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MG MGB Technical - Still hunting source of fuse blowing

Okay, I'm still trying to find the source of my #3 fuse (3rd down) blowing. Each time I think I've fixed something it blows within minutes off test drive hasn't blown yet while setting. I've installed a circuit breaker fuse to use while testing.

I'm using the wiring diagram (the one for the B with 3-wires going to the headlight switch) to follow wires from fuse and never found a problem. Well yes I found little problems along the way that I thought may be the problem, but all didn't help.

So now I'm under the dash (boy I wish this were a big pickup truck!) finding the occassional little bare wire spot, which I tape up upon discovery.

So now I've really found a PROBLEM. There are 2 white wires (1 w/a female end, 1 w/a male end) that attach to the back of the tach (to the right side if looking up at back of tach from floor). These 2 wires melted together. I've pulled them apart and separated them from harnass. No other wires appear damaged at all. One of the white wires is easily replaceable as it joins up with 3 or 4 other wires about 12" away in a twist connector.

Its the other white wire that stays up in harnass as the harnass runs on bottom ledge of dash that is my problem. I just can't get the harnass out, so I can't determine where the white wire is going so I can replace it.

Does anyone know where this wire is going to end, so I just bypass it with new wire?

Does anyone have any ideas why these 2 wires would get so hot?

My wiring diagram doesn't include any white wires coming off the tach, but the tach is on the 3rd fuse that keeps blowing. So I'm thinking this is all connected!

Thanks for your responses and great help on these things.
R.W Anderson

What is the model year of your B? A short will cause wires to melt. Make yourself a couple of enlarged copies of the wiring diagram, one to make notes on as you check the circuits, and one for reference. Wiring diagrams are available at http://www.advanceautowire.com click on stock schematics. They will pobably be able to help you with a new harness if you need one.

Disconnect the battery and start physically tracing the wiring. Use a continuity tester, and meter to check resistance of connections.
Kimberly

Not sure of your car year but I think you will find the white wire you are chasing runs from the ignition switch to the Tach. Based on my 69, the other white wire you have may well run to the coil. Unless these are continuous, ie either through (into the circuit within) the tach or joined together outside the tach, I believe you may find she will not turnover.

The short could well be within the tach itself. Is the tach grounded off the back via a black wire running to the chasis? There was a change made to this wiring route in cars later than mine I think. have a look at Paul Hunt's webpage. There are leads there to a discussion of this.

Good luck
Regards
Roger
Roger T

For clarification, my MGB is a May 1972 model. I will check to see if the 2nd white wire is running to the coil and/or iginition switch. I haven't checked to see if the engine turns over with the 2 wires disconnected from the tach. I'm a little nervous about turning anything on at this point, as I don't know how long the two white wires were melted together.

FYI: the white wires appear to have short (1/2") light brown lines every so often, haven't seen these on any schematic yet, so not sure if the white wires are original.
R.W Anderson

Intermittent fuse blowing is about the hardest electrical thing to track down. the 3rd fuse down is the green circuit, with many components and branches. About the only way to do it is to put in-line fuses in series with each of the (usually) two connectors that go to the green side of that fuse, and bridge the original fuse. Which in-line blows, and more importantly which circuits stop working when other on the other fuse are still working, tells you where to go next. This would normally be the 4-way bullet connector nearest the fuse, inserting the fuse in series with each of the outputs from the connector to see which one blows the fuse, etc.

If it is kind enough to blow the fuse as soon as you turn on the ignition, then replace the fuse with a high-wattage 12v bulb like a headlamp bulb. If the short is present the bulb will glow at full brilliance, but limit the current to a safe value. It is then a matter of pulling and replacing wires down each branch seeing what causes the lamp to dim and what no longer works when the other parts of that circuit do.

The whites are not fused so that is nothing to do with the fuse blowing. On a 72 they will be feeding 12v from the ignition switch to the coil via the tach pickup. If they are melted then it looks as if the wire going to the coil shorted out at some point. They should be plain white, if they are white with a brown stripe then it is probably not original, which could mean a PO had them fry before, and simply replaced them without actually finding the cause, and it happened again. The engine *should* turn over with these wires disconnected as they only feed the coil, nothing else.
Paul Hunt 2

Excuse my not logging in. Don't have password with me at this location.

I've been reading more schematics and following more wires. 72B

I've confirmed what one stated above, that the two white wires from the tach are not fused. I've finally gotten enough of the wiring harness apart to trace one melted white wire to the coil. The other white wire is connect beneath the dash to 2-3 other white wires and 2-3 other wires.

