Starter Motors for MGB's with Ballast Resistor Ignition.

[Vic Butler]

There are queries from members who have replaced the starter motor on rubber bumper B's with ballast resistor ignition and then experience starting problems which weren't there before.
The original starter motor has a connector on the solenoid which feeds directly to the positive side of the coil. The wire is white/light green. When the starter is activated 12v is delivered to the coil bypassing the ballast resistor. When the engine is running the current to the coil is via the resistive wire.
It seems that some replacement starters don't have this connector on the solenoid so I'd advise anyone contemplating replacing the starter to check that the replacement does have the relevant connector.
The fact that has to be considered is if everything was working before the starter motor swap and it's not afterwards then the fault must be something to do with the fitting/wiring of the starter itself.

[Michael Barclay]

Hi Vic
The following just occurred to me that may be a solution to the problem with these new motors with missing terminal.
The purpose of the White /green wire coming from the starter solenoid to the + side of the coil is to apply full battery voltage to the coil when the starter motor/solenoid are engaged .Could the same effect be accomplished by running a wire from the contact on the starter relay (The one that runs down to the solenoid) and the + terminal of the coil. Thus when the starter solenoid is energized full battery voltage will go to the coil and as far as I can see achieve the same effect as existing White/green wire from solenoid to coil. This would avoid the need for the extra terminal on the starter solenoid and reduce the frustration people are experiencing .
I don’t have RB car to try this on but maybe someone who does could comment.
Best regards
Mike Barclay

[Vic Butler]

Morning Mike.
I can't see why that wouldn't work, it's still a 12v supply which would be turned off when the engine is running.
However, an easier solution would be to piggy back the terminal on the solenoid that takes the power supply. I suspect that's where the connector supplied if asked for would go anyway.

[Charles Farran]

In the possible solutions you suggest is there not a danger of the non ballastered wire then feeding a supply either back to the starter solenoid & or starter relay when the engine is running as the positive side of the coil is being fed by at least 6 Volts via the ballasted wire? The non ballastered wire is only energised to 12 Volts when the starter solenoid & starter relay are activated . Not sure if you may energise the starter solenoid etc with 6 Volts when the engine is running & potentially burn out the solenoid & the ballast resistor. Use of a diode might stop current flowing down the non ballast line if my concern is valid.
I may be talking rubbish, but a great believer in keeping wiring to original format (other than more use of relays & fuses).
Some owners simply revert to a 12 Volt coil as per the older MGBs & don't use the ballastered wire route.
Cheers
Charles

[Vic Butler]

No, it wouldn't Charles. The starter relay becomes inactive when the key is released and the run relay is activated, thus cutting off all power to the solenoid. It's the set up used with the original starter motor.
With the rubber boot covering the wiring on my BGT, I can't see how the wires are connected to the solenoid.

[Paul Scott]

Good Evening,
I did a bit of investigating last night and came accross this

https://www.v8register.net/subpages/V8NOTE320.htm

My own 75 B has the Ballast system but does not have the extra relay found on the later RB's.

Wh gn wire from starter to coil - on the same terminal at the coil it is fed by the ballasted wire. Looking at the Diagram shownin the above link. It does look like the starter has a second terminal similar to a relay built in to send the 12v to the coil whilst cranking.

Regards
Paul

[Charles Farran]

Hi Vic,

My last post may indeed not have been correct. However i was essentially referring back to the situation where the incorrect starter motor (without the additional terminal to jump the ballast resistor out of circuit) was to be used.I had in mind the information contained in Rick Astley's excellent manual "MGB Electrical Systems" albeit coming from another direction.
With credit to him he says on adding a ballast resistor "In order to use a ballast resistor on an MGB/C which was not originally fitted with one,requires that a connection be made to the starter-motor to jump the ballast resistor out of circuit during cranking and so apply the full system voltage to the coil. Changing the starter-motor for a later type is not usually practical and it isn't possible to simply join a wire from the starter relay or solenoid to the coil.That would certainly provide the requisite full voltage to it,but it would also mean that with the ignition on,current from the coil would flow back down the wire and keep the solenoid,and hence the starter-motor,energised. A method of isolating the reverse current from the coil is needed,and either a diode or an additional relay will achieve that objective" He goes on to say that you may have difficulty in obtaining high current diodes & suggests wiring more than one in parallel to get round the high current issue.

The later type starter motor on my 1980 has 3 terminals. The main one where the main battery cable meets the main browns to the alternator, starter relay & live supply to the fuse box etc. , a second one which is where the white/brown feeds in from the starter relay & the third one which is the white/light green that is the 12V feed to the coil that is energised on cranking.

Cheers

Charles

[Vic Butler]

Charles, is the coil connector on the starter motor separate from the others or is it combined in a "clump?"

[Charles Farran]

Hi Vic,
The second & third terminals are each "stand alone" from the first main one (which is a heavy duty threaded bolt where the main battery & brown feeds attach with eyelets as opposed to tags for the white/ brown (second) & for the white/light green (third). The third terminal must be linked internally to the starter motor internal wiring as it is activated at the same time as the starter motor is cranked. (All three terminals are insulated from the starter motor body which obviously forms the earth return). The white brown will activate the solenoid (only on the later pre-engaged starters) which then pre-engages the gear before applying full turning power.
I agree with you that the front relay is the ignition relay, with the starter relay behind. I believe the starter relay was moved to this position on later RB cars due to the location of the brake servo with the dual circuit brake system. The leads that supply these two relays are very close together & it is possible to place the respective operations of ignition & starting the other way round as long as the correct combination of leads is maintained. It is always worth checking to see which is which. (Starter relay has Earth ,White/Red(X2),Brown(X2) & White/Brown ; Ignition relay has Earth,White(X2),Brown &White/Brown). (I know i should use "N" for Brown but thought it useful to be clear to others)!
There are discrepancies between wiring diagrams produced by Bentley & Haynes etc. - the above is what is on my car - for example Haynes indicates that the single White/Brown on the starter relay is White / Red !
Sorry if i went off topic a bit, but the weather has necessitated being indoors for a while!
Cheers
Charles

[Vic Butler]

Thanks Charles. I am using the wiring diagram from the official BL Repair Operation Manual.
There is no diode between the coil and starter motor solenoid connection. There must be something internal that stops current feedback when the engine is running. The wire with the eyelet is obviously the alternator wire.
I think the replacement starters don't have the same circuitry as the original.
I've noticed that starter motors are advertised as being suitable for B's from 1967 to 1981 when only rubber bumper ones have ballast resistor ignition.