One trick you may use is to wire in a 12v light in place of the fuse that keeps blowing. If it lights brightly, you have a short. You can then move wires around and see if you can find the issue. Move the wires around that you suspect. Disconnect the battery anytime you are messing with the fuses or connecting the test light (I use insulated alligator clips). One slip while installing fuses or hooking clips up can result in a blown fuse.

One trick you may use is to wire in a 12v light in place of the fuse that keeps blowing. If it lights brightly, you have a short. You can then move wires around and see if you can find the issue. Move the wires around that you suspect. Disconnect the battery anytime you are messing with the fuses or connecting the test light (I use insulated alligator clips). One slip while installing fuses or hooking clips up can result in a blown fuse.

Fuse lights up then blows with lights off key off. Same thing right? There’s definitely a short somewhere.


12/71 2.6 block E88 heads triple weber 3/2 Mallory Abarth Tokico 1-3/4 sways

yes but with the light on you can monitor the testing with less risk to the existing wiring. the light introduces resistance to the circuit and protects the wiring from constant loads (at least until the fuse blows). The light will be on but the brightness will vary as you turn things on and off and hopefully enough of a change in brightness for finding the short.

One more thing, you may want to remove the access panels and take a look at the tail light sockets and wiring, IIRC have read about issues with shorts back there.

Just now, Dave WM said:

One more thing, you may want to remove the access panels and take a look at the tail light sockets and wiring, IIRC have read about issues with shorts back there.

Those would not come into play with the switch off. The OP said he messed with the radio before his problems started. Considering the circuit has constant power, it would be tempting for someone to tap into that circuit for the memory functions of the radio. The fact that the radio was touched recently makes that area a prime suspect.

I hope the OP will follow the troubleshooting in post #8 and let us know the results. If more diagnostics are required at that point, it can be performed in a measured approach instead of shotgunning ideas.

Awesome! Might take me awhile but will certainly report findings.


12/71 2.6 block E88 heads triple weber 3/2 Mallory Abarth Tokico 1-3/4 sways

Oh ok, I remember my brake lights (I was thinking brake lights, not sure if that is the same as "parking" lights) being active regardless of key on or off. my bad.

I also see the tail lights are not hot (+ goes thru switch under pedal, ground is continuous, I am used to seeing the ground be the switched circuit) unless the brake switch is activated. oh well.

ha timely post one of my brake lights is burned out!

Edited October 25, 20177 yr by Dave WM

Both the brake lights and parking (aka running) lights have constant power.  Neither depend upon the key being in the On or Start position to operate. They are on completely different circuits, though. 

Again, diagnostics were posted to isolate the problem. Once those results are known, then we will know whether the problem has an easy fix or more diagnostics are needed.

I have a similar issue:  I have spent the last couple of months bringing my '72 back to life from a long slumber.   When it came time to check all the lighting, I noticed none of the running lamps worked at all.  Checked fuse...yep, blown.  Replaced fuse without it blowing (20A, switch off).  But, turning the light switch to position one produced no light and the fuse becomes too hot to keep a finger on the terminal (but fuse did not blow).  I traced the green/blue wire to the right side firewall junctions and get continuity to three wires at the junction block with the green/blue wire.  One of them is a large gauge red wire.  I don't think this is correct, so I need to start at the switch which I will do tonight.  Will remove the switch and confirm it behalves as indicated on the wiring diagram.

Any other suggestions?  Might help both me and the OP.

Incidentally, I removed the fuse block and it is perfect...no loose connections or discolored (from heat) terminals.  Headlights work, dimmer does not (might be relay...need to check that, or switch...will check when checking combo switch).

5 hours ago, David F said:

I have a similar issue:  I have spent the last couple of months bringing my '72 back to life from a long slumber.   When it came time to check all the lighting, I noticed none of the running lamps worked at all.  Checked fuse...yep, blown.  Replaced fuse without it blowing (20A, switch off).  But, turning the light switch to position one produced no light and the fuse becomes too hot to keep a finger on the terminal (but fuse did not blow).  I traced the green/blue wire to the right side firewall junctions and get continuity to three wires at the junction block with the green/blue wire.  One of them is a large gauge red wire.  I don't think this is correct, so I need to start at the switch which I will do tonight.  Will remove the switch and confirm it behalves as indicated on the wiring diagram.

Any other suggestions?  Might help both me and the OP.

Incidentally, I removed the fuse block and it is perfect...no loose connections or discolored (from heat) terminals.  Headlights work, dimmer does not (might be relay...need to check that, or switch...will check when checking combo switch).

It doesn't sound like anything the OP is experiencing. You need a different set of diagnostics, similar to what Dave WM described in post #15. Buy a test light and post back when you have it.

Thanks, but I don't need a test light to show/tell may anything that my VOM won't tell me.   And, a fuse that blows and a fuse that gets too hot are very similar...both are drawing too much amperage.  Why might be the difference.  I just need to trace each circuit feed by the the running light fuse.

I have verified my combo switch works as it should and I got the mechanicals of the dimmer switch working properly.

2 hours ago, David F said:

Thanks, but I don't need a test light to show/tell may anything that my VOM won't tell me.   And, a fuse that blows and a fuse that gets too hot are very similar...both are drawing too much amperage.  Why might be the difference.  I just need to trace each circuit feed by the the running light fuse.

I have verified my combo switch works as it should and I got the mechanicals of the dimmer switch working properly.

You don't even know what your meter is telling you. That is obvious from your first post. You don't even know the difference between a dead short and an overloaded circuit. What you're lacking in knowledge, however, you're making up with attitude.

If you want to drop your attitude and learn how to diagnose your problem, let me know. There are reasons for the instructions I give, and I don't necessarily explain them fully. Then again, I don't need to.