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Electrical Gremlin

6

67Boomer

Guest
Hopefully someone out there can give me some advice on an electrical gremlin I have in my 67 big block convertible. I am just coming to the end of a two year restoration on the car which included restoration of the gages and panel, removal of underhood wiring harness (this harness was unwrapped, checked, then re-wrapped), all lights, etc.
Everything is back together and works fine except for one fuse. The fuse block says this is the 10 amp fuse for the back-up lamps and gauges. As soon as I turn the key to the "on" postion, the fuse immediately blows. I've already checked for any obvious wiring problems, but no luck so far. Can somebody provide a step-by-step approach to finding my gremlin?
 

kbuhagiar

Well-known member
Joined
May 1, 2002
Messages
80
Location
South San Francisco, CA
Corvette
1963 Convertible 327/300 3TL
If the fuse is popping IMMEDIATELY after turning the key on, you probably have a ground fault. Rather than troubleshoot-by-blown-fuse, get a hold of a volt-ohmeter and verify by looking at the circuit going out. You should get a very low resistance reading (just a few ohms) to ground. Now, it's been my experience that most electrical faults show up after some portion of the equipment or vehicle have been 'disturbed'; in other words, the problem is most likely (but not necessarily) located somewhere where you just did some work. Since you mentioned the instrument panel, my first thought would be to disconnect or somehow clear the instrument panel harness from the rest of the wiring harness, than take another reading at the fuse panel. If the ground is gone, take a close look at the instrument panel assembly. If the problem is still there, then your next step would be to further isolate the circuit, a process that will try your patience and your physical dexterity.

Let me know what you find, and I'll try to assist further.
 

WayneC

Well-known member
Joined
Jan 5, 2002
Messages
472
Location
Santa Barbara CA
Corvette
63 roadster project
I haven't done this myself, but I've heard it's possible to buy a recycling circuit breaker that fits into a fuse holder. By using that, one can alternately disconnect/reconnect one device (load) at a time until the circuit breaker stops cycling when a particular device is disconnected.

Since the Corvette is mostly fiberglass, the most likely cause is a hot 12v lead incorrectly connected to a grounding post, rather than a wire being pinched/cut by metal, although it's possible to inadvertently put a screw through the harness and thereby connect a hot lead to a ground lead. Black leads (with or without a tracer color) are grounding leads. I'd first doublecheck the ignition switch wiring, then check to make sure the wire lead that connects to the top center rear of the dash is actually a grounded lead (not a 12v feed), then at the backup lamp switch to see if the leads are on the correct posts and the switch actually works properly.Then move on to other load devices to check that the leads are correctly attached.

One other method of checking is to make up a long test line with a 10 amp fuse in it (leave the old 10 amp fuse removed from the fusebox), then individully test any devices on the circuit at the device by removing the "hot" harness lead and connecting your test lead (which has it's other end connected to the plus side of the battery)... if the device works without blowing the fuse, it's not likely that device (or it's wiring) is the problem... but if the fuse blows on more than one device in this testing, then the problem may be in the harness.
 
6

67Boomer

Guest
Thank you both for your help and insithts! Last night I made a major breakthrough...I've traced the problem to the pink wire that attaches to the sending unit on the bottom of the fuel tank. When I disconnect that lead, the fuse stays and all else on the circuit seems to work!

Any information as to how the fuel sending unit functions and what the pink lead does would be appreciated. In the meantime, I'll continue to trace the wire. Thanks again!!
 

WayneC

Well-known member
Joined
Jan 5, 2002
Messages
472
Location
Santa Barbara CA
Corvette
63 roadster project
You have email. The pink (on some midyears it's light green) wire is 12v feed from the ignition switch. It goes to a terminal marked "I", while the tan wire goes to a terminal marked "S"... perhaps you may have connected it to the wrong post on the fuel sender during your restoration? If you did connect the pink wire to the "S" terminal, I'm afraid you may have fried your tank sending unit... see my email for some troubleshooting procedures.
 

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