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Here goes: I have a 1977 MGB, de-smogged.
The battery was changed and suddenly the ignition light goes on, the speedo won't work, the tach won't work and when I pull up the hand brake the starter will not disengage (or will engage when it is pulled up when it is running)

Guys, any ideas?

Chris; If you don't have a wiring diagram for your car go to Advance wiring. On the left menu click on stock schematics. Find the late US Market MGB diagram from Haynes.

On the right side of the the diagram near the center you will see a diode in the handbrake circuit. The diode is probably bad. The diode can be removed and the handbrake will work OK but you will not have any light indications for brake troubles or handbrake on.
If diode removal solves the problem the diode is bad.
The diode should be replaced if you want those functions to work. I think your speedometer has a separate problem. The other problems should clear if the diode is bad.

I don't have a rubber bumper car so I cannot tell you where the diode is located. I would guess it's somewhere under the dash, but that's only a guess. Someone will tell you.

Clifton Gordon

A bad diode will cause the starter to engage when the handbrake is pulled up.
Above is a link to a site with wiring diagrams. Make yourself an enlarged copy.

The diode has a white/red wire and a green/orange wire connected to it.

Does anyone know the part number or where I can get one?

Anything better than 20v at 1 amp should suffice, at a cost of pennies. Spotting the item in the Parts Catalogue is bad enough, let alone Moss, where it will probably be ten times as much.

If the diode blew when the new battery was fitted its possible the battery was conencted the wrong way round initially, in which case other diodes like in the alternator may also be damaged. Hopefully it is just a coincidence.

On a RHD car it is high up on the firewall behind the dash on the right-hand side.
Paul Hunt 2

Try Radio Shaft or a better Electronics Supply store (where the Ham Radio guys and electronics geeks hang out). Give them the specs as Paul mentions above, and you should find your diode. Ask for an industrial version. Ask the counter guy to also show you which is the Pos. (+) end of the diode (it will likely be marked) and then use your old diode as a guide or if that doesn't work, follow Dan Master's schematics from Advance Auto wire, I am sure he probably shows polarity of this diode in the handbrake circuit. Mostly, just take your time doing the fix and you will probably come out OK.
Bob Muenchausen

After looking at a wiring diagram, I can't find any reason to keep the diode. It looks as though the only thing the diode does is turn on the dash brake light when the ignition switch is in the start position. Does anyone see a reason why this diode can't be removed from the electrical wiring and the wires (input white/red and the output green/pink permanently disconnected?

How would I know if the battery is connected incorrectly? The Car starts.
I still cant seem to find this elusive diode on the car. ARGGH......

Can I switch the leads from neg to positive to determine if it is correctly attached? Will it cause more damage?
Sorry for such stupid questions but I had a mechanic do most of the work on the car and am afraid he changed the red wire to negative, though I am not sure.
Damn, I need to find a good MG mechanic in South Carolina. ARGH

Battery negative is connected to ground by being bolted to the body. Battery positive is the big wire that runs to the starter. DO NOT switch or you WILL cause major damage!
The diode should be under the RH side of the dash behind the glovebox. Some cars have two diodes, the one in question is the one with Green/Orange wire coming out.
FR Millmore

One more stupid question?
If I did incorrectly hook up the battery, would it still operate?

The battery will, and the car might run, but things with solid state components may be unhappy. Alt/reg, electronic ignition, radio, tach, possibly fuel pump, can all be damaged by ass-backwards hookup.
FR Millmore

Before I start the car up again I would like to be able to assure myself that the battery is connected to the correct polarity. Can you help? I do not want to cause any more damage than I might have already caused.

The ignition light already is lighted, the radio does not work; the handbrake engages the started, the clock and tach do not work, although the engine does run and the car will start. The red wire is connected to the positive side of the battery and black to the negative, although I am somewhat concerned that the mechanic changed the red wire to increase its lenghth.
I guess I should have paid more attention to the work being done to the car when it was done.


To quote from my post above: "Battery negative is connected to ground by being bolted to the body. Battery positive is the big wire that runs to the starter."
The cables were originally both black, so DON'T GO BY COLOR! You can SEE where the ground cable is bolted to the body, this should be from the negative post of the battery. Since the long positive cable to the front of the car is rarely replaced, but the short ground cable is frequently replaced, I suspect that your negative ground cable has been replaced by a red one. Therefore, it is likely that you have in fact connected the battery backwards.
FR Millmore

Thank-you for all your help....
I did incorrectly connect the polarity. I checked out the connections and determined that the mechanic did change the negative ground to a red one and I mistakenly thought that would indicate the positive terminal. ( I have now labeled them).
Now, if I can determine the damage I have done to my car and repair it I will be a happy camper.

Just start it up and see what works. Many of these things may not have been damaged, depending on how they are made, which can vary. The diode problem which started this discussion may be perfectly OK. Most likely thing to have a problem is the internal regulator in the alternator, so check that the red IGN light works correctly and check for actual charging, with a voltmeter.
FR Millmore

This thread was discussed between 28/06/2006 and 03/07/2006

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