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MG MGB GT V8 Factory Originals Technical - Will a fuse here solve this dangerous condition?

My CB V8 has what I believe is the motorola 55-amp alternator. The hot connection to the alt. -- i.e. with the heavy brown wire attached -- is literally about 1/32" from the passenger-side valve cover. Seems to me that in an accident if the alterator bracket got bonked even slightly, there would be a major fire -- this is unfused and carries literally all of the current used by the car.

Would inserting a fuse in the brown wire that goes from the starter to the alt. make this safer? If so, what would be the appropriate fuse size? 60 amps or so, or is that too high to be useful?

Any advice appreciated.
Harry M. Peeters

Harry, yes the attaching stud on the back of the alternator could cause a major short. There are small plasitc/rubber caps for this exact problem that slips over/through the wire and covers the connection. An inline fusible link would add extra protection, but is generally not needed as the heavy output wire usually goes directly from alternator to battery/starter. A short on this circuit would only burn this wire. It is good measure, however, especially if the wire is bundled up in a harness.Good luck.

Where would you put the fuse? It's the alternator that is generating the current - assuming the engine is running that is - and if the stud is shorting a fuse in the wire would not help. I cut the stud back as far as I could on mine and enlarged the hole in the alternator cover so that the plastic boot sits inside and the connection has as much clearance as possible. About 1/4 inch now compared to nothing before.
Geoff King

The alternator is connected to the battery so if you short out that lead while the engine is running you suck current from both. You will get far more current from the battery than the alt, which has over-current protection, so the fuse (if that is what is best) should be installed between the potential (ho ho) fault and the solenoid terminal. Any seriously overheating wire in a loom will melt all the other wires around it into a solid mass, with the possibility of further shorts and hence further burnt wires in other parts of the loom, so the fuse should be before any loom taping.

Paul Hunt

Of course, if you have a fuse you can place it anywhere in the wire, even after some loom, 'cos it stops it overheating in the event of a short. Doh!
Paul Hunt

Paul, many thanks -- what size should the fuse be (on the big brown wire between the starter & the alt.)?

1.Get a little square of rubber or plastic and glue it on to the rocker cover behind the stud.
2. There arent enough fuses on any MGB. Add an extra set when you fit the headlamp relays!!
Uncle BoB
Bob Hemingway

80-100 amp fuse should be ok.

I will drap out the amprobe tomorrow to see what kind of amperages the starter requires.
Michael Hartwig

I wouldn't fuse the line from the battery to the starter, just the line to the alternator. An accidental short taking the alternator off-line is one thing, but not being able to start the car is quite another IMHO.
George B.

Guys, not talking about fusing the starter cable -- rather I'm talking about the heavy brown wire running from the starter to the alternator. Seems to me 60 amps would be enough, e.g. for everything besides the starter. But I do recall that when the electric fans are going, radio, heated rear window, big headlights, all that kind of stuff adds up so maybe 80 for good measure. I just want something that will blow if there is an insanely obvious short, like the alt. hitting the motor.

If you're going to fuse the alternator, then it's a matter of adding up the wattages and dividing by 12. As far as an electric fan, my gigantic SPAL 16" fan pulls nearly 20amps, but it's the biggest fan I found. Most others draw around 10-15. But does it really matter what size fuse you use? Since you are only using it for short circuit protection and not overcurrent protection. You could use a fuse the same rating as your alternator. Most cars average around 63amps. On the topic of not fusing the starter, because you can't get it started again. What about using a circuit breaker? They cost a small fortune, but you never have to change fuses. Most good car audio shops will have them.

Michael Hartwig

Fusing the starter is not an issue, Michael, but fusing the alt lead is.
Paul Hunt

The point is the battery pos terminal the starter pos terminal and the alternator terminal are all at virtually the same potential. None of these points need fusing - only protecting from shorts. The current drawn by the vehicles various accessories are drawn through different paths and fuses depending on whether for example they are swithed by the ignition - usually a white feed or permanently on, brown leads. That lead may also run from the starter to distribution. I have relocated mine to pick up from the alternator and then run to 8 fuses where I distribute current to its destinations. Those fuses protect individual or groups of feeds from inadvertent shorts. The alternator is protected from over voltage and over current. If you want to, fit an ammeter in between the alternator (my case)or the starter and the distribution to the car.
The ammeter should be a 60 amp one if you have anything other than a standar V8 alternator which struggles to give more than 40 amps. The ammeter will indicate the current flowing from the battery/alternator common point. What the starter draws is immaterial as long as you have suitable battery that can stand the high current drawn during its use.
bob p

This thread was discussed between 31/10/2000 and 03/11/2000

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