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MG MGB Technical - Still having electical/ingnition problems

1972 MGB-GT..2 problems...1) Ignition light flickers sometimes faintly other times brighter. Flickers more and brighter in unison with/when hitting road bumps. This started about 4 months ago.

2) Car occasionally runs rough like misfiring, stumbling, hesitating, and on three occasions has died completely while driving down the road. Frist two times it died was during heavy rain storm driving thru heavy back splashes. Had to have my wife come and tow rope me home. 2nd time it died and then after a time I was able to get it started and it was all I could do to keep it running to get home. If I did not keep the rpm's up it would want to stumble and die. That was what happened this last time also.

I feel that I might have a loose and or corroded connection somewhere in the ignition wiring. Which is causing power loss to the coil? Which in turn is causing the rough running and stalling.

Is there a ground for the ignition system that I should be checking for a solid connection. I have checked the battery ground, I cleaned it up and reconnected it.

What other grounds should I be checking and where are they located.

Other then that, it has been suggested that I check out the wiring under the dash coming out of the ignition switch.

I wish I was a little better at diagnosing electrical prolems and not so intimdated by it all.

Any help and suggestions would be greatly appreciated.

Thanks,
John

John Fraioli

First, make sure that your distributor is set up properly. Check to make sure that the points are gapped to .015". Next, check the condition of your cap and rotor. If they are old or show signs of deterioration then they will suffer and fail when used in a wet climate. Start here and proceed to check the rest of the ignition system afterwards. RAY




rjm RAY

Why does everyone think it must always be a ground? It rarely is, there are relatively few earth connections compared to 12v connections.

First thing to do if the engine is cutting out is look at the tach while the engine is still spinning. If that has suddenly dropped to zero then the problem is in the ignition LT circuit i.e. ignition supply through the switch, coil, points, distributor ground wire (!) and their connections. If the ignition warning light is on while the engine is still spinning then you have lost the 12v supply through the switch onto the white wire.

If the tach is still registering, then the problem is either HT or fuel.

The ignition warning light will glow if there is a potential difference between the brown/yellow wire at the alternator and the white wire at the ignition switch. This can either be because there is an internal fault inside the alternator causing the brown/yellow wire to be either higher voltage or lower voltage than it should be as in the case when an output diode has failed open-circuit, or because there is a bad connection at the brown wires at the solenoid or ignition switch or anywhere in between, inside the switch itself, or the white connection from the switch, including any connections between the main harness and the ignition switch harness in those wires. So plenty of opportunity for bad connections, only one of which is an earth or ground.

I'm not a betting man, and I like to fully diagnose a fault rather than jump in to particular connections, think you have cleared the fault, only for it to pop up again on some dark and stormy night. But where the brown wires connect to the battery cable at the solenoid is usually a good bet, or any multi-plug between main harness and ignition switch harness as I say. I've diagnosed intermittent faults like this by putting a multi-meter on the centre console and connecting it to, say, the white where it comes out the ignition switch. If the meter flickers when you get the problem then move back through the switch onto the brown. If it doesn't then move forwards to the white at the fusebox and the coil, and so on.
Paul Hunt 2010



The ignition light was flickering on and off and/or dimming and brightening on mine and it turned out to be a sloppy loose spade connector at the alternator.
JM Morris

Ok, Here is an update. The rough running stalling problem is resolved. When I took the car out 2 nights ago to see how it was running, It still had the ignition light flicker problem but seemed to be running smoothly for the first 10 minutes. However, I noticed that the temperature gaurge was runnig hot. So I drove it a few more minutes and then started to notice the rough running and missing again so I headed back home (I was pretty close to home). When I pulled into the garage and shut the car off I heard some hissing under the hood. So I opened the hood and noticed that the heater valve was dripping badly onto the dizzy. So that is why the engine was missing. So I replaced the heater valve and wiped out the coolant from the dizzy cap and blew out the coolant that was in the dizzy with compressed air. Tonight the car runs smooth again.

However, I still have the flickering ingnition light. I drove the car for about 12 miles and made note as to when the glowing was worse. It pretty much stops glowing while I am accelerating, and then flickers and glows when letting of the accelerator. Road bumps don't seen to bother it as much as I had previously thought unless it is in a decelleration mode. So no flicker when accelerating but slight flicker when shifting gears and when letting off the accelerator pedal. Twice during tonights test run it stoped the flickering for a about a mile, then starts up again. The last mile back to home it did not flicker at all. So it seems to correct itself from time to time. It also does not glow or flicker too much (if at all) while the car is at idle. So it really just seems to flicker on letting off the pedal and or while braking. I did also notice some very faint flicker in unison with the turn signal blinker, or when tapping the brake.

Paul, Thanks for the sequence that I need to do to help locate the problem. I would like to e-mail you for a few more details.
Just one question for now where is that distributer ground that you did talk about located. I assume it is one of the wires I see inside the dizzy no?

Thanks to others for your suggestions I will check the three prong plug that goes into the Alternator once again even though I did take a look at that when I had the alt out a few months ago when I brought it in for testing.

Hopefully the above finding may be a clue to others as to what is going on and what might be my best way to fix the problem. I have some long distance club driving events coming up soon and need to get this sorted out.


John
John Fraioli

Hi John,

I would check the brushes in the alternator. I'm not sure how you would access them as some alternators need to be disassembled to get at them, whilst others have a removable control box which includes the brushes.
You may think that, unlike generators, alternators don't have brushes, but they do, to the slip rings, which energize the "field" winding on the rotor.

P.S. Or more basic yet, the fan belt slipping (had this one happen to me once )

Herb
Herb Adler

Everything points to a bad connection - the fact that it occurs on deceleration more than acceleration i.e. the engine tilted slightly more one way than the other. Also being more noticeable with the indicators or brake lights, which put more current through the white and brown wires leading to the solenoid and hence will drop more voltage in a bad connection which will cause the light to get brighter, as will the headlights.

The distributor ground connection should be a very flexible brown braided cotton wire connected between the points plate and the distributor body, with an additional one between the spade connection on the side and the points plate of 25D4 distributors. However problems there will affect running, causing the tach to flicker in time with any misfire, but won't be the cause of a flickering ignition warning light. Email me by all means, but I shall be offline for a few days now.
Paul Hunt 2010

This thread was discussed between 16/05/2010 and 20/05/2010

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