MGB Register Forum

The original round black Ignition Switch Relay has just given up the ghost on my 1976 Roadster, the relay is the one furthest towards the front of the inner Driver's side wing, on my UK Rubber Bumper car.

What size or type of replacement would be suitable?

Moss seem to stock the black 'round can' OEM ones, are they available from anywhere else ?

Or would a later more modern type electronic black square plastic box one do the job?

I also have several 6RA type to choose from, anybody know what capacity is required (20amp?) - would one of these be ok to use?

Many thanks in anticipation,

The front relay is the "run" relay and mine packed up a couple of years ago. I bought an original round one from Brown and Gammons but when later I wanted a spare as the relays are interchangeable I could get only a square one. It's a Lucas SRB402/B CHM68 relay.
If you can get the original round one then do so but the aforementioned relay is a direct replacement.
When I changed mine I removed one wire at a time and ensured it was on the correct terminal on the replacement relay.

Thanks for the reply.

The old round black can type relay has the markings 30/5 - so I assume any 4 pin 30amp relay would be ok. (If 30 refers to the max amperage?)

In fact I fitted a 20amp Lucas 6RA aluminium can type and it still works ok.

I will endeavour to obtain an OEM one though!

Philip, on my 1978 car, the 30 pin is actually marked 30/51. Just wondering if you have misread yours.

I suspect that any old 4 pin relay that has 85, 86, 87 and 30 or 30/51 pins would work.

I replaced the relay with the 6RA I had recently bought new.

Everything seemed fine - I now have an ignition light and circuit, the fuel pump, heater blower and wipers run as normal.

I went to try to start the Car and found it would only fire untill I let go of the ignition switch - then it stopped!

If I continue to hold the key in the start position the engine runs ok(albeit with the starter still churning away!), but as soon as I release the key and it returns to position one, the engine cuts out.

Time and time again this happens, but it won't continue to run?

I swapped the new 6RA relay with the rear-most black round original relay, but get exactly the same result.

Seems like the ignition circuit is cutting out, as soon as the engine has fired, after every turn of the key?

Anybody have any suggestions Please?

That has something to do with the balast system Philip.

You should have a 6v supply going to the + side of the coil with ignition turned on. It will get 12v whilst cranking. Do you have a test light?

Regards
Paul

Philip, have a trawl through the topic "Starter Motors for MGB's With Ballast Resistor Ignition" and see if there's any help there. The topics are rather similar.

Paul / Vic,

Thanks for the speedy replies, much appreciated!

Unfortunately, after a full mechanical refurbishment last summer, the Car has stood outside for the last 8months, waiting for the overdrive o ring seals to be replaced, as the car wouldn't reverse or engage overdrive when it got warm.

I drove it onto the ramps at that time with no other issues.

I replaced the o/d o rings and refilled the gearbox with oil, ready to remove it from the ramps when these electrical gremlins started!

The wet horrible weather over the winter months has really taken its toll on all the engine components, the new alternator, stat housing, servo, cleaned and serviced carbs all have turned white with aluminium oxidation, so is it possible that the broken relay has suffered a similar fate?

And maybe, during wiring loom untaping, wire tacing, and terminals being unplugged and refitted, that the the ballast resister has been inadvertantly blown up or just failed at this moment in time - who knows?

I'll trace the white and light green connections from the coil/ solenoid/Ballast resistor to try to establish if the resistive wire has failed tomorrow and report back.

I'll trawled through the forum topics but can't t find anything related to removing the ballast resistor system and replacing it with a 12v coil system - which is the way I'd like to go with this (if its the resistive wire causing the problem).

Have any topics been written on this subject do you know??

What do you think?

Philip, the ballast resistor is not a physical component but a length of resistive wire in the loom.
Switch on the ignition and use a multimeter or voltmeter to see what the voltage at the positive terminal on the coil is. It's the one with the 2 white/light green wires. If the reading is around 6v then the resistive wire is ok, if it's zero then the wire has failed.
Have a read of the topic Ballast Resistor Wiring for a 1977 MGB.