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MG MGB Technical - Cleaning engine, tachometer, smoke

Well, this is embarrasing.

In preparation for using the MGB in a tech session we decided to 'gunk' and spray off the engine. We've done it before, and only needed to wipe the inside of the distributor to get it started again.

This time, after it started, it died. Tried wiping it down again, and on attempted restart smoke from the wiring by the alternator (between the coil and the main harness) and smoke under the dash... and it also filled the tachometer. The white terminals on the back of the tachometer are melted, I'm about to open the tach but it is fairly black inside.

The white wire is definetly fried, both sides of the tach, probably towards the ignition switch as well, not sure. The car has a hall effect breakerless points set, not installed by me. The white and white with black wires going to the coil doesn't, a red and a black wire do instead.

How much damage do you think I'm going to find?

warmly,
dave
Dave Braun

Ouch. The white wires inside the tach are fried too. Replace them, or get a replacement tach?

warmly,
dave
Dave Braun

Dave - I don't remember what year your MGB is, but the white wire you are referring to is the wire that supplies 12 volts from the ignition switch, through the tach to the + side of the coil. You obviously have a short in or at the coil (possibly the points in the distributor stuck shut), that fried the white wire. The white wire inside the tach is nothing but an extension of the wire going from the ignition switch to the coil It can be easily be replaced, but you have to make sure that you wind it through the middle of pickup transformer (the open wire coil that the white wire goes through) in the same direction as the burned wire and that the sleeve is on the same end of the new wire as the old one. The same caveat applies to the wire from the ignition switch to the tach and the wire continuing from the other end of the wire in the tach to the coil. if you get any of this reversed, the tach will not trigger properly. Cheers - Dave
David DuBois

Thanks Dave. The MGB is a 1970. Based on your commments it was a case of shorting out the electronic pick up or the coil terminals with water. I'm going to snip out the existing wire and replace it with new the entire length.

The difficult thing will be to figure out which way the wire winds in the tach. It is pretty melted. I also need to clean it out without disturbing the coil windings. I'm going to try to locate a picture of how it is supposed to look, or a wiring diagram. I thought once I saw a positive ground instruction on changing the polarity of an electric tach. Perhaps I can find that.

Here is an image.

sure appreciate it,
dave

Dave Braun

Found a picture referenced on MG Experience from Andrew Bird of what the loop is supposed to look like.


dave

Dave Braun

Dave - Wow, you don't do things half way - although if you had left the ignition on a bit longer you could have imparted a charcoal color to that wire ;) Go with the wiring on Andrew's tach and you should be ok. Cheers - Dave
David DuBois

Yeah, well, I just rewired it, and the ignition wire from the starter switch thru the tach to the coil... and melted the wires to the pertronix.

I didn't install the pertronix, so I'm not sure what the drill is, but I'm refitting points and condenser right now.

dave
Dave Braun

Installing the points did it... although the distributer has seen better days. The locking flange is cracked, and the PO who installed the pertronix 'tossed' the rubber fitting with the spade terminal. I was able to jury-rig an isolated fitting, and after a momentary confusion when the starter relay wire AND the distributor backed out I was able to temporarily start, time and run the car. I adjusted the fuel gauge full mark while in there, and I'm now having a dickens of a time getting the tach back in its little home.

~sigh~

Now that I know every thing works, I'm going to pull the distributor out and send it to Jeff Schemmler.

warmly,
dave
Dave Braun

Dave - "Now that I know every thing works, I'm going to pull the distributor out and send it to Jeff Schemmler."

A very wise course of action. Jeff's work is first rate. Cheers - Dave
David DuBois

This thread was discussed between 20/06/2009 and 22/06/2009

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