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MG MGB Technical - Starter solenoid puzzle

Late MGB with pre-engaged starter (Lucas 2M100).

Turning the ignition key just produced a ‘clonk’ from the starter solenoid. At the second attempt it started ok. A couple of days later the same thing happened. The battery is ok, so I suspected the solenoid. Dismantled the starter and solenoid and found the mechanism between solenoid and starter shaft was a bit sticky. I cleaned the old grease off and re-lubed the moving parts.

After reassembling, I decided to bench test the starter and put it in the vice. I connected the battery negative to the starter case (earth), and to test just the solenoid I put the supply positive onto the solenoid ‘start’ spade terminal (the one normally supplied from the starter relay). I was expecting the solenoid to energise and push the starter cog out, but nothing happened. Checked the current – the solenoid was taking 11 amps. I checked that everything was assembled correctly and moving freely. I then connected the positive supply to the main stud on the solenoid; linking the positive to the ‘start’ spade terminal produced a result – the solenoid pushed the cog out and the motor ran. I tested it a few times and then put it back on the car.

I’m still puzzled as to why the solenoid didn’t operate and push the started cog out when the supply was applied only to the solenoid. Any ideas?
Brian Shaw

The main cable is on all the time, and powers the starter and the solenoid. The solenoid is like another relay, it needs to be switched at the lug that comes from the starter relay. By linking them together the solenoid moved.
Dave Braun

these might also help with info;

http://i1.ytimg.com/vi/lf82vFQc410/mqdefault.jpg

http://i1.ytimg.com/vi/kmh2qZgfv1Q/mqdefault.jpg

http://www.youtube.com/watch?v=R6CR_8efA88

http://i1.ytimg.com/vi/LG0ZuPiF2XQ/mqdefault.jpg

Nigel Atkins

If the starter relay or ignition switch put out an earth to the solenoid operate spade, then it would need 12v on the battery cable stud before the solenoid would operate. But that's not the case, the starter relay or ignition switch puts out 12v, and the other end of the solenoid winding should go to the case. Therefore it should have operated as Brian expected, but not spun the motor.

Are you sure you used the right spade? Rubber-bumper starters have two spades, the extra one being for the coil ballast bypass circuit. Putting 12v on here would do nothing, regardless of whether 12v was on the battery cable stud or not. I suspect you put your 12v on the two different spades at different times.
Paul Hunt

Paul, I was definitely putting the 12v onto the solenoid winding terminal, it's a 1/4" spade. Next to it is a 3/16" spade marked 'ign' which I know is the ballast bypass. Each time I touched the 12v wire on the 1/4" terminal there was a spark, and as I said, I measured 11 amps flowing. I removed and re-fitted the solenoid a couple of times just to make sure it was properly coupled to the engagement lever.

At one stage I removed the solenoid and put 12v onto it on the bench. The plunger pulled in, but not quite far enough to close the main contacts. I have a spare identical solenoid (old, unrefurbished, but known to be working) which behaved the same when 12v was applied.
Brian Shaw

I've just had another look at the solenoid. I've taken some resistance measurements (see photo). SOL terminal to E (earth = solenoid casing) is approx 0.8 ohm. This is the dc resistance of the solenoid coil. Resistance between STA and E = 1.05 ohm, and between SOL and STA = 0.25 ohm. These measurements were taken with the solenoid detached from the starter, so there must be some internal connection between the solenoid winding and terminal STA.

Brian Shaw

Brian

There will be 2 windings in the solenoid, the closing winding and the hold on winding. The closing winding, as the name suggests will pull the armature in, and the hold on winding adds a bit extra once it's pulled in to keep it there.

See image for explanation.

The link below is to a document put out by the company beginning with F whose name shall not be mentioned on these pages but it's very useful none the less.

Page 9 shows the pre-engaged starter, it shows a Bosch unit but the Lucas unit will be virtually identical.

http://www.belderok.net/transit/manual/Manual%20Vol%202/STARTING%20SYSTEM.pdf

Bob


R.A Davis

Interesting, and it figures (early overdrive solenoids had a similar arrangement), although the workshop manual makes no reference to it. However the pull-in winding will be short-circuited by the solenoid contacts once the solenoid has operated, by having 12v both sides thus drawing no current, leaving just the hold-in winding to keep it operated. There is no need to add a bit extra once it has operated, the objective is to reduce the current passing through the solenoid windings to the minimum necessary, to reduce heat as well as battery drain (although by this time the motor is powered so that is negligible).

I know I didn't have a problem testing a new starter before fitting it, so I must have connected 12v to the battery cable stud then linked that to the operate spade from the word go.
Paul Hunt

Thanks for the info. That seems to have solved it. I've attached a simplified schematic. For the pull-in winding to energise it needs the motor to be in circuit. When the main contacts close the pull-in winding de-energises. When I did the bench test I didn't have the link between solenoid and motor connected, so only the hold-in winding would energise.

Brian Shaw

Paul

You're absolutely correct, I hadn't bothered to work the circuit through and I'd only read the description above the diagram.

An academic point of interest, I'm sure is just for clarity, is the 2 coils are shown wound in opposite directions. The Left-Hand rule tells me that can't be right.

Bob
R.A Davis

Well it *could* be, the hold-in would subtract about 10 amps-worth of magnetism from the 50 amps-worth of the pull-in, but it wouldn't make sense for it to do that. Draughtsman's frolic :o)

Incidentally the Haynes sectioned drawing of the early pre-engaged shows the two windings, hadn't noticed that before, but again no other mention that I could see.
Paul Hunt

PS. Not only make no sense, but as the pull-in winding de-energised on closure of the contacts the magnetic field would reverse to the lesser value. As it passed through zero the solenoid would start to release, and as soon as the contacts opened the solenoid would try to operate again, causing chattering. So definitely an error.
Paul Hunt

This thread was discussed between 26/08/2013 and 30/08/2013

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