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MG MGA - Another electrical problem

The other night, I noticed that my ignition light was on full brightness, even at speed.
I looked under the hood this morning, and found that the yellow wire running from the generator to the voltage regulator was fried.
When I jumped the wire, the light immediately went out, so a fried wire seems to be the culprit. However, I'm hesitatnt to just replace the wire without knowing what caused it to fry in the first place.

My wiring loom is new - about 2 years old. Allcircuits and accessories are working.
I recently installed a driving lamp, but that was simply a matter of plugging into the existing fog lamp bullet connector and grounding.
Is this a sign of a bad voltage regulator, or something else?

Thanks in advance.

Joe
Joe Cook

Joe - Both wires on the generator go to the regulator, which one was fried. If the lighter (smaller wire) was fried, then there is something wrong in the generator, like a shorted field winding. If the large wire was fried, then something in the regulator or elsewhere in the car shorted, pulling a very large amount of current. If this is the case, I would suggest replacing the wire, but hooking it up through a 15 amp fuse, then start turning on various electrical loads in the car (lights, wiper, heater fan, brake lights, etc), first one at a time and then all at once until you see if the fuse is going to blow. If it blows, the last thing you turned on is where the short is. Good luck - Dave
David DuBois

Dave,
The gauge of both wires seem to be about the same. It's the wire that attaches to the lower terminal (with the large spade connector) that goes to the D post on the regulator. Is this the "larger" wire? The other wire (yellow/green) is OK.
Thanks again.

Joe
Joe Cook

By the way, car is negative ground.

Thanks again.
Joe Cook

Joe - That particular wire whoud be the British equivalent of a 12 AWG wire in American terms, I beleive that they call it a 28 strand wire. If it is the same as the wire going to the samll terminal (field that could be the problem, the wire would be too light to handle the current. If both of the wires are the heavier gauge, then the problem is a short somewhere in the car (the heavy wire to the field would not be a problem, just over kill). I think at this point, I would suggest calling British Wiring <http://www.britishwiring.com/> and ordering a piece of wire of the proper gauge and color, along with the correct size female spade lug and replace the wire, temporarily splicing in a 15 amp fuse (you can get a blade type fuse and holder from NAPA). Just tell Leslie what you need the wire for and she will let you know what you need - they are great people to do business with. Once the wire and fuse are in place, then start turning things on as I suggested previously until the fuse blows or everything is on and the fuse is holding. If the latter occures, I would run for awhile with the fuse in the circuit to see if perhaps there is an intermittent short somewhere that comes and goes. Good luck - Dave
David DuBois

Thanks again, Dave.

This will have to wait until the weekend, as my office is about 200 miles from home. I was poking around the Britiish wire website last night - I'll give them a call.
I was going to do exactly as you recommend last weekend, but I couldn't find the proper size spade lug (1/2"?) anywhere. I tried auto stores, electronics stores and home centers without any luck. Drives me crazy when such an inconsequential part causes such headaches!

Regards,

Joe
Joe Cook

Joe - Again, British Wiring. They are probably one of the best finds that I ever made (through these BBSs I might add). Cheers - Dave
David DuBois

OK, I replaced the fried wire with a new one that now has an in-line fuse - no joy. The light is still on. The fuse has not blown. The points in the voltage regulator seem to be OK (not fused, and good spring action). I double checked my rewiring via a continuity tester. I also checked the seating of the ignition light.

What's puzzling is the car starts and runs fine, so I don't think I'm running solely on the battery.

Any other ideas?

