240z Fuse Box Replacement - Feedback Request - MSA, ST, Etc.

[peng155]

The basic problem is that as the car ages... so does all of the electrical wiring in the car, as the wiring ages it starts to oxidize.. as the wiring begins to oxidize, the resistance of the wire increases, and the current passing through the wiring begins to experience higher resistance, higher resistance results in the wiring getting hotter then it was designed to.

This is what is happening with the parking Light circuit, and headlight circuit, current passes from one side of the fuse (The battery, or source side) thru the fuse and then on to the load (the headlights).

the weak links... say the combo switch, and the rivets securing the fuse holder to the fuse box experiences more resistance then it was designed to (the rivet get hot, and melt the fuse box plastic, the contacts in the combo switch degraded) ... the only way I think (not positive).. to totally repair the underlying cause of the problem is to replace electrical wiring with new non-oxidized wire.

What Dave's harness upgrade does is to place a relay into the circuit to take the high current load off of the factory headlight circuit, instead of having say (Just guessing here) 15amps or more of current passing through the factory electrical wiring... only a few milliamps of current is passing through the factory wiring to power up the rely.... and the main current to power the headlights goes through the relay circuit in Dave's harness upgrade.

Edited October 6, 2014 by peng155

[jfa.series1]

BvtnDave -

Phil provided an excellent description of Dave's harness benefits. These are really great upgrades for your car and will potentially save your combo and turn signal switches from burnout. So... DO add them to your MSA order for the fuse box, your car will thank you!

Also, change out your old, tired turn signal and emergency flasher cans for electronic units similar to this one: Novita/Hazard Warning Flasher (EL12) | AutoZone.com

These units are available at almost any auto supply store. No more pokey turn signals!

[Jarvo2]

Here's how I DIY'ed the headlight relays. I'd suggest anyone not comfortable with electrical work, soldering, etc. to just purchase the MSA version.

Dave's 1972 Datsun 240z: Headlight wiring harness with relays

[santamaus]

I know this thread is a little old, but it came up on a search for replacement boxes.  I just installed the headlight upgrade harness (which is awesome and easy, btw!), and noticed that my parking/tail lights were out.  Traced it back to the fuse box--yay.  Saw the cover was head distorted and the inner clip was slightly melted, as was the metal inside the fuse, but the not in the middle where it should have gone out.

So--question is, what causes this particular fuse (and the inner side of it) to consistently overheat?  Have I unknowingly solved the problem with the headlight upgrade?  It seems silly to just replace the box only to have it overheat and fail again because I haven't solved the problem that caused it in the first place.

[SteveJ]

The headlight relay harness does not impact the parking lights at all.

I posted this a few years back: 

The best way to keep the circuit intact (IMHO):

1. Change all of the exterior markers and interior dash bulbs to LEDs. (You will lose the dimmer function unless you change the rheostat to a PWM, but that's a topic for @Captain Obvious to describe.)
2. Change the 20A fuse to a 10A fuse. (This limits the current and heat build-up.)
3. Clean the contacts (front and back) on the fusebox. (This should get rid of hot-spots in your fusebox.)

[zKars]

Time and corrosion that slowly builds on the contacts, creating ever increasing resistance at the fuse/clip contacts and even within the wire crimps at the connectors. This creates increasing heat. Which accelerates the corrosion, which accelerates the heat, etc. That particular circuit operates at a fairly high current level that makes it the first go. 

BTW the fuse selection itself does nothing to limit current, it is just a switch that blows when the current in the circuit exceeds its melt point. Voltage and resistance in the circuit determine the current. V=IR 

[SteveJ]

The fuse is a current limiting device by definition. One of the problems with that circuit was that it was designed to have too close to 10A draw, so the engineers selected a 20A fuse to prevent nuisance trips. This gives the opportunity to build up more heat at the fuse clips. At close to 20A, that sucker will glow in the dark. I have seen it.

Using the 10A fuse in conjunction with the other steps I listed will reduce the opportunity for heat to build up to the point of melting the fuse box. If the fuse is blowing at that point in time, it could mean it's time to clean the corrosion from the fuse box and light sockets again.

For owners of 73 and later cars, there is another incentive to drop the fuse to 10A (after swapping over to LEDs). The gauge of the wire at the 9-pin connector on the combo switch is too small to support 20A of current. I had a short at one of the bulbs in the gauge light portion of the parking light circuit. Since the current in the short was too much for the wire, the 9-pin connector was damaged. A properly sized fuse (in this case 10A) would protect the wiring and connnectors.

[santamaus]

Thanks for the speedy reply!  I'll swap those for LEDs, for all the obvious reasons.  Pretty much all that I saw are listed as 1156 bulbs with less than 5w draw--I'm guessing from your post above that the sockets are identical between the 67/89/1156 bulbs--they're just different wattages?

My rheostat is not awesome, so it's no worries replacing that--did you have a recommendation on something that fits easily?  I'm guessing with full LEDs I'll need to swap out the flasher unit as well, looks like that's in the steering column, so I'll find out what will fit there when I take the headlight switch out to clean it.

[SteveJ]

57 minutes ago, santamaus said:

Thanks for the speedy reply!  I'll swap those for LEDs, for all the obvious reasons.  Pretty much all that I saw are listed as 1156 bulbs with less than 5w draw--I'm guessing from your post above that the sockets are identical between the 67/89/1156 bulbs--they're just different wattages?

My rheostat is not awesome, so it's no worries replacing that--did you have a recommendation on something that fits easily?  I'm guessing with full LEDs I'll need to swap out the flasher unit as well, looks like that's in the steering column, so I'll find out what will fit there when I take the headlight switch out to clean it.

Are you referring to LED or incandescent bulbs for 5W draw from an 1156 bulb? Phillips lists the incandescent at 26.9W. The wattage of the different types of incandescent bulbs are listed in the spreadsheet I attached to the post in the other thread.

There is not a ready replacement for the rheostat. The bulbs I used aren't overwhelmingly bright, so I have no need to dim them.

If you replace the turn signal bulbs, you will need to replace BOTH flashers. I have been using these for 5 years: https://www.amazon.com/gp/product/B011BTMDQM There is one on the driver side under the dash, and the other is on the passenger side.

 

[santamaus]

Yes--LEDs, I saw a range of about 2W to 5W, mostly depending on lumen output.  I knew there were two flashers, I didn't know they where in two different places.  Did you end up just bypassing the rheostat and bridging the two sides?

[SteveJ]

I just put the rheostat on the lowest resistance, and I leave it there.