Loom fire - advice welcome

[KM]

To set the scene: Spitfire Mk2 with alternator. Have just replaced a seized speedo. The newly sourced one is from a more recent model as it has the clip off connector for the speedo cable rather than the screw on type. Everything else is visually the same including the main beam warning light, oil pressure warning light and ignition light. I used the voltage regulator from the old instrument. I connected everything up (apart from the earth for the ign and main beam lamps which were loosely in position) and decided to test before putting back in dashboard and refitting. Turned on the ignition key and the red and green lights illuminated. Now to check the main beam: turned on the dash mounted light switch and flicked to full beam. Within seconds there was a cloud of smoke from under the dash on the left so switched everything off immediately. Because I switched on both at the same time I couldn't tell whether turning on the lights or the main beam caused the problem. Two wires have burnt themselves out and melted their insulation in the loom. One is a fairly thin black earth wire that runs from the back of the fuel gauge through the bulkhead to earth at the coil mounting. This is in a bad state and actually melted itself in one place. The other is a thick brown wire that runs from the back of the dashboard lighting switch to the B connector on the control box in the engine bay. Its pretty difficult to say whether the lighting wire melted its insulation and the fuel gauge earth wire or something else which is why I am asking for advice. I don't want to connect it all back together and have the same problem again.

Before the speedo was replaced everything was working fine. I did notice that the black earth wire where it connects to the back of the fuel gauge was not as tight as it could be. The main beam indicator bulb did not blow and neither did any of the fuses.

Could this possibly have been caused by not earthing the speedo properly before testing or should I be looking somewhere else. Back later to check on responses. Going out to buy wire!

[RobPearce]

I wouldn't have thought the loose earth on the speedo would cause that. It might have resulted in other trouble, including probably making the blue light not work, but would not have melted the big brown wire. That was caused by something that could take a fair current shorting the light switch to earth.

When you say "before putting back in dashboard", was the speedo out of the instrument panel, or was the panel out of the bodywork? The master light switch is in the panel quite close to the speedo. My bet is that you had something metal making contact with one of the unused terminals on the light switch.

 

[KM]

I'll check for the obvious shorts. It just seemed a strong coincidence that it happened at the same time as the speedo replaced. The speedo was sitting in its hole in the dashboard and the dashboard screws were all removed so the dashboard was loose but more of less in place.

[KM]

Removed the two melted wires and checked all the connections, some were loose but not sure whether that was due to me grabbing wires when turning off the ignition wasn't working. Will test tomorrow in light with help from someone standing by over the battery connection.

[Velocita Rosso]

Two things ..."MK2 with alternator" but  to " B on control box".....which is with dynamo set up?

The other  thing, Rob says....the main light switch has a capability, as with mine, to short out on the metal dash frame.....I would say that is what happened

[drofgum]

Kevin,

It would seem to me that somehow the brown wire from the B+ terminal got shorted to that black wire. The available current on the brown wire is limited only by the resistance of the connections. That is plenty to overheat both the smaller black wire and the brown wire.

                                                                           Best of luck,

                                                                           Paul

[KM]

Its all back together now and working apart from the main beam warning light is on with dipped beam and visa versa. Shouldn't take long to work out. Plenty of spare wire and electrical connections for RBRR!