Thanks. (Beautiful day here - I can't believe I'm not able to drive!)
Joe Cook

Joe - Now it's time to check the output of the generator. Disconnect both wires from the generator and short the two terminals together with a short piece of jumper wire. Connect a multi-meter, set to read volts, between the jumpered terminals and ground (if you car is wired for positive ground, connect the black [negative] wire from the meter to the generator and the red [positive] wire to ground and just the opposite if the car is wired negative ground). While monitoring the meter, start the engine and slowly increase the revs. The meter reading should rise with the revs to 20 volts (and continue upward if you let it). Drop the revs as soon as the voltage goes above 20 volts as it will continue to rise quickly to the point that it will burn out the generator if you let it. If you get the above indications, it is time to get the regulator checked out. If the voltage doesn't rise, or if it goes up a short way even to 12 volts) and no further, then you have a faulty generator. In either case, it is time to take the regulator, or generator (or both) to a good auto electric shop (NOT Auto Zone, Shucks, Kegans or other discount outlets with a tester in the back) and have them work their magic on the offending unit. Good luck - Dave
David DuBois

Did as you suggested, and got NO reading from the meter. checked the meter against the battery to make sure it's working; battery reads about 10 volts. Sounds like a generator problem. Thanks for staying with me on this, Dave!
Joe Cook

David why do you go to all that trouble to check the dynamo output when simply putting the volt meter across the battery would achieve the same result?
Bob (robert)

Bob - If one reads 10 volts across the battery, as Joe's does, how do you know what the cause is? It could be a battery that isn't taking a charge, a cable that has corroded terminals, a regulator that is set very low or is non functional, or no output from the dynamo. Following "good Navy troubleshooting procedures" (that I learned back in the dark ages when they used valves in radios), I just always start looking at which ever end if furthest out and try to eliminate things that could cause the problem. When I find something that can't be eliminated, it becomes the candidate for repair or replacement. If the Joe's dynamo had shown good voltage from it, I would have told him to check the regulator next (besides "good Navy troubleshooting procedures", I had a similar problem on our TD that turned out to be the dynamo. You know the old story, once burned, twice cautious).

Joe - When you take the dynamo in for repair, take the regulator with it. It sounds very much line you had some shorted field winding in it, which could have damaged the regulator also. Next thing, when you get a good generator and regulator back, make sure that the fan belt is adjusted to run VERY loose. Due to the bronze bushing at the back end of the generator, if the belt is run tight (like is done on all modern cars with alternators), it causes excessive side pressure on that rear bushing, which in turn, causes accelerated wear on the bushing. The result is that in a very short time, you wind up with an armature rubbing against the field pole pieces and, If left in that condition for very long, will eventually wear into the field windings and cause the wire to the field terminal to burn up. I did an article on the fan belts for T series cars for the exact same reason, and in there I called for almost a full inch of slack in the fan belt between the generator and water pump pulleys. The belts in the MGAs and early MGBs don't have to be that loose (they are much more flexible than the ones used on the T series cars), but there should be between 1/2 and 3/4 inches of slack between the same points. Before someone cautions about that being so loose that it will cause the belt to slip, the generator does not produce the back torque or resistance to rotation that the modern day alternators do when current is being drawn, so there is very little rotational resistance for the belts on cars with generators to have to work against. Cheers - Dave
David DuBois

Yes I can see your point I suppose it is a matter of deciding which is furthest out. I suppose with my way of working my first check is voltage at the battery, if that is not increasing I have a fault with Dynamo or control box. You go straight for the jugular which is fine. I suppose it is the LAZY steak in me.
I wonder if it is something to do with the difference in cultures. The American military way of life and the British "do as you like" "I don't give a to55!"
Bob (robert)

Bob - I was being somewhat facetious about my "proper Navy troubleshooting procedures", but it has stood me well over the years. I'm not sure that looking at the battery first is being lazy, although in Joe's case it definitely confirmed that something in charging circuit was way out of whack. I have troubleshoot problems starting from either end and when I really want to save time, I start in the middle and keep cutting each leg in half as I prove or disprove a problem, but that has sometimes led me down the garden path and many extra hours of work. I guess that you just take your choice and go with the flow. Cheers - Dave
David DuBois

This thread was discussed between 11/06/2005 and 19/06/2005

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