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MG MGB Technical - Smoke!

Serious problem with a wire from ignition on steering column...becomes hot and has burnt the plastic coating off - thought I had solved the problem by isolating with new tape but same happended again in 2 minutes.
It is the wire in white with light green stripe. According to Haynes this wire serves the radio - however the radio is working. Has been fine for a whole year of ownership.
Any suggestions very much wished for!
/moss
Moss

The short may be further down the wire from where you are seeing the insulation melting as a result.
Can you disconnect the radio and see what happens? With mine I have a 1.5 A inline fuse and I would recomend that.
Do you have or can you make a test light? If you do you can see if current is flowing with the radio disconnected, this would prove there is a short circuit before the wire reaches the radio. You would need to disconnect the Green white wire from the ignition switch and put the test lamp in series. If it lights with the radio out of circuit then its shorted somewhere.
Stan Best

Hi Stan, thanks for response. Have just checked the wiring diagram - there should be a 'line fuse' but I cannot see one connected anywhere. Have just taken the radio out - and been for a spin - the wire is not hot anymore. I will check the circuit for a short though...how do you make a test light?
/Moss
Moss

Hi Stan, thanks for response. Have just checked the wiring diagram - there should be a 'line fuse' but I cannot see one connected anywhere. Have just taken the radio out - and been for a spin - the wire is not hot anymore. I will check the circuit for a short though...how do you make a test light?
/Moss
Moss

A test lamp - I think I'll buy one - I have the feeling that it will not be the only time I have to check a circuit...
Moss

I remember, (fortunately it was not on the MG1) having connected the battery terminals the wrong way around ( not saying this is your case! ) But first thing I noticed on turning on the ignition was smoke coming out from behind the dash, 'Yipes'! I screamed turned everything off double quick, yes I had fried the radio, but everything else, Thank The Lord was saved! You may find that the short is at the radio, or along the cable to it, usually where it gets hot the earth is occuring. Mike
J.M. Doust

Hi, usually people make their own. Its useful even if you have a meter. A 5 watt or so incandescent bulb in a holder with a wire on each contact is foolproof. I use my LED timing light but you need to get polarity right with that. Mine has an insulated croc clip on both leads.
In this case yuor results so far tend to show a short inside the radio. With a test light you could put it in series next to the radio and it would light brightly. You could then measure volts to ground either side and full on the supply and a few millivolts the other would tend to show a short in the radio. Removing the radio and trying again with the light at the ign switch end if the light goes out then the wire is not shorted If you then clipped the test light in where the radio was and turned on the ignition and it lit brightly with no problems this would confitm it. Ign on then off instantly I think ,we dont want a fire! I would also laeve the light in place and wiggle the wire just in case disturbing it has moved the short away from ground. The ideal would be to examine its entire length, but thats not easy. Even if the original problem was in the radio you may find damage in the loom means it would be better replaced,a counsel of perfection , or possibly just a new wire run to replace the green/white. Finally you can run the radio out of the car and see if it works, maybe from the battery via a hook up with a 10A fuse if that goes bang its a clue the problem is in the radio! If the radio works fine and a meter shows it drawing 1 or 2 A then it throws suspicion back on the wire.
Stan Best

All starting to make sense - have bought my tester - now where to I put the ground connection? On the metal steering frame where everything is bolted on?

I think the wire is live as I was out in the car this morning and the wire touched metal by accident (ignition on) and there was spark. I have also found the line fuse on the radio (taken the radio console out - my goodness what a lot of wires and cables) part of the power wire encapsulated in a black tube - opened the tube and cannot see a wire through the glass.

This got me thinking about why the fuse would blow and suddenly a wire getting too hot. Could it have something to do with the fact that both front indicators in the bumpers (rubber bumber model) are out at the moment - the wires to them are not connected? The rear indicators are blinking faster than normal?
/Moss
Moss

No, no connection with the flashers, the lower load causes the flash rate change, and warns if a bulb blows.
Earths can look good, but not be connected. I would put one clip on the metal frame and then the other to 12v. If you get the light on, its an earth. Then you can put it on the w/Gn wire and see whats happening. It sounds like you have an in line glass fuse, and it has blown. It would be inside the plastic tube which would have contacts to the wires.Is it a glass tube with a metal cap either end? If so yes the fuse wire should be visible. When it went it should have protected the wire. If not then the short is before the fuse. If you can see 12 v at the input to the fuse then its OK to that point (but wiggle it to make sure, it may be close to an earth and only shorting when it moves.
Stan Best

Hi Stan, what do mean by 'then the other to the 12v'?
There is a metal cap at each end of the glass tube - each end of the power wire is fixed here.
/Moss
Moss

Just a 12v anywhere. You can see some bare metal, but you dont know if its an earth. Well if you clip one side to the possible earth, and the other to 12v the lamp will light if it is an earth, no light, its not an earth. The 12v must go to theign switch, be careful not to accidntally short it ground with the clip. Good luck.
Stan Best

White/green is not fused. It comes off the ignition switch in the accessories position, then on some years goes to an in-line fuse, becomes green/pink, and feeds things like electric washers, wipers and heater fan. On other years it may well go to a radio inline fuse. The white/green smoking means either the short is between the melting and the in-line fuse, or the inline fuse is much too high a rating so the short could be beyond the fuse i.e. in the green/pink or radio circuit.

You only need to switch the ignition off to isolate the white/green at the ignition switch, not remove it. You also need to switch off anything the accessories position powers or your test-lamp could well 'see' an earth through that leading you to think the wire itself is shorting.

When using a test-lamp it is safer to use a purple supply i.e. rear connection bottom of fusebox, courtesy light etc. so that if the clip does short to earth it will only blow that fuse and not damage whatever it is connected to.

Most metalwork around the car is likely to be at earth, but always test your connections and test device by connecting it to a known good earth and 12v supply first. I.e. if you are looking for an earth connect the flying lead of the test-lamp to a 12v supply such as a purple wire, then touch the test lamp on some metalwork somewhere to make sure the 12v supply is good. If you are looking for 12v connect the flying lead to a likely earth point then touch the test-lamp to a 12v circuit such as a purple or brown wire.


Note that the original flasher stops flashing and lights the good bulb continuously if one bulb has failed or been removed. More modern (than the MGB) flasher units do what you describe i.e. flash at double-speed with one bulb out. Both are a bulb failure warning device. MGB owners sometimes fit the more modern type as it is quick and dirty way of getting round a slow flash problem with the original type of unit, but as the problem is usually bad connections leading to dimmer lights than they should be, you will still have dimmer lights with the replacement flasher.
PaulH Solihull

No more hot wire! There are two 'line' fuses from the radio - the power wire and one to the battery - never met that before. The fuse from the power wire was blown. Anyway mo more hot wire with smoke. Have taken the radio out and now waiting for the new carpet set to arrive before putting everything back to gether.
/Moss
Moss

Some modern radios have an "extra" wire that becomes live when you first turn the radio on to power an electric arial. Many also have a "permanent" live to retain the pre-set stations.
Michael Beswick

This thread was discussed between 10/07/2011 and 24/07/2011

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