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MG MGA - Fuses

I am having trouble with the 35 amp fuse blowing whenever it feels like. When this happens the following units fail to operate. Wipers / Fuel gauge /heater motor /and turn signals. I have tried to isolate the problem by only hooking up one at a time but as it is intermittant. This did not work. Can I add a extra 4 block terminal and put one wire on each to see which one fails. If so what size fuses should I us as I do not want to fry the wires. Bill
Bill Mason

The fuses used in the MGa are slo-blo type fuses--actually 17/35 amps. They'll pass a continuous 17 A, and tolerate a momentary surge up to 35. However, they'll only pass a 20 A or so load for a minute or so before it blows. Since your problem is intermittent, you might try putting 15 A inline fuses in each circuit and see which blows. Adding a permmanent 4 fuse block might also be a good solution....I've never really thought the original two fuse system was really adequate.
R. L Carleen

Bill

The stop light switch is also on the same circuit.

Steve
Steve Gyles

So is the wiper motor.
Andy Bounsall

Bill, hook each circuit up one at a time in series with an ammeter to check the current draw of the device. Let each device operate for a period of time and watch for a surge in current on the ammeter.
John H

Since the problem seems to be intermettent--there's most likely a short that only occurs with vibration, G forces or expansion/contraction. I'd carefully check all the wires on those circuits in areas where they pass through bulkheads. Also check all wire terminations to be sure they're not frayed. Finding intermettent shorts can be a real headache. Isolating it down to one circuit is a good start...but operating with these different circuits disconnected for any length of time is a real safety hazard--especially the stop and turn indicators. It's also fun to know how much fuel you've still got onboard.
R. L Carleen

Take a look at any wires in the vacinity of the clutch and prake pedals. One individual had a similiar problem and it turned out that a wire was hanging down where it would rub against the brake pedal. It eventually wore through the insulation and would then short and blow a fuse ever so often when the brakes were applied. Good luck - Dave
David DuBois

Break open the fuse and look at the ribbon element inside. If the center, narrowed section of the fuse looks good but the ends look blueish, or the solder has melted, then the problem you have is the fuse block. Replace it with a new one.

The contact area is brass and tarnishes over the years increasing the contact resistance. A bit of engine oil coating the surfaces doesn't help either. The spring tension also weakens. These all add up to a local hot-spot on the fuse element.

This is a common failure mode for this size fuse passing this large a current.

Chuck
Chuck Schaefer

Last time I saw that one, it was related to the brake light circuit. The wire insulation has been rubbed off by intermittent contact with a sharp metal edge. Sharp braking would move the wire to the metal edge (an electrical ground) and energize the circuit at the same time, resulting in a short circuit. This one could be located anywhere between the brake switch and the brake lights. Happy hunting.

Karl


Karl Leclerc

This thread was discussed between 07/06/2006 and 08/06/2006

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