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MG MGB Technical - Starter Confusion

The intermittent starting problem on my '72B that I have had for the past month evolved into a no starting situation last weekend. Through advice on this site (thanks Paul & Dave) I tested the starter relay and found the brown wire had 12V @ all times, the brown and white and red/white had 12V when the key was turned. I could both feel and hear the relay click, with no sound at all from the solenoid. I concluded it was the solenoid, and removed the starter this evening and tested it on the bench. I connected jumper cables from the battery to the starter, and crossed the 2 large copper lugs with a nail, and the starter motor turned over easily, with a few sparks. Does this indicate the starter motor is good, and indeed the solenoid is bad? I neglected to take voltage readings at the starter while it was in the car, perhaps I should have. What is strange now, I re-connected the battery, but could not detect any voltage at the wires that connect to the solenoid. Should I see anything there with the starter removed? None of this makes sense, because the started easily spins on the bench. The batteru cable is 2 years old, and is heavy gauge welding cable so I am confident that is good. In the prior 8 years I have owned the car, I never encountered a starting problem. Did have alternator problems this spring, changed that and it puts out 13.8-14.2 volts at high idle.
Joe Dufresne

Bridging the large studs on the back of the solenoid bypasses the solenoid contacts, and so if the motor spins OK that does tend to confirm the solenoid is the problem.

However you should also bridge the battery cable stud to the spade on the solenoid (try one then the other if there are two, but if they are different sizes it should be the larger). This should cause a clonk from the solenoid, the pinion to move out, and the motor to spin. If you don't get the clonk or the movement of the pinion then it is the solenoid winding that is bad. If you get the clonk and the pinion movement then the solenoid contacts are bad, but there could also be something stopping the pinion movoing as far as it should which may well be stopping the contacts closing.

Incidentally you don't need jumper cables to test the starter on the bench, it takes a fraction of the currenty spinning freely compared to when cranking an engine, so much so that ordinary wire can be used, although it may get a little warm.

With the starter out and the battery reconnected (not a good idea, incidentally, unless you are absuluteluy sure that the unconnected battery cable can't touch anything) you will only have 12v on the large cable coming from the battery, not on anything else, unless you connect the brown wires at the starter to the battery cable.
Paul Hunt

I connected power to the large lug on the solenoid that the battery cable attaches to. I bridged to the large spade, nothing, to the smaller spade, the starter turned over. Now I am not so certain the solenoid is bad, but had ordered a complete starter that should now have arrived at the local auto parts store. I still question the relay, when someone turns the key, I defintely feel the click, the metal relay can does vibrate strongly. I have never been under the hood holding on to the relay when the key is turned, is this normal?
Joe

I don't know whether this is a follow-up to an earlier thread, but recently I did give diagnosis info for the starter relay and the final part of that is to ensure that even if it *does* click, you should confirm that 12v is present on the output spade and wire connector i.e. the brown/white wire. The brown/white miust be connected to the relay and the solenoid, anything else isn't a satisfactory test of the relay. Even though you get the click the relay contact could be faulty. If it is completely open-circuit then you won't see 12v on the output terminal with the wires disconencted. But if it is only high resistance then you will see a full 12v with a meter on the output terminal with the wires disconnected, but with the wires connected this voltage will drop significantly, quite possibly to below that at which the solenoid will operate. If you *don't* have 12v on the output terminal with wiring connected to the relay and the solenoid, then you also need to check the voltage on the brown terminal and wiring connector at the relay while the relay is still operated, in case it isn't a problem with the relay contact but there is a bad connection to the relay or further back in the brown wiring. In this case the voltage on the brown terminal or wiring connector will drop significantly.
Paul Hunt

Replaced the starter w/solenoid, still no go, will order a starter relay tomorrow. The only other possibility is in the wiring harness, but the way it has acted, at first not starting every 3rd or 4th time, gradually getting worse, the load noise and vibration from the relay lets me believe this 37 year old device has gone bad.
Joe

Could still be a bad connection at one end of the brown/white or the other, between wire connector and spade or between wire and wire connector if they are after-market. Much cheaper than a new starter and relay :o( Personally I'd have said wiring connections are more likely to get gradually worse, components more likely to suddenly fail.
Paul Hunt

The starter relay was the problem! The car now starts easily every time. Just one more problem, when the car quit starting 10 days ago, the radio also quit. I changed the starter solenoid, mistakenly and suddenly the radio worked again, but still the car would not start. The old solenoid and starter were OK, but I wasn't absolutely sure until I replaced them. Changed the starter relay tonight, it starts up but now no radio. Power to the fuse. Very curious, the good news is the person who installed the radio 8 years ago warrants the installation for life. I have an appointment, and perhaps he can solve this problem. The radio works, I heard it 2 days ago! 8 years of ownership, and all the electrical problems came due this spring. Most are solved, it now starts, it charges correctly, the directional flashers now work properly, the brake lights work correctly and now have a Dave Dubois relay installed to protect the switch in place. Get the radio playing tunes, and I should be good for the summer! I suppose I have learned something in all this, and that is old switches, alternators and relays have a finite life span. I guess 37 years or 106,000 miles is it!.
Joe Dufresne

Too bad the replacements won't last nearly that long. RAY
RAY

This thread was discussed between 30/04/2009 and 06/05/2009

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