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MG MGB Technical - Dud starter?

OK, Have I got a dud starter? It's an inertia type on a 3br engine. I read many posts throughout the BBS archive, then did some tests.

Voltage at solenoid 12.7v before trying to start. Drops about 2 volts when starter is spinning. About 0.2v less at the starter itself. Pinion doesn't engage - just "whirrs"and the teeeth are touching against the ring gear but not engaging. Tried the same with a prtable fully charged starter power supply - no difference.
I used a jumper lead to duplicate the power cable from solenoid to starter. I also did other checks to confirm that the engine earth was good, and bendix was free.
So I am assuming the starter itself must be at fault. I removed it. Pinion is slightly "polished" where it has been in contact with the ring gear. Ring gear looks sound with no obvious damage. (I had tried rotating the crank to a number of spots to ensure I was not dealing with a worn ring gear.

Have I missed anything? I must say the starter, whilst spinning, did not seem to exhibit the violent start up i would expect. I have read that if connected to 12v when sitting on the bench, it will take off if not restrained.

John
PS the starter is rotating the correct way too!
John Minchin

Since the volt drop is before the solenoid, so must be the resistance.
I would check all the connections around the battery(ies) including the earths.
With the starter spinning it should generate more back EMF than when cranking so I woiuld expect less volt drop.
Stan Best

Sorry, just re-read the post and a 2v drop when cranking would not be surprising, however the starter is not engaging so it seems odd to get that much drop. The Bendix style starter can take 100s of amps when cranking, it should just be one or two if spinning without turning the engine. I will measure the resitance of my one and post results.
Stan Best

Sounds like the pinion is sticking on the spiral-cut shaft. Many years ago I had a Mini do that, all the talk was this area should be scrupulously clean and not lubricated, so I took it off and cleaned it, put it back and it worked - for a couple of weeks. Went through the process again but more carefully and this time it only worked for a week. Then an old hand told me to put a drop (literally) of oil on the spiral-cut shaft, which I did, and had no more problems.

Having said that like Stan I'd expect virtually no volt drop if the motor wasn't turning the engine i.e. the motor was just spinning freely. 2v is the drop that you would get when cranking the engine with a good battery. First measure the voltage right on the battery posts when this is happening (each battery if twin 6v), and compare that with the voltage at the motor terminals. If that's only 10v both ends when the motor is just spinning freely then the battery is knackered.

If the battery terminals show 12v but the motor terminals show 10v then you have bad connections. Measure the volt-drop between the 12v battery post (not its connector) and the starter terminal when the motor is spinning, and again between the battery earth post and the motor body. Ditto the two posts that carry the link cable for twin 6v batteries. Even when cranking there should be less than a volt dropped in total, and you should be able to get it down to 0.5v with all good connections. Should be negligible if the motor isn't cranking the engine.
PaulH Solihull

I just put the DVM across the starter on my car. Its so close to a dead short that its hard to say anything definitive. I measured 1.2 ohms across 2 points on the block ( head studs either end) then 1.4 ohms from the post on the starter to a head stud. So its maybe 200 milliohms. Some of that is quantisation error in the DVM and up to 400 milliohms is the leads and probe contacts. Its a 200 ohm range.
BTW, when I said 1 or 2 above, I meant amps flowing when the starter is not cranking, not volts dropped. Sorry if that wasnt clear.
This stacks up with the workshop manual numbers which are several hundred amps with the starter stalled.
Paul is 100% correct about lubricating Bendixs. Back in the 60s an MGB GT owned by someone at work would not crank. I said ,based on my experience, put a drop of 3 in 1 on it and problem solved. The roar of recieved wisdom that dirt would jam the mechanism was quite loud then. But dry the car wouldnt go.
Stan Best

Problem solved!

Removed the starter and connected to battery on the garage floor. Motor spins the correct way (as i had noted) but the pinion assembly stays at the end of the shaft. The bendix is the wrong hand! The DPO provided a motor with a damaged pinion, and a brand new assembly. Externally it looks identical, but the bendix is different - what car is this from??

Anyway, the DCO (C=current) is now swapping the bendix over and reassembling. Proving it pays not to assume anything!!

Thanks to all for your advice; I will be using it I am sure, as no doubt I am not out of the woods yet

John
John Minchin

I didnt know handed Bendixes existed! Maybe the DPO was the same?
Stan Best

Voila!!

John Minchin

Amazing - maybe it's for engines that rotate in the other direction, don't some Japanese do that?
PaulH Solihull

Too bizarre! Must be because you are on the upside down part of the world. Just think about getting the matching backwards turning starter motor in error!
Actually, I have heard that some Honda rotate the wrong way. Maybe including the ones the Rover/Sterling were created from.
There is another thing: A few cars mount the starter backwards in the bell housing, with the starter next to the gearbox. That would take a "wrong" starter and drive for a "right" engine. Seems to me that some Lotus are this way.

FRM
FR Millmore

Honda-engined Rovers did indeed rotate the other way, for example the 212, but the 214 had an 'English' engine that rotated the normal way.
PaulH Solihull

This thread was discussed between 03/07/2011 and 05/07/2011

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