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MG MGB Technical - Wiring improvement Suggestions?

Im no electrician! Gimme nuts and bolts any day.

Should I put a 'mains fuse' in the main feed line from the alternator??

Iv got the loom (harness) out so any sensible modifications will be a lot easier to do now!

Does anyone have worthy suggestions? (I have a headlight relay question thread also)

Thanks in advance.
Innes Bint

Either a battery cut-off switch or *two* fusible links - one in the brown(s) up from the solenoid to the cars electrics and the other in the brown(s) also up from the solenoid to the alternator. One in the alternator feed alone is marginally the least necessary of the two and only protects against an internal short in the alternator or the brown(s) feeding it. That still leaves the browns feeding the rest of the cars electrics.

Whilst a combination of battery cut-off switch and turning off the engine will protect the harness from further damage if the problem is a short in the brown wires or alternator, damage will still occur in the time between seeing/smelling smoke and acting. Fusible links *should* protect against any damage. Loom in or out shouldn't make that much difference, they need to be in the tail going down to the alternator, and easily accessible with the harness installed i.e. above the cable clip. However that still leaves the section of brown wire between the fusible link and the alternator unprotectd while the engine is running, unless you add a 3rd one at the alternator.

Bear in mind that if fusible links do blow while the car is underway you will lose all electrics, including headlights if driving at night on unlit roads, and all that implies. Really you also need to separately fuse the headlights (one per filament, with or without relays) fed from the solenoid and *not* from the fusible links to guard against this. Either that or put separate fuses in each branch of the brown circuit (i.e. fusebox, hazards, ignition switch, starter relay, ignition relay, other lighting) plus individual fuses for each headlamp filament all fed off the fusible link. Any short on a brown wire will then blow its fuse and not the fusible link, the fusible link being a last-ditch protection. And you can *still* lose main and dipped beams, unless you have the presence of mind to pull on the headlamp flasher.
Paul Hunt 2

Hi Paul. Thanks for the tips (and photo of your relays on the other post).

Quote: "Really you also need to separately fuse the headlights (one per filament, with or without relays) fed from the solenoid and *not* from the fusible links to guard against this"

Am I correct in assuming the "fusable links" you refer to is the factory 'fuse block'? And therefore, I need to fuse upstream?
Innes Bint

I have just added this image. If you would care to "photoshop" or "MS Paint" where exactly you mean Id appreciate it.

Cheers


Innes Bint

No, 'fusible links' refers to additional high amperage fuses in the main brown wires near the solenoid, 40 or 50 amps or so. These prevent harness damage in the event of a short on an unfused circuit, of which there are several. I've shown two fuses in red in the attached picture. Both browns from the alternator must go through one fuse, and both browns feeding the rest of the cars electrics (i.e. ignition switch, fusebox etc) should go through the other. Having said that there is something to be said for fusing the two wires to the ignition switch and fusebox separately as in the event of a short that blows a fuse inspecting the blown fuse (and the non-working circuit) will show which 'branch' has had the problem. The down-side is limited space where the tail comes up from the solenoid on cars with pre-engaged starter, particularly the V8. Great care must be taken when adding components to the brown circuits, you don't want to *increase* the likelyhood of harness melt-down.

If this, and all the other aspects of adding fusible links and consequently separately fused circuits for the headlights is too much for you, then at the very least you should add in-lines to the fuel pump and the overdrive. These are much simpler to deal with and just involve the insertion of fuses with bullet or spade tails (depending on year) in convenient locations that already have bullet and spade connectors. See http://www.mgb-stuff.org.uk/wn_electricsframe.htm and click on 'Wire Colours, Fuses and Terminal Numbering' and 'Fuses', then the appropriate links in the text.


Paul Hunt 2

This thread was discussed between 11/11/2007 and 12/11/2007

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