[69geeto]

So, my 69 GTO is blowing the 20A fuse for the running lights. I check the load side and see .8 Ohms. (VERY low.. and all are LED bulbs). If I turn on the headlights, the resistance goes up to maybe 10 ohms (and the fuse will then hold). I've never seen anything like this one.
Any thoughts? Nothing has changed over the last year, and it just started happening.

[mzbk2l]

Does it blow as soon as you put a new one in, or as soon as you turn the lights on, or just at random intervals?

[69geeto]

Pretty soon as it's turned on. With .8 ohms or less, the current is hitting close to 20A. Just very strange that that resistance goes higher when the head lights are turned on. I may need to look at the light switch.

[mzbk2l]

If the LEDs don't have built-in resistors, they may show almost zero resistance. If the headlights are still incandescent, do they add their resistance to the circuit when they're turned on? (Hence your higher readings with the headlights on.) The only LEDs I've installed (other than interior lights) are the daytime running lights in a 2005 Tahoe that liked to go through a bulb every month or so until I switched to LED. Those were Phillips and advertised as plug & play. I've never checked resistance in that circuit, but it hasn't had any problems in the 6 or so years they've been in there.

[69geeto]

I'm sure the led's have built in resistors. I did check the resistance and it was very high (low current draw). They were installed over a year ago and all was well until last week. I'll keep plugging away. I think pulling the switch might help troubleshoot.

[Bill Hanlon]

Trying to read resistance (ohms) on an active (has voltage applied) circuit isn't valid. The ohmmeter injects a small voltage into the circuit to be measured and gets real confused when other voltage is in the circuit.

[69geeto]

I pulled the fuse and measured the load side with a fluke meter. The load side is in fact less than 1 ohm (way to low) which would draw 12 amps.. Ya, can't measure resistance with voltage applied.

[Geoff]

Sounds like one of the LED lamps has developed an internal fault & is drawing excessive current, which is blowing the fuse. Remove the LED lamps one at a time to see if it fixes the problem.

[Greg Reid]

And don't keep blowing fuses. Pull the bulbs one at a time and check the resistance after each one. If the resistance is still low after all of the bulbs are out, start checking the sockets and wiring for shorts.

[69geeto]

Well, I found the problem. Wire pinched between the right parking light connector and the frame. Intermittent short took a while to find.
Thanks for all the help and tips.

[69geeto]

A quick picture of the wiring .

[69geeto]

Also should mention that the dash lights and running lights are separated at the light switch. So pulling the switch wires and checking resistance separately, helped narrow things down considerably.

[mzbk2l]

Thanks for the followup. Was that something that had been apart recently, or did the wire finally just rub through after 40+ years?

[69geeto]

The car was restored in 92 and not touched since. So 32 years of rubbing that wire to break through.
This one had me stumped for a while. 😆 I'm getting too old to lay on my back looking up through the fender!!