goes from the fuse block to the brake light

https://mirafiori.com/forum/viewtopic.p ... 970%201971

Your issue could be a short somewhere in the wire, a bad/broken bulb or contact point, etc. I'd start with the tail light assembly (as it's the most easily accessible of what's controlled by that circuit) and check for pinched wires going to the wiring harness or on the light assembly boards.

Happy hunting and embrace the frustration you'll experience while learning a ton about these cars. They're simple yet elegant in their own way but not necessarily logical when it comes to electrical stuff (coming from someone who's still relatively a novice).

[spider2081]

I have determined that the white wire is causing the fuse to fail.

I'm guessing you removed the white wire from the fuse and the fuse does not blow. I think this fuse also powers some of the gauges as well as the turn signal lights. I think leaving the light blue/white wire connected to the fuse and having a good fuse would let the turn signal lights and the gauges work.
Does the white wire terminal have 2 white wires in it? If it does you might consider cutting one (in an easy spot to re-splice it) to further isolate the electrical short.
Does the car have an electric coolant fan or is the coolant fan electric? I think if there is an electric fan the fans control relay is powered from the white wire on this fuse also.

A car this old could have had some wiring modified by a previous owner or during maintenance so finding the short could take time and patience.

[Chadsorangespider]

I'm going to assume that you also don't have a functioning fuel gauge, tachometer, turn signals, engine temperature gauge, or cooling fan as they appear to share that circuit (Fuse I or 9).

You are correct about these other gauges not working. My hope is after finding the short they will start working as well. I will start like you said with the grounds. Thanks for the direction.

[Chadsorangespider]

I'm guessing you removed the white wire from the fuse and the fuse does not blow.
Does the white wire terminal have 2 white wires in it? If it does you might consider cutting one (in an easy spot to re-splice it) to further isolate the electrical short.
Does the car have an electric coolant fan or is the coolant fan electric? I think if there is an electric fan the fans control relay is powered from the white wire on this fuse also.

Yes the wire that blows the fuse has two white wires in it. There is an electric fan by the radiator. I think I will start with reseating any ground wires I can find then further test by cutting one of the white wires as you suggested.

I might be in a honeymoon phase with this car but solving these problems is exciting and fun…so far.

[18Fiatsandcounting]

A few thoughts in addition to the good advice offered above by Alex and Spider2081:

1. If the fuse blows as soon as you turn on the ignition but without your foot being on the brake pedal, then it's likely not an issue with the brake lights. On the other hand, if you turn on the ignition and then step on the brake pedal and the fuse blows, then yes, something in the brake circuit is likely at fault.

2. Fixing the grounds on these cars is an excellent idea in general as noted, but it's not the cause of your fuse blowing. You've got a short somewhere.

3. On my '69 spider, Fuse I (#9) is powered by a large pink wire and then has three connections "downstream" of the fuse: Two white wires and a white/blue wire. One white wire does to the stoplight switch on the brake pedal, and one white wire feeds the instrument cluster gauges. The white/blue wire feeds several things, the brake warning light on the center dash, the back up lights (through the associated switch on the tranny), and the fast idle electrovalve in the engine compartment. Any one of these three general areas could have a short, so you could unbolt the fuse box, turn it over, and disconnect each of the three wires (two white and one white/blue). If you have an ohmmeter (and with the ignition off), you can measure the resistance between ground and the each of those three wires. If you see very low resistance on any of those wires to ground, that's the wire that has the short. You can also do this by seeing which wire causes the fuse to blow when you connect it, but that may require going through several fuses!

Hope this helps.
-Bryan

[Chadsorangespider]

Alright I was able to narrow it down. When I disconnected the fuel gauge wire connector fuse did not blow, also the tach, oil pressure, and water temp gauges started working as well and brakes and turn signals.

So any ideas on repair vs replace the fuel gauge. Do I test each wire coming out of the wiring harness?

[spider2081]

When I disconnected the fuel gauge wire connector fuse did not blow

Great job. I would remove the gauge from the dash and visually inspect all the wire going to the connector. Especially the wire that mates with the cars side of the connectors white wire. Maybe the issue is a wire that connects the injector gauge to the connector.

[18Fiatsandcounting]

Agreed, excellent sleuthing!! I'd also check the sending unit in the gas tank. You could have a short in the wires connected to it, or the sending unit could have an internal short to ground. In that state, it would blow the fuse as soon as the fuel gauge is turned on (i.e., ignition turned on). Try disconnecting the two wires to the sending unit, and see if you still get a short. If not, then it's likely the unit is bad. If you still see a short, then it could be a wire to it that is inadvertently grounded.

-Bryan

[spider2081]

When I disconnected the fuel gauge wire connector fuse did not blow

Great job. I would remove the gauge from the dash and visually inspect all the wire going to the connector. Especially the wire that mates with the cars side of the connectors white wire. Maybe the issue is a wire that connects the fuel gauge to the connector.

too early in the morning I don't know how I made this a new post. I tried to change the word "injector" to "fuel"

I don't know how to delete a post.

[18Fiatsandcounting]

Agreed, excellent sleuthing!! I'd also check the sending unit in the gas tank. You could have a short in the wires connected to it, or the sending unit could have an internal short to ground. In that state, it would blow the fuse as soon as the fuel gauge is turned on (i.e., ignition turned on). Try disconnecting the two wires to the sending unit, and see if you still get a short. If not, then it's likely the unit is bad. If you still see a short, then it could be a wire to it that is inadvertently grounded.

-Bryan

You can't delete a post, although I would like to delete this one that I made above. Please disregard what I wrote, as it is wrong. Grounding the wires to the fuel sending unit will either turn on the low fuel light or cause the gauge to read full (depending on which wire), but it won't cause a short and blow out a fuse. Sorry for my mental lapse!

-Bryan

[Chadsorangespider]

I found the short!!! It was in the fuel gauge as stated before. There is a yellow and black wire that connects in two place with a small jump wire and the second jump wire causes the short. It tested with continuity on the multimeter and appears to be connected intentionally between the black and white wire and where the second yellow and black wire connect. The also appears to be a small resistor that is under the metal piece where the fuel reserve indicator light is and the yellow/black wire shorts out.

What would be the advantages/disadvantages of taping this wire off and closing up the dash?

https://photos.app.goo.gl/PCHaD6ARX5xohNDy9

[spider2081]

I have a few old style fuel gauges. 2 have a diode ( not a resistor) soldered exactly where you mention.
The third does not have the diode installed. Diodes 2 failure modes are to short or to open. It appears the one you have is shorted.
I'm guessing the diode is an upgrade from the earliest gauges. Thinking as low levels of fuel sloshed around in the tank the fuel senders switch would bounce on and off, causing the low fuel warning light to flicker. With out the diode the gauges needle might bounce erratically. I think the diode sort of dampens the needles reaction to the senders bouncing.
Leaving the Yellow/black wire off the low fuel warning light will prevent the light from operating,as it powers the light.
Replacing the diode is the ultimate repair. I think a 1N4001 diode would suffice. They cost less than $1

[Chadsorangespider]

I have ordered some diodes. I will update when they come in and I attempt the repair.

Thank you to spider2081, 18Fiatsandcounting, AlexD for you helpful advice and willingness to share your knowledge. Hope I can give back in the future.

[spider2081]

I didn't think of this for my original post but if you cut either lead on the diode I think everything will work. The fuel gauge needle may be over active when the fuel is below around a 1/4 of a tank.