Ballast Resistor wiring for 1977 Mgb

[Lavo01]

Hi everyone, I'm a newbie to this forum so please be patient. The starter motor went on my MGB. I replaced the starter but then the engine would turnover and start for maybe 2 seconds before dying. I ave since replaced the points, condenser and the two ignition and starter relays but still no change with the starting. I have read that the above symptoms point towards a failed ballast regulator. With this in mind, I have purchased a ceramic regulator and tried to wire it in but I really have no idea how to wire it up. This is what I've done, I've connected a wire between the ignition post on the starter motor to the positive on the coil. The green and white wire from the positive on the coil to the positive on the resistor, the second post on the resistor is connected to the positive on the coil also and the negative on the coil to the condenser. The car will still not start and the regulator gets hot with the ignition in the on position. Can anybody please help. Cheers.

[Paul Scott]

That is definitely wrong.

The balast goes between fusebox and coil. The original is hidden in the loom and changes colour from the fusebox to the coil.

White and pink seem to ring a bell for some reason

Please see wiring Diagram for your year.


https://www.google.com/url?sa=t&source= ... 78Or0yO6ua

Or just google mgb wiring diagram advance autowire should come up.

I will take a look tomorrow if an answer does not arrive over night..

Regards
Paul

[Vic Butler]

There are 2 white/light green wires on the coil positive terminal , one comes from the resistive wire, the other from the starter motor bypassing the resistive wire when the engine is cranking.
At the fuse box, to confuse matters even further, there are 4 while/brown wires on the same terminal. 1 goes to the resistive wire, one to the fuse for the cooling fan, one to the ignition relay, the front one on the inner wing and one to the fuel pump.
To bypass the resistive wire you'd have to wire the ceramic resistor to the coil positive terminal after disconnecting the white/light green wire from the resistive wire and to the terminal on the fuse box with the 4 white/brown wires. You won't need to work out which of the 4 white/brown wires at the fuse box goes to the resistive wire if you have disconnected the correct white/light green wire from the coil and insulate the terminal.
I presume the relays are wired correctly.
Does the starter motor have a terminal on it to bypass the resistive wire when the starter is s the engine? This will be a white/light green wire. Some aftermarket starters don't have the terminal for the direct feed to the coil.
The wiring for the ignition system on later MGB 's is somewhat confusing with wires being the same colour.

[Paul Scott]

Good Morning,

Does the engine die instantly when you release the key after starting?

The ballast resister only comes into effect after you release the key and feeds the coil with around 7v. If you have a multimeter you can check to see if the coil is getting power via the ballast. White Light Green on the coil is positive - with the ignition switched on you should get a 7v ish reading.
The +side of the coil gets fed by 12V whilst the car is cranking usually from the WLG wire coming from the starter motor.

Unfortunately I cannot find a definite wiring diagram for your car. The Autowire Haynes version does show the relays but this does not match up exactly with Vic's explanation of the Ballast connection.

Shows White wire from the relay to ballast then comes out white / LG at the coil.

Has anyone changed the coil? If it has a 12v coil it will not run with the ballast due to the drop in voltage.

Paul

[Vic Butler]

I used the wiring diagram for a 1977 MGB as shown in my official BL repair operation manual.
It would appear that replacing the starter motor has caused the problem.

[Michael Barclay]

Hello Dave
Both Vic and Paul have given a good description of why your car will not start. I think you are on the right track with failure of the ballast resistor. Your car is fitted with a 6 V coil which under normal running conditions is supplied with power via the ballast resistor. The resistor drops the 12v down to 6 V to feed the coil. To aid cold starting, the ballast resistor is bypassed when the starter motor is engaged by the starter solenoid applying 12 v to the coil until the key is released back to the normal ignition position. It sounds to me the 12 v is being applied when you turn key to the starter position hence the reason it fires for a few seconds then when you release the key back to the normal setting the engine stops because power is not getting to the coil via the ballast resistor.
I would suggest the following. The ballast resistor is really a resistive wire. Disconnect the new ballast resistor and wiring put things back to original. Use a mulimeter and connect it to the + side of the coil (White/light green wires) and ground. Turn on the ignition and measure the voltage. This should be around 7v.If nothing the problem is probably with the ballast resistor. Then with mulitmeter still connected turn the key to the starter position and measure the voltage at the coil. This should be 12v. If not then you are not getting a feed from the starter motor solenoid.
My suspicion is 12 v is being applied when you turn key to the starter position hence the reason it fires for a few seconds then when you release the key back to the normal setting the engine stops because power is not getting to the coil via the ballast resistor.
As Vic mentions there are 2 white/lightgreen wire connected to the + side of the coil. One of these comes from the Starter solenoid to provide the 12 v when cranking. The other connects back into the loom via the ballast resistor (which is a resistive wire) and appears as one of the 4 white and brown wires connected to the fuse box.
I would suggest removing both white /green wires from the coil and separate these to work out which one comes from the starter solenoid and which comes via the ballast resistor. Reconnect the one from the starter solenoid. Then temporarily connect your replacement ballast resistor between the +coil connection and 12V in the fuse box (same position as the 4 x white/brown wires. Turn the ignition switch and measure the voltage. Hopefully you will have around 7 volts. Then turn the starter and hopefully the car will start. Let us know how you get on so we can advise if none of the above helps.
Best regards
Mike Barclay

[Vic Butler]

Mike, when my BGT had the same symptoms it was down to failure of the ignition relay (front one on the inner offside front wing.)

[Michael Barclay]

Hi Vic
I agree entirely with your comments. I also think you are right about the terminal being missing from the replacement motors thus no 12 v getting to the coil for starting. . I don’t have starter motor to look at but it could be that the terminal needs to be transferred from the old motor to the new one.
It’s curious that most of these problems seem to relate to the RB cars. As far as I am aware the starter motor is the same pre-engage unit (whether in Lucas M418G or2M100 form) fitted to the B since approx. 1969. The only real difference was the change from the good old faithful 6RA starter relay to the black round type (Probably a BL cost saving exercise) and the introduction of the 6 V coil and ballast resistor ignition system. This leads me to wonder how many starter motors have been prematurely condemned to the bin when it could be a simple relay or wiring fault. This is further confused by the number of different wiring diagrams/wire colours for the MGB. (I have 19!)
Unfortunately I do not have access to a RB MGB otherwise I would be prepared to put together a simple diagnoses guide to checking out the starter and ignition circuits but I am sure through the forum we can all help our fellow member’s out.
Best regards
Mike Barclay

[Vic Butler]

Morning Mike.
A simple test would be for someone else to turn on the ignition while you listen for a click from the front relay. No click means either a duff relay or incorrect wiring if replaced.
A little while ago I had a WOSP high torque starter fitted to my Land Rover. The instructions with it stated that if a connector for the cold start for a ballast resistor ignition was required then one could be supplied. My 1977 BGT still has its original starter but I can't have a look at the wiring to it as it's covered by a rubber cover.