I'm proposing to replace the long wire that runs from the tach to the coil with a new wire. And I'll do the same for the shorter wire that runs from the tach to the group of wires.

The two green wires that are clipped together on the back of the tach appear fine.

I'm also going to, as one poster suggested, provide a better ground between the tach housing and the car. I think I'll simply attach a wire to one of the threaded spindles coming out of the tach for mounting and then ground it to the frame.

Of interest during my testing is I found that the white wire that runs from the tach to the coil is grounded. I would put one test lead on the wire end that I removed from the tach and I was checking to see if this white wire ran to any of the other white wires, going to the ignition or going to the wiper arm/OD arm. No matter what I touched the 2nd lead to it showed conduction. I could touch one lead to the disconnected white wire from the coil and the other lead would go to anything metal and it would show full conduction.

As I traced the white wire from the tach to the coil I did find other melted areas, so it is no surprise that this wire is grounding out.

What caused the wires to originally melt I do not know. Any ideas? There are no flucuations in the fuel or temp gauge to indicate there is a problem with a voltage regulator or relay.

Does anyone see a problem with my running a new from tach straight to coil?

Does anyone see a problem with my running a new ground from the frame of the tach to ground?

Yes, I still haven't seriously thought about replacing the whole harness - yet! I'll put in some fall hours before breaking down and spending 400-600 on a wiring harnass.

Some neighbors are thinking I should have kept my sailboat!!!! Lucas hasn't been nice lately!

Thanks again for any assistance
R. W. Anderson

Hi.

The white wire that goes to the coil will have a low resistance to ground (not a short, but nearly) if the engine happened to stop with the points closed.

Try it again with the wire disconnected from the coil.

I have seen insulation breakdown in the wire that goes between the coil and the distributor, it is in quite a hostile location, but I wouldn't have expected a short there to have allowed enough current to flow to cause insulation melting.

The original problem that caused the white wire to melt may have been an internal short in the coil, though I have never heard of this happening.
It could have simply been the result of a PO putting the white wire on the wrong tag of the coil in the past.
If the coil to distributor wire is showing signs of melting this may be what happenend.

I can't see a problem with replacing the wires as you describe.

Don
Don

Not sure if it is related but will let you in on an experience of mine with a shorted white wire. The several wires that run down the corner of the engine compartment and head to the back of the car (one of which is white and goes to the fuel pump) go through a couple of soft metal fold over guides down near the clutch slave cylinder. I had melted my main harness down and was replacing it when I thought I would clean and inspect and re-wrap the wires going to the back of the car as well. In one of those fold over guides I found that all three wires had been rubbed bare and that's where it was shorting and had caused my entire dash harness to meltdown. Got very lucky because if I hadn't replaced that I would have melted the new harness. No electrical probs in several years now. Also an inexpensive battery cutoff switch (NOT the Moss one) can save fires, etc. I was able to shut mine battery off immediately when the melt down happened.

That said...a new wiring harness is completely worth it. No more spending time chasing old wiring system faults. I put it off for several years and in retrospect would do it for any old car I buy in the future unless it had been done.
J.T. Bamford

White wires were never fused from the factory.

The tach should have a black wire on a smaller than usual spade rivetted to the case, as long as the other end of that wire is making a good connection to ground along with the black wires from the other gauges (usually under one of the knurled thumbwheels) there should be no need for another, and it won't be an issue in the melting of the white wires, but it shouldn't cause a problem.

A short between coil and distributor will only stop the engine, it won't cause melting of the white wires from the ignition switch or tach, unless the coil is the incorrect item (too low a resistance) or is itself faulty. This is because the points are supposed to represent a full short to ground when closed anyway. The primary resistance for a coil on a 72 should be about 3 ohms. However if the ignition is left on with the points closed i.e. engine not started you will get about double the heating effect in the coil and wires. If you have a rubber bumper coil (measures 1.5 ohms) this will double the current, and as well as damaging the points and overheating the coil could cause the white wires to get hot, they could be carrying up to 9 amps.

The only thing that can cause the white wires on the tach pickup to melt is if the one between tach and coil has shorted out to the body somewhere, or as I say the coil is (was) too low a resistance. Replacing the white wires between tach and coil and ignition switch at least is a good idea, I thought on a 72 it should go to a female bullet on the back of the tach with a male bullet on the wire. The other white from the ignition switch should have a female bullet going to a male bullet on the back of the tach. But any current through thw whites at the tach will also have been going through the ignition switch and the brown wire connected to that.
Paul Hunt 2

This thread was discussed between 26/09/2006 and 01/10/2006

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