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MG MGB Technical - emergency brake engages starter

When I start my 76 MGB with the emergency brake engaged- it starts fine, but after releasing the emergency brake and then re-engaging it while the engine is running- the starter motor engages the engine!
I can even turn the engine off and with the key in the accessory position, by raising the emergency brake handle, I can start the car. Looking at a wiring schematic- there does not appear to be any wires connected to my emergency brake- so I looked under the car and had someone release and engage the brake to see if these was any binding of any wires by the brake cable at the rear axle. I did not see any wires near the vicinity of the rear axle that could be binding by the cable assembly. Any ideas where I can start looking to fix this problem are appreciated.

The brake-test diode is short-circuit, which allows 12v from the handbrake circuit to energise the starter relay. Look for a black tubular gizmo a couple of inches long behind the dashboard with a white/red wires one side and green/orange the other and disconnect it. The only side-effect of this is that if you crank with the handbrake down the handbrake warning light will no longer glow, although it will with the handbrake up! And I'll bet you didn't know it did that anyway. If you want to replace the diode any 100v 1 amp (or similar) diode will suffice, you want to connect it such that it allows current to flow from the white/red to the green/orange, not the other way.
Paul Hunt 2

Get rid of the diode in question and insulate the loose wires. All this diode does is turn on the brake dash light when the ignition switch is in the start position. The brake dash light will come on if you have the hand brake pulled up or if there is a pressure imbalance in the hydraulic system because it gets power from the green circuit.

Take a look at the wiring diagram for your B and you will see what I am refering to. Wiring diagrams are available at
Click on stock schematics.

I disconnected the wire and have ordered 10 diodes (at a whopping $.37 each) from DigiKey and will replace the existing diode with a new one shortly.

I always wondered about those warning lights. Driving with the handbrake on seems to be the sort of thing that you'll notice without an idiot light. But what about if the brake cylinder imbalance switch is activated? How obvious is it when driving that half your brakes have failed? I guess it depends on which half goes since the dual circuit is arranged front/back. I am leaving the master cylinder warning switch out of my car since my switch broke and the damn things always seemed to leak anyway (I replaced it with a bolt and copper washer). I always wondered if I shouldn't fix it properly.
Simon Jansen

This thread was discussed between 07/07/2006 and 12/07/2006

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