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MG MGA - Blown Fuse


When I press on the brake pedal, I blow the 35 amp fuse - which shuts off brake lights, turn signals & fuel gauge. I realize that it is a short somewhere. Any idea how to quickly diagnose the problem ?
DA MacFarlane

The short will be after the brake light switch going toward the back of the car on a 1600. Visually check the wiring that goes along the right frame rail for any external damage. If no damage found check inside the rear lights. On a 1500 the brake light signal goes to the turn signal control box on the firewall then to the rear. get a wire diagram or check here
You can unplug all the wires that join in the RR corner of car to determine if Fault is there forward or there back.
R J Brown

Also check around the brake pedal. I know of one person who had a wire dangling near the brake pedal such that a bare spot on the wire would touch the brake pedal whenever he applied the brakes and blew the fuse. Cheers - Dave
David DuBois

Getting closer. When I turn on the ignition, all is well (wipers, heater, turn signal ) . But when I pull the light switch on, fuel gauge goes to zero. With the lights off or on, when I push on the brake pedal , the fuse blows eventually - not immediately - and then , of course, nothing. Once the fuse is blown, if the lights are on and I push on the brake pedal, the lights dim. The only change has ben a paint job - with the plinth(s) removed and "re wired ".

I think that I know where to start but any help would be welcome - we're finally getting summer and I need to drive !
DA MacFarlane

We need to know if you have a 1500 or a 1600 as there are major differences in the stop lamp wiring.

The delay in the fuse blowing is probably because the short is near the rear so you have a lot of wire in the circuit. Wouldn't do it too much or you may burn up the wiring.

Jeff Schultz

Reconnected the snap connectors incorrectly at the back, or one is loose and earthing?

Steve Gyles


1961 MGA 1600.
I will be checking the wires to the rear lights tonight. It really seems to be the only place that has been fiddled with...
Thanks for all of your help.
DA MacFarlane

Is that a slow-blow 30A fuse you're using in there?

Do all the lights work properly- the proper filaments lighting in the 1157 bulb corresponding to either the parking circuit or brake?

Do you have a wiring diagram and a multimeter? My guess is that the problem's in the plinths. Make sure the brake circuit isn't backfeeding into the parking circuit somehow- like plugging a brake bullet connector into a parking 4 way connector or something.
Mark J Michalak

1600 makes it a lot easier. It cannot be near the brake pedal as the switch is hydrolik and near the starter. The short will be between the brake switch (see picture)and the rear of car. The wire to the switch is green. the wire from the switch is green with a purple stripe. In the RR corner the single G/P wire joins 2 G/P wires. Those 2 wire go to each brake light bulb. The ground wires are black from the bulbs.

R J Brown

Check the cabling from the dip switch is not sitting directly under the pedal and being sandwiched between pedal and floor and being damaged by the depression of the pedal. I had a headlamp switch act as a fuse because of this a few years ago. The wiring loom is actually much too long here, and the pedal eventually cut into the insulation and created a short to ground.
dominic clancy


Mark - none of the lights are working except for the correct turn signal lights and raer lights - until I put on the brakes-then poof ! I belive taht it is just a common 25A fuse from the PO.It's been in for years.
RJ - When I purposely "shorted out" the hydraulic switch, the fuse blew, so I'm pretty sure it will be around the plinths. I just spoke to the shop that reassembled the car after painting and he metioned that they had "alot of problems" getting the rear lights to work. Weird thing is that they worked for a month or so, so I would guess that it's a bad connection or ground or else it would've blown quickly if it was incorrectly wired in.
DA MacFarlane

How about a collapsed filament in a brake light bulb causing the short circuit? It happens. Checked the bulbs?
Bruce Mayo


Looks like loose connections at the rear bulbs - everywhere ! The PO has actually soldered some on - poorly & has some weird connectors on a couple. Can't believe it lasted this long ! Off for a spin to see if they now hold together and get some bullet connectors. Thanks all.
DA MacFarlane

This thread was discussed between 02/06/2010 and 03/06/2010

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