MG-Cars.info

Welcome to our Site for MG, Triumph and Austin-Healey Car Information.

Parts

MG parts spares and accessories are available for MG T Series (TA, MG TB, MG TC, MG TD, MG TF), Magnette, MGA, Twin cam, MGB, MGBGT, MGC, MGC GT, MG Midget, Sprite and other MG models from British car spares company LBCarCo.

MG MGB Technical - starter won't stop

Greetings: I replaced the starter on my '76 MGB with a rebuilt starter about a week ago. After installation, it cranked fine for two or three tries. The engine hadn't been run recently so I didn't expect it to start easily. On the fourth try, after releasing the ignition key, it continued to crank. By the time I pulled the + battery cable the battery died. I charged the battery and tried again. It cranked ok for about 20 tries. Engine started & all was well. I've checked both the ignition switch and starter relay and they're both normal. Today, it did the same thing again after two attempts. Anything else I should check before returning the starter as being faulty??
Thanks to all.
Bill
W. L. Meyer

Either you have a faulty relay or starter solenoid. RAY
rjm RAY

If it does it again, disconnect the output from the starter relay and see if it stops.
Dave O'Neill2

If disconnecting the starter relay, then pull the starter and take it back to the place that you purchased it from. Cheers - Dave
Daved DuBois

One of the wires in the wiring harness down from the stater relay to the starter shorts to the chassis causing the starter to engage.
werner haussmann

http://www.mgb-stuff.org.uk/braketext.htm#balance

Read the entry about the 1976 H/B (ebrake for you guys?) warning circuit.
There's a diode in the circuit that, if dodgy, results in cranking all the time.
RoadWarrior

"One of the wires in the wiring harness down from the stater relay to the starter shorts to the chassis causing the starter to engage."

Not so, the wire sends 12v down to the solenoid. If that shorts out to the chassis you fry the wiring. However if it shorts to the battery cable stud, or some other source of 12v, *then* it will cause continual cranking.

It's either a coincidence and either the brake test diode or starter relays have chosen that moment to fail, or associated wires to short to something, or the solenoid in the new starter is sticking. Personally I always check electrical stuff on the bench before installation.
PaulH Solihull

I do not think it will be the diode, as the result from this failing is the starter spinning all the time with the ignition switch on position 1 AND with the handbrake on. This is not what the OP describes.

As the solenoids on these starter motors are generally bomb-proof, my money would be on the starter relay on the way out, especially if it is the original one.
Hal Adams

Once the starter begins to spin, and continues because the diode has gone short-circuit, it doesn't matter where the key then is, even in your pocket, it will continue to spin. Dropping the handbrake *will* stop it, although you would have to have great presence of mind to think of that at the time, and I'd say that the handbrake usually *is* on when you are cranking.

On the face of it as soon as you turn the ignition switch off the starter should stop, as removing 12v from the green circuit, which is going backwards through the diode to hold the starter relay operated, should release the relay and stop the cranking, but it is isn't as simple as that. If it were, it would be much less of a panic to remove the battery cover panel and undo a clamp to stop it, as remembering to drop the handbrake is unlikely to be the first thing one thinks of. What makes the starter crank continuously under this fault condition, even when the ignition is off, is that the ignition ballast bypass circuit on the solenoid is putting 12v onto the coil during cranking. When the diode fails, even when the ignition is turned off, this 12v comes backwards through the ballast resistance onto the white/brown wire from the ignition relay, which is also feeding the green circuit via its fuse, and that is enough to go through the closed handbrake switch and the short-circuit diode to hold the starter relay operated.

I do agree it is unlikely to be the diode itself, as it seems intermittent.
PaulH Solihull

Wow!: Thanks so much for all the response. Had no idea something that seems so simple could be so complicated. Not being that familiar with the electrical scheme, I would have never thought of the handbrake. I still haven't resolved the problem, but may have some time tomorrow to get back into it. I've had this car for 10 years and never had this problem until the day I replaced the starter. Seems like the odds are a bad starter (rebuild).
W. L. Meyer

Why did you replace the starter?
Dave O'Neill2

.....Wow!: Thanks so much for all the response. Had no idea something that seems so simple could be so complicated. Not being that familiar with the electrical scheme, I would have never thought of the handbrake. I still haven't resolved the problem, but may have some time tomorrow to get back into it. I've had this car for 10 years and never had this problem until the day I replaced the starter. Seems like the odds are a bad starter (rebuild).....

It is not complicated. It is made complicated by people going off at tangents. From your symptoms, it is not the handbrake!

Everything points to the relay or solenoid as RJM Ray suggested in the reply to your question. Look at the relay first. Withoiut sounding patronising, do you know where the starter relay is?
Hal Adams

Dave: I replaced the starter because it had been intermittent for a couple years. It always started, just took several, or more, tries. I got tired of that.

Hal: Yes, I know where the relay is. I have checked both that and the ignition switch with a multimeter and both behave normally, over repeated cycles.
Thanks
W. L. Meyer

"I have checked both that and the ignition switch with a multimeter and both behave normally, over repeated cycles."

You need to be doing that when it fails. Difficult I know when it is intermittent, but on more than one occasion I have dealt with that by positioning a small multi-meter in the cabin connected to the circuit I'm currently monitoring, then using the results of that to decide what to monitor next, until I prove which causing the problem.

However if you are saying that the output wires of both the ignition switch and relay showed 12v going *off* when you released the key, but the starter continued cranking, then it must indeed be the starter i.e. a mechanically sticking solenoid.
PaulH Solihull

OK The second rebuilt starter is in and everything seems back to normal, with repeated starts. Just in time to store it for the winter! Thanks and Merry Christmas to all!
W. L. Meyer

This thread was discussed between 17/11/2012 and 01/12/2012

MG MGB Technical index

This thread is from the archives. Join the live MG MGB Technical BBS now