1972 Possible short very warm fuse

[gotswap]

So I am a new proud owner of a 72 240z and have a couple questions

How warm do the fuses get while being used ie Headlight and Parking/Running Lights.

I have not popped a fuse yet but when looking at the fusebox the AC and Parking Light slots are melted, if it is a short somewhere I imagine it would pop a fuse immediately, could be wrong though. I know it is not any of the light subharnesses because I unplugged them all and the fuse was still quite warm. Are there any likely areas where the harness is routed that is a common issue for wearing off shielding?

Thanks

[doradox]

This is a classic Z problem. The fuse is not making a good connection with the holder. Too much resistance and that causes the heat. It will eventually melt the fuse inside one end, but not in the middle. It won't look "blown" at first glance. Clean the holder and the fuse and try it again. I've seen this on pretty much every 240 I've seen except, oddly enough, the 72 I own now.

Steve

[gotswap]

I was hoping that is what it was. From reading some other "similar" posts that was going to be my next course of action. Am I right in assuming that it is for sure not a short due to it not popping the fuse?

[Nissanman]

It is usually by chance that we detect how warm fuses actually are when passing their rated current.

Cleaning contacts has been mentioned [very important], but you might check all the connections to the fuse box for tightness.

The factory either riveted or spot weld the wires to the fuses and if these are loose and/or corroded, they will introduce a high resistance joint which can cause excessive heat.

Just a gentle wiggle will be enough to determine if they are tight and safe.

If you have a Multi-meter, you could measure the fuse to wire resistance and see how they shape up.

As for the fuses themselves, a normal 3AG fuse may need to pass up to 200% of its' rated current before rupturing.

That is why it is important to replace a blown fuse with one of the same rating or slightly lower rating.

Also, you could squeeze the fuse holder tabs to grip the fuse tighter, thus ensuring a low resistance contact.

Unfortunately, this "old technology" fuse system has some inherent problems!

The more modern blade style ATC fuses are designed to introduce a significant friction component to the fuse contacts and are not susceptible to the physical breakdown that a tubular glass fuse suffers.

[gotswap]

Measured the resistance at the fuse holder and then all the way along the path to the multifunction switch and everything checked out fine. I then reseated all of the connectors, at least the ones I could find. I did this at the fuse block on the steering column and at every light socket this morning trying to track down a potential problem. I am fairly certain that the harness is good.

I did what you all recommended and cleaned the fuse holder with some 1500 grit and wala, no more heat.

Found a couple more that were getting a bit warm so I'll tackle those tomorrow after work. Do you think it would be beneficial to drop the cash on one of the aftermarket fuse boxes?

Thanks for the help!!!

[TomoHawk]

If you want to clean the whole fuse box system at once, I'd recommend using Evaporust. It will remove rust & oxidation and not harm the wiring, plating, insulation, rubber, or plastic parts. then a lttle DeOxit on the contacts to improve the last of the surface and add some protection.

[AggieZ]

You should check behind the fuse box and see if an oversize wire has been run into the fuse holder.... or an additional wire into the same fuse holder. Also, check the rating of the fuse. Its probably an oversized fuse put into run an extra electrical demand like an amp for the radio or electrical fuel pump. Even if the wires in and out, and the fuse are rated ok, the melting copper around the fuse indicates that the original copper is cheap. The solution is to re-route the added electrical component through a completely independant line and new independant fuse. Or im not half as smart as I think I am.

Edited August 5, 2008 by AggieZ

[gotswap]

All the wiring is in excellent shape and the only aftermarket electrical item on the car is a new headunit. The fuses are also the correct size per the fuse box cover.

I went to checkers last night and picked up all new fuses and some of the electrical contact grease so I should be set for awhile.

I am still wondering if the aftermarket blade style fuse box is worth it...

[Arne]

The aftermarket fusebox is not a bad idea, but in reality it doesn't truly address the root problem, which is the convoluted way that the lighting circuits route the current, leading to excessive heat in the fusebox. This is a terribly common problem with 240Zs, and is best addressed by using relays to get the heavy current out of the factory wiring, fusebox and combo switch. Search this site for 'headlight relays' to see volumes of posts on this subject.

[Mike]

If you don't care about keeping the stock fuse box, I highly suggest that you replace it with a new unit. MSA sells a really cool fuse box that is built on a circuit board design and uses the newer style spade fuses. I installed one and it looks/works great!

http://www.thezstore.com/page/TZS/CTGY/SEIC04D

[gotswap]

I might consider it since the

[gotswap]

I might consider it since the original fuse box that is in the car has seen better days.