Keep blowing out 20AMP fuse to tail-lignts on 72 240Z

[landmizzle]

Hi all,

The good news is I finally purchased a new (to me) '72 240Z and have her at home. The bad news is I can't drive her at night because the 20AMP fuse to the tail-lights (third fuse from the bottom on the right hand side in the fuse box.) burns out after about 3 secs once I turn the head-light combo switch to the #1 or #2 position.

I knew about this problem when I bought the car but figured it was just a bad switch. Nope. I've replaced it and that didn't help.

I'm not an electrical expert so I'm hoping someone on this forum can point me in the right direction.

Here's what I know. This fuse has a green-white (gw) wire running to it. It feeds the tail-lights, running lights, and dash lights. When I put a fresh fuse in, the dash lights come on briefly then the fuse burns out. The tail-lights and rear running lights (side markers) also come on but are very dim. The front parking lights and side-markers do not appear to come on.

I've looked at the wiring diagram and have observed that the dash-lights, rheostat, tail-lights,license plate lights and rear marker lights are feed by the gw wire. The front parking lights and side marker lights are feed by a green-blue (gl) wire.

On the head-light combo switch there is a plug that on the switch side has a gw wire. It ties into the plug and connects to a gl wire on the other side of the plug. If I jumper the fuse that keeps blowing, the gw wire on the combo-switch gets very hot and well begin to smoke within about 5 secs. The hot spot is just before the plug on the switch side.

I assume a high resistance is developing somewhere in the circuit and thus the smoking gw wire. I really don't know where to go from here though. Does the hot spot develope near the short point? Since the gw wire connects to the gb wire near this point, does that mean there's high resistance in the gb wire?

The connectors in the combo switch plug look fine (i.e. no corossion.). All other electrical components work fine including the head-lights, turn signals, etc. The hazard lights worked when I first inspected the car but didn't when I picked it up. Don't know if this could be related.

I've checked the bulbs and connectors on the tail-lights, license plate light, and left rear marker lights and they look fine. I haven't checked to the front parking lights and marker lights yet as far as the bulbs and sockets are concerned.I've also followed the gw and gl wires around the body and engine bay and haven't found any obvious cracks in the wiring from what I can see. Certainly haven't checked every inch of wiring yet though.

I've considered removing the fuse box and taking a closer look at it but, that will necesatate the removal of the center console. The fuse box from top side looks fine with no melted plastic or corroded contacts. I can remove the console for a closer looks but I wanted to get some advice before I start blindly removing parts that may or may not be involved in this issue.

If anyone can give me some hints/advice on what may be wrong or what I should do next to better isolate the problem, I'd be extremely grateful.

Best Regards,

Landmizzle '71 and '72 240Z

[MikeW]

I'm no electrical expert but a blown fuse is an indication that there's a short circuit somewhere. Electricity normally flows from the battery to through the fuse to the light and finally to ground. Since the light has a lot of resistance not a whole lot of current flows and therefore the fuse doesn't blow. If, on the other hand, if the electricity is somehow getting to ground before the light there's almost no resistance, too much current flows, and the fuse blows. This is what the fuse is supposed to do in this situation. To verify it you'll need to pull the fuse (or leave in the blown fuse) and using an ohm meter see if there's resistance between a known ground and the hot side of the lamp. Normally this would be open. If you get a reading you've got a ground somewhere in between. Testing with the switch on or off should tell you which side of the switch the ground is on.

[Daniel]

First let me say excellent excellent effort!!You have a good line of thought going there.I would suggest a couple of things.First go to the lights that dont work and find out why.That needs repair anyway and may be your demon.If not remove all the bulbs and see if it pops with no load.Those sockets could use cleaning and wd40'ed so it 's not a waste of time.I have seen bulbs look good but short.I wouldn't worry about the dash bulbs until last resort.Also remember your headlight hi'lo switch is in this mess also.How does it feel and look.If no blow with the bulbs out reinstall a couple at a time to test.Your doing great! Most of the problems with ANYTHING broken will be found where human hands were able to touch it.Always try those first.

[landmizzle]

Howdy,

Thanks for the compliment!

I found the problem! The front passenger side marker light was incorrectly wired. I couldn't tell at first because the wires where covered in undercoating or some such goo. Following advice given, I disconnected the front marker lights and the problem went away. Bingo! Narrowed it down.

Thanks to both of you folks for your input. I really appreciated the quick response.

Now, I get to go for a ride after the sun sets. The wife wants to drive the Z now too. She's never cared to drive them in the past but since this ones an automatic, she's all jazzed about the idea. Go figure.......

Best Regards,

Landmizzle '71 and '72 240Z

Originally posted by Daniel

First let me say excellent excellent effort!!You have a good line of thought going there.

[Daniel]

Great!! Glad you're fixed. Notice that the problem was within the "human hands can touch it"area.Seems they did huh?