[Mgs] 80B, melted white-green wire from ignition switch

leon at zaks.com leon at zaks.com
Fri May 27 09:09:00 MDT 2022

Steve - I fully agree with PaulH. Though the switch may now be 
compromised but it's highly unlikely it was the cause. The cause is a 
short circuit in the green/white (G/W) wire. Though there is no fuse 
indicated in the wiring diagrams for the car there may be one in the 
lead from the radio to the bullet connector of the G/W wire. If there is 
a fuse and it is not blown then the short is between the fuse and the 
ignition switch. The G/W does not go through the firewall.

There are some mildly sharp edges in the steering wheel supports that 
the wires run near - could be rubbing caused it or more likely someone 
working on the radio pulled it a bit much.

Also - having had a few ignition switches apart the way it is built it 
would be very hard for it to be misaligned. What can happen is where the 
shaft from the key mechanism can wear the plastic of the actual switch 
and cause it to not connect properly - the "If I wiggle it it works" or 
"If I wiggle it it doesn't work" syndrome.

I would replace the switch - after you find the problem. Last week I 
just purchased an aftermarket switch for my b - BPNorthwest has it for 
under $20, part number BHA5292.

Here's my comments on what connects to what and why - I'm doing this to 
help anyone trouble shooting and ignition switch and also to reinforce 
it in my head.

Brown (Br) wire is the 12v feed and is always hot.

Slate is the anti-run on - power only when the key is off.

G/W is accessory and running. The radio is connected to G/W so you can 
turn the radio on without the car running.

White (W) is to the fuel pump (inertia switch first), overdrive, 
ignition system (with ignition relay on later models) and it feeds the 
Green wire going to most instruments, turn signals and the like.

Red/white (R/W) is the power to the starter relay, seat belt module, 
hand brake, alternator warning lamp. On some models this wire doesn't go 
directly to the starter relay - it goes to the seat belt module so you 
can't start the car without buckling your seat belt :)

Purple is the connection to the horn - it works opposite of the other 
wires - in the end of the connector coming from under the dash it is 
always 12v. The purple wire from the connect on the switch should go to 
ground when the horn is pressed.

Here's a table of what should connect when on the switch.
Position 1 is off, then progressively 2 is accessory, 3 is running, 4 is 

You can test the switch by disconnecting the plug under the dash - 
connect one side of a meter or powered continuity tester to the brown 
wire and checking each of the other wires. Where there is an X you 
should get a connection. If not, the switch is bad. If you get a 
connection on a spot that does not have an X the switch is bad.

Position     S  G/W    W     R/W   Br    Purple
1            |  X |        |      |        |    X  |           |
2            |     |   X   |      |        |    X  |           |
3            |     |   X   |  X  |        |    X  |           |
4            |     |        |  X  |   X   |    X  |           |

leon ...

On 5/27/22 3:55 AM, PaulHunt73 wrote:
> The wire is smoking because whatever it's feeding is shorted to earth, 
> so you will have to follow that wire to the radio.  If you have a fuse 
> in the wire going into the radio then it's not the radio itself or 
> that would have blown.  Assuming there IS a fuse in there and not a 
> nail or something.  If no fuse then it could be the radio so 
> disconnect it and try again, and if it still smokes the short is 
> between that point and the visible melting of the white/green.
> It's not the switch that is faulty in this respect, although having 
> been feeding a short-circuit the contacts in the switch may have been 
> damaged.  It does sound as if the switch is iffy as well, maybe 
> because of the short having been present in the past. So you need to 
> find and fix the short before changing the switch or you are likely to 
> damage that switch as well.  That wire should only go from the switch 
> to the centre console for the radio ending at a bullet connector, it 
> doesn't go via the engine compartment.
> PaulH.
> On 25/05/2022 01:52, Steve Shoyer wrote:
>> Hi - the radio in my 1980 MGB had been flaky for a while.  It would 
>> work with the key in the accessory position but not in the run 
>> position.  I could wiggle the key backwards and the radio would come 
>> on, but if I wiggled too far the engine would turn off.  I assumed 
>> that the ignition switch wasn't positioned properly but as long as I 
>> could get it to work I forgot about it by the time I got home.
>> Until the radio wouldn't work at all.  No power was going to it. I 
>> tried to trace the white-green circuit and found that the white-green 
>> wire was melted about 2" from the ignition switch harness connector 
>> and the insulation had bonded the white-green to the other wires in 
>> the bundle.  The other wires didn't seem to be damaged, and I was 
>> able to peel the wires apart without any problem.  I repaired the 
>> burned section of white-green and tried it again, this time resulting 
>> smoke coming from the steering column.  The white-green wire had 
>> melted along most of its length.
>> I had a spare ignition switch (not sure of its history, but maybe it 
>> came from a parts car) and swapped it in.  Strangely, it had a splice 
>> on the white-green wire near the connector.  It also started smoking.
>> I tried to check the old spare switch (before it melted) to see how 
>> the switch worked but I didn't learn anything.  I also tried to see 
>> if there was voltage on any of the wires in the connector that mates 
>> with the ignition switch, thinking that power comes into the switch 
>> on the brown wire and is distributed to the other wires depending on 
>> the switch's position.  I grounded the black lead of my multimeter to 
>> the chassis and started looking for DC voltage on the other wires.  
>> At first I didn't see anything, but when I touched one of the wires 
>> (I think white-red) the starter kicked in.  I stopped there.
>> Any ideas as to what might be causing the wire to melt?  I didn't see 
>> any obvious problems with the wiring harness in the engine 
>> compartment, but it's very possible that I missed something. Also, 
>> when replacing the ignition switch, should I get an aftermarket 
>> switch or the Lucas switch?  The Lucas switch is about twice the cost 
>> of aftermarket so I'm leaning toward the aftermarket switch, 
>> especially if it might just burn up as soon as it's installed.
>> Another odd thing is that the car ran fine when the radio wasn't 
>> working at all.  I didn't get smoke from the wiring, although by then 
>> the white-green wire was burned away before it left the ignition 
>> switch harness.
>> Thanks.
>> --Steve (1980 MGB)
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