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MG MGB Technical - Electrical Gremlin

On todays 40 mile ride in the country in my '72B, I noticed when I came to a stop the car would lose gauges (tach, temp & fuel), directional lights and brake lights. As I got the car moving again at a steady speed, the tach came back to life, and the gauges returned. Before everything came back, the tach needle bounced wildly, then settled to the normal reading. When the gauges were operating, the lights worked as well. This would take a few minutes of cruising before everything came back. Eventually, I needed to stop again, and the problems returned. The only thing I did this past winter was to have the distributor rebuilt and the car is otherwise running great. I also added a brake relay (from Dave Dubois) last year to solve persistent brake light failure problems. Any clues? I have checked all the leads at the distributor, coil and brake switch relay. All are clean and well connected.
Joe Dufresne

It's either the N feeds to the IGN switch, or the W feeds from the switch. The tach activity indicates the problem is before the fuseblock, which is the other likely place for the rest of the symptoms.
Most likely is the bunch of connectors right under the steering column, but you should go through the whole car. You will find cracked sleeve connectors, lay in about 20 singles and 10 doubles to do the whole car. They are not worth cleaning; even if they are not now cracked they will be soon!

FRM
Fletcher R Millmore

The tach takes the input from the white wire between the ignition switch and the coil or the or a white/black wire from the coil, but it gets it's power in both cases from the green circuit. An intermittent fuse connection could be the problem, however the sleeve connectors are far more likely to be the problem. I've found the outside still looks sound, but the metal on the inside just crumbles to bits when you remove the wires on both ends. There should be two of them in the white circuit, one under the dash near the steering column (it's probably a triple in your case, with five white wires; one source from the ignition switch, and four outputs), and the second on the engine side of the passenger footwell/firewall above the starter (a double here, with three wires). The one in the engine bay is more likely to be your problem. Look at that one and the white to green fuse.

On the other hand, if you have overdrive and the overdrive drops out when everything else goes awry (assuming it's engaged), then look at that connector under the dash.
Dave McCann

True, forgot about the power feed for tach. By all means check the fuseblock, both the clips and the rivets that hold the contacts in. Nevertheless, I find about 80% of cars have bad connectors at the steering column bunch. Easy to tell usually since they feel warm to hot while under way.

FRM
Fletcher R Millmore

I examined the connectors closely under the steering column, replaced connectors one at time and tried, still no go. I checked the connection on the inline fuse, cleaned and still no go. The connectors under the steering column were in good shape. One thing I did notice was the negative pole on the coil can be wiggled, the entire pole moves when you touch the wire. I was checking the spade connectors, and noticed this. The nut was tight. The positive side of the coil with the red wire doesn't act like this. I don't believe this is a problem because the car runs well. Could there be an internal problem in the coil causing this slight movement? Have changed out most connectors under the hood in the past few years. Will keep trying, electrical diagnoses is my automotive achilles heel. Handy with a wrench, limited ability with a multimeter beyond the basics. Never had a problem like this in 9 years of ownership. Keep those suggestions coming, perhaps someone out there has run into this exact problem.
Joe Dufresne

If the engine is continuing to run normally it won't be the feed from the ignition switch but the green circuit, and by the sound of it a loose fuse or spades on the fusebox. If only some fused ignition circuits are affected then it could be a loose green circuit bullet connector.

The inline fuse on a 72 only feeds washers, wipers and heater fan.

Loose terminals on the coil will affect the tach and the running, but nothing else. originally terminals were rivetted and could work loose overtime, later ther used studs and nuts. If the whole stud is turning then the problem is internal to the coil, but if it isn't causing running issues then I'd ignore it, but remember it id you *do* start getting running issues and replace the coil. If the nut is just loose then tighten it.
Paul Hunt 2010

I am begining to believe it is in the fuse block, have ordered one and will let everyone know if this solves the problem. Thanks all for your advice, I also talked to Bob Ford @ Britek who recently ran into this problem, and found it to be the fuseblock.
Joe Dufresne

Joe:
You gotta use that ol' Yankee ingenuity and logical thinkin' first. As you increase the RPMs of the motor, the lights and gauges get brighter. This means that your alternator is cranking out the power whenever it's speeding up. It's most likely your battery. You let your little ol' B sittin' in the barn all winter and you never checked her battery?

Remove the battery cables; clean off the battery posts; then clean off the battery cable terminals. Add distilled water to the cells and put it on a low charge overnight.

Your little "B" is 38 years old. Did you ever replace the positive battery cable from the battery to the alternator? You had better do this asap. Those cables are good for 30 years or so and then copper oxide begins to take it's toll. You have had this cable for 38 years now. Make sure you converted the two 6 volt batteries for a single 12 volt unit that fits into the R/H battery box. Put a thin piece (1/4 inch thick nominal)of open cell foam over the top of the battery to insulate the posts from the battery cover.

Invest in a battery trickle charger for those long cold winter days when you're not drivin' your B.

http://www.amazon.com/Schumacher-SE-1-Trickle-Battery-Charger/dp/B000H94F7S




R Murray

The *lights* get brighter, but the ignition warning light shouldn't, as it should have the same increased voltage both sides regardless of battery, battery cables etc. (which will affect cranking). To get the light to glow there has to be different voltages each side. That can only come about by bad connections from the solenoid on the brown through the ignition to the warning light on the white, or problems inside the alternator which result in their being different voltages on the brown/yellow to the warning light and the brown to the solenoid. And it usually happens with increasing electrical load, not increasing revs.

Nothing in the fusebox will cause it, but the fusebox will affect the tach, directionals, brake and reversing lights etc., but not the engine.
Paul Hunt 2010

Replaced the fuse box today, problem solved. I noticed when I had the old box in hand that several of the lugs were very loose, did not notice that with the fuses in place. Thanks for all the advice!
Joe Dufresne

This thread was discussed between 03/04/2010 and 10/04/2010

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