MGB Register Forum

Dave,

You can't determine the "Voltage" as such of the coil. The difference is the internal resistance of the coil itself.The later RB cars were fitted with the low resistance coils (described as Ballasted) & are called 12V Ballasted but in use are fed 6V , but given a 12V feed on cranking to assist starting, particularly in our colder climate.The good book (with credit to Rick Astley) indicates1.3 to 2 Ω for a Ballasted one and > 3 for a Non Ballasted one for standard coils rather than performance coils. On cranking, when the IGN terminal on the back of the solenoid is made live it sends 12V to the coil & current will take the easiest route that way rather than through the Ballast resistor route. The good book describes this as "effectively jumping the ballast resistor out of circuit".

Charles

Thanks Charles.

The securing clip for the coil does say Lucas DLB102, which is a ballasted coil. May not be the one supplied with the coil.

Meter readings between low tension terminals 1.5ohms and between low and hi tension 9kohms, which I took to indicate "6v" coil.

The running voltage 11v is a mystery, but as it seems to have been like that, and running OK for ages, I will wait until I can get the car on my lift to have a look at the solenoid connections, lockdown permitting.

I wonder if you disconnected the feed from the back of the starter motor to the coil,leaving the ballasted feed to the coil & then see if you could start the engine. If so, then measure the voltage at the coil & see if it has now dropped to close to 6V when running. That would then question why the 12V supply is not being cut off when the ignition is on & the engine running. I take it your fuse box is oriented the correct way round with the leads all connected to the correct terminals?
Cheers
Charles

I can't get at the solenoid connections at the moment Charles so that will have to wait until I can get the car on my lift.

I'll check out the fuse box connections though.

Dave, the ignition relay is switched off when the starter is operated thus cutting off the ballast resistor circuit. When the engine has started and the key released the starter relay is switched off and the ignition relay back on. The 12v supply direct from starter motor to coil must be cut off at the solenoid as that wire is not connected to any of the relays (see previous posts on this subject.)

Dave,

I'm not sure if this has been mentioned yet, but have you checked that the wiring to the starter & ignition relays on the inside wing is all present & connected to the correct terminals in their correct groupings? The original relays are identical & it is easy to jumble the wires between the two.
(This may have no bearing on your problem but i am a great believer in checking the easy things first, particularly when it is easy to do without crawling under the car). It's just strange that you have a 12V supply to the coil when the engine is on ,but only 6V when the ignition is only on & a varying voltage from low to high when cranking ( this last aspect may be correct).
I'm not also sure whether your relays are original or modern replacements (forgive me if i have missed that piece of information if already supplied) that might be allowing current to flow - discussed earlier in this thread.
Vic - i agree with your post of 4.59pm as "the great book" i think glossed over this point & looking at a detailed wiring diagram for 1978 & later confirms it - so grovel grovel back from me!!!! At least we are all learning more about our cars!

Cheers

Charles

Hi Guys
I took another look at the wiring diagram and Charles is correct .The 6v from the coil would still be connected to the solenoid during normal running. The addition of a diode would sort this (Readily available from RS). but after looking at a simpler solution I came across this relay that may work and would be an easy swap with existing starter relay.
https://www.12voltplanet.co.uk/make-and ... 0-amp.html
There is that has a double output pins. See attachment. The wire from the solenoid could be connected to pin 87 and the wire from the coil to 87A. When the ignition switch it turned the start position 12 will be applied to both the solenoid and the coil then when released because they the relay contacts are isolated from each other there will be no voltage feedback from the coil to the solenoid during normal running conditions. Could be a cheap and simple solution to the replacement RB starter motor issue.
Best regards
Mike Barclay

Evening Charles.
I agree about the book glossing over the factory fitted ballast resistor ignition. He goes into detail about converting to a ballast resistor ignition but no mention is made of the standard system. It's aroused my curiosity now so I'll try and find out how the standard system works with the cold start connection.

Vic,

I think the explanation is shown by looking at a wiring diagram, certainly for a 1978 & later Bentley version which is easier to understand as it is in colour. The trick must be in the ignition switch , that in the cranking position activates the starter relay ,but deactivates the ignition relay as you alluded to in your post of 4.59pm.When the ignition switch springs back to the running position,then the ignition relay is activated again.Dave appears to have a supply of 12V at the coil when the engine is running, which suggests the non ballasted feed must be getting a 12V supply from somewhere!

Cheers

Charles

Wow. Some stuff here.

Vic. Aye, the 12v ignition supply line needs to be checked out, but I can't do this until I'm allowed out of lockdown owing to difficulty getting at the solenoid. You and Charles appear to disagree about whether or not the 6v ignition supply line is cut off during cranking.
Edit. I see Charles post at 8.05pm

Michael. That relay might be just the thing, considering that I do not appear to have a 12v supply to the coil during cranking. I'm going to check out the 2 relays and wiring tomorrow.

Charles. I have a round and a square relay, the square one is a replacement I think. Have you any links to the detailed wiring of these relays? Anyway, tomorrow I'm going to give them a coit of thinking about!
Edit, I hadn't got to your 8.05 post when I started this one!

The fact that I have 11 volts at the coil when running is still a mystery.

Wish I hadn't chipped in on Vic's thread now!!!!!

My lad who is a self acclaimed auto electrician is coming round to give me a hand. They say that 2 sheep heads are better than none I think.