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MG MGB Technical - mystery wire

I have a 75 b. There is a solid green loose wire with a connector on the end that is taped togather with the wire to the temperature sending unit on the engine. The "mystery" wire is thicker than the one going to the temp sending unit. It also is covered with a black plastic sleeve.(both wires are green) It also is hot...it arcs when touched to metal. This car has had to emission control stuff removed. Could this be something left over from that? The ignition warning light also never goes out. It dims alittle when the car is running, but according to the manual it should go out. Any advise would be welcome.
cooper

Cooper. You need get a copy of the factory wiring diagram and have it photocopied, then expanded so you can see it better. I have mine expanded 300%, then laminated so I can draw on it with a water erasable marker when tracing circuits.

Generally speaking, green wire circuits are hot when the ignition switch is in the run position. It is a fused circuit coming off the third fuse from the bottom, rear side. The input is the white wire circuit which is unfused power going into the front side of the fuse box.

You need to find out where the hot wire is coming from and disconnect it or find out where it should be going and connect it up properly. If you still have the Z-S carb, you might make sure that the wire going to the induction heater is still hooked up properly to it. If you have a Weber conversion, this may well be the wire to the Z-S induction heater.

Yes, the ignition warning light should come on when the ignition switch is turned to the run position and should go out by the time the engine revs are 1,000. If not, you have a problem in the white wire or charging circuit. Make sure the thin, brown wire, with a yellow strip, is firmly connected to the alternator as this is part of the ignition warning light circuit.

As a quick check for function, measure the battery voltage at the battery terminals. It should be 12.0 to 12.5 volts. Start the engine and check the system voltage at the battery terminal clamps. It should be 13.5 to 14.5 volts, preferably closer to the latter figure. If it is not, you have an alternator problem. If it shows good, you may still have an amp output problem. However, that normally shows up by a dim ignition warning light that glows slightly when operating the turn signals, lights or horn, rather than a bright, full time light.

This may help to get you started. Les
Les Bengtson

thanks for your help. Yes, this car has a weber conversion. If the wire was to an induction heater on the old manifold would it be ok to just cap and tape it up? As far as the ignition light....it will burn bright when first turned on...then once car is running it dims about 50% and sorta flickers.

again thanks for the advise
cooper

Cooper. Most people just tape them off, but make sure you do a good job of it so it cannot spark. I remove the wire myself.

Use a volt meter to see what it can tell you about system performance. Radio Shack has some inexpensive analog VOM (volt-ohm meters) which seem to work just as well as my more expensive digital ones. The ignition warning light should come on when you turn the ignition switch to the on position and wait a few seconds for the fuel pump to run. When the engine is started, the light should go out almost immediately showing that the car is operating off the alternator. If it only dims and/or flickers it indicates a problem, normally in the alternator or the connection to the alternator. Check to make sure the plug with its three wires is firmly connected, then do voltage checks. Rebuilt alternators are available from a number of sources and several people have installed other brands of alternators with good results. I stick with the Lucas because I have had good luck with them over a long period. Les
Les Bengtson

"It also is covered with a black plastic sleeve"

Seems to be the induction heater supply. Mine is covered this way too.
M Gaudreault

This thread was discussed between 29/08/2005 and 30/08/2005

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