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MG MGB Technical - Power loss and blown fuse

'76 MGB. After starting off and getting on highway at cruise speed, the engine loses power for less than a second and comes right back. When this happens the tach needle swings wildly. After this occurs a few times the fuse (third one down in the block) blows, which takes out the tach, fuel and temp gauges, etc. Engine runs fine after that although I haven't gone very far for fear of what next. The whole situation has happened the last two times I drove it, which were the first times out this year. Any suggestions will be greatly appreciated.
Bill
Bill Meyer

Start tracing the green wire circuts suppling the instruments back to the fuse, you have a short somewhere, could be a wire pinched against the body by a clamp or component. Since the car runs fine after the fuse blows it would appear the ignition system is working
John H

Also sounds like a bad connection in the white circuit, which means the fuse isn't blowing straight away but pulling down the ignition voltage instead. A common place for intermittent shorts is the tail feeding the brake light switch, but it could be anywhere in the green circuit and there are many branches. One option for diagnosis is to separately fuse the two green spades connected to the output of the fusebox, then you will know which of those two main branches is causing the problem, then progressively move the two fuses along that circuit to each branch until you isolate it.
Paul Hunt

Thanks> I took Paul's advice and fused each green leg separately. As luck would have it the leg with the most components is the one with the short, i,e. the leg with the stop and reverse lights is o.k.
Two questions: Does anyone have any experience with the other components regarding the most likely place for me to start? Again the problem occurs after about 5-10 minutes of running and at speeds of 45-50 mph and above.
Also, can someone describe the physical locations of the EGR Service Counter and the Induction Heater.
Thanks again.
Bill
Bill Meyer

Bill,

I'm wondering if one thing that might be contributing is the absence of a ground wire (black coating) from each of the instruments to the body. On my 69 each instrument is grounded, though some cars may be sequentially wired with a black wire going to the body at a single point.

Your tach would be a good starting point as that seems to play-up prior to the short happening.

Regards
Roger
Roger T

The induction heater is between the Zenith carb and the manifold, and indeed that loose wire snaking over the engine is another known cause of shorts. Can't help with the service counter, I'm afraid. Other than that it could be any wire chafed through and shorting when vibration reaches a certain pitch and strength.

Lack of a ground wire won't cause blowing fuses, only non-working components. Apart from the tach the other instruments only use the ground for night-time illumination. Could be a detached ground wire flapping around and touching a green wire though. Personally I think the tach swinging and engine cutting out is merely a symptom of the short temporarily staving everything else of power before the fuse blows, and that will only happen if there are bad connections in the white circuit back to the ignition switch, in the switch itself, or the brown feeding it.
Paul Hunt

If the short is in the white circuit, what explains the fact that after the fuse blows, the engine runs fine without any power drop out?
If the problem is in the green circuit and since it occurs at higher speeds, it seems like the tach might be a logical place to start since it is the only affected component related to speed. Make any sense??
Going out there now to start "exploring".
Thanks to all!
Bill Meyer

Success!! Thanks to all. I got lucky and found the problem in about 20 minutes. The spade connector to the induction heater had come loose and was just flopping around. Impossible to see because a PO had converted the carb to twin SUs and, I think as part of that conversion, installed a metal heat shield between the manifold and block. The connector looks like its been shorting intermittently for some time. It appears very difficult to re-connect so I just taped it off. I never run the car in cold weather and don't know how much affect it has in any case.
Thanks again to all, especially Paul.
Bill
Bill Meyer

This thread was discussed between 16/06/2008 and 19/06/2008

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