MGB Register Forum

Charles, I have as a supplement to my Repair Operation Manual a wiring diagram for a 1979 B. The ignition wiring is identical to the 1977 diagram. A white wire from the ignition switch goes to the ignition relay in both diagrams. The 1979 diagram has 3 inline fuses so I checked my MG and it has 3 inline fuses under the relays. The fuse with the white/brown wire entering from the radiator end is the fuse for the heated rear window on GT's and it takes its power supply from the ignition relay. This fuse is number 44a in the wiring diagram. The inline fuse with the green wire entering from the radiator end is for the cooling fan thermostat. This fuse is number 44 in the wiring diagram. The third inline fuse with brown wires at both ends is for the hazard warning lights. This fuse is number 70 in the wiring diagram. Fuse number 44a is not in the 1977 diagram.
The wiring diagram for early rubber bumper B's shows just the ignition relay and no inline fuses.
Now, my October 19th 1977 built BGT has the inline fuse for the heated rear window not shown on the 1977 wiring diagram, yet the BSH alternator configuration is as the 1977 diagram.
Confusing or what???

Good idea Charles. I'll swap em over today, and report back.

Vic, you'll be wearing your spade out if you're not carefull

Charles, grovel time again. Comparison of the 2 wiring diagrams in brighter light shows that the ballast resistor circuit in the 1977 diagram comes straight from the fuse box but from the ignition relay in the 1979 one. The white wire to the relay from the ignition switch is in both diagrams, just the ballast resistor circuit white/brown wire is altered.
That'll teach me to peer at wiring diagrams in dim light late at night.
It doesn't explain the anomalies in the wiring on my B and I'll check to see if the ballast resistor circuit comes off the same relay connector as the white wire.

Hi Vic,

I too have the Rep Op Manual M refAKM4070 with the supplement for 1979 Cars.

I agree the earlier diagram doesn't show an in line fuse for the HRW. There are many discrepancies in various wiring diagrams both from the BL sources, Bentley & Haynes. In the ROP it says at the top of each key to the diagrams "Several of the components listed in this key may not be included in the specification of all models" - guess that works the other way round as well i.e. your car had an in line fuse for the HRW but the diagram wasn't updated until some time later. I think I had a discussion with you before on this with regards to the brake failure set up for the dual circuit set up.
Incidently I have just checked my car,( albeit a roadster where I know there is a free lead that would have fed an HRW), and it has 3 in line fuses as you describe. (The Haynes diagram refers to the W/N feeds to these in line fuses as just W)!
Over the years i have come across one or two procedures in the ROP , where talking to others is has been suggested that the manual perhaps hadn't been updated for the later models -don't ask me what as i can't remember!!!!

You & i are going off topic so i had better stop!!!!

Cheers

Charles

Vic,
I was typing my last post & sent it before I saw that you have sent a further one at 11.54 today!
Cheers
Charles

Hi Vic,
Per the Haynes diagram, the white (poss W/N )wire from the ignition switch meets the white (poss W/N)wire that feeds the ballast resistor (that goes to the coil) on the same terminal on the ignition relay. See my earlier post re colour as it's the Haynes diagram .
Cheers,Charles

With the Land Rover now at the fitter's for the new bulkhead to be fitted I turned my attention to the neglected MGB GT. I measured the voltage at the positive terminal of the coil. With the ignition switched on it's 6.3v, cranking it's 9.7v and running it's 11.1v(?) As it's running fine I'll ignore it.
Aside, the fuel pump made the occasional tick and I could smell petrol. There was nothing dripping underneath and the carbs weren't flooding. Removing the battery cover I could see the inlet and outlet unions were wet. Grovelling around under the car with a 13/16 AF socket I carefully tightened them which cured the problem.
(I wish I hadn't started this topic!!!)

Thanks for that Vic, and good luck with the Landie job.

I swapped over the 2 relays this aft, with no difference in operation or readings.

You've pretty well confirmed my readings in the ignition switch run position, but we differ on the cranking position, which does point to something amiss at the solenoid which I will sort as soon as I can get to my car lift.

I think that will do for me on this one. I'll report when sorted.

Thanks all for the very usefull posts.

Dave,

Well done so far - we will just have to wait to see what transpires at the solenoid end.
I have looked at some more archive material and established that the Voltage at the coil on a non ballasted system will read a steady 12V whereas on a ballasted system will fluctuate between 12V & 6V which explains why i think both you & Vic recorded readings of circa 11V.

Cheers,
Charles

I've got the answer. The starter motor solenoid was redesigned to include the IGN terminal when the ballast resistor ignition cars were introduced. When the starter is cranking the engine a resistor is shunted out of circuit to boost power to the coil. When the starter is released the ballast resistor circuit is brought back into use. So, when replacing the starter on a ballast resistor ignition MGB, make sure there is the IGN terminal on the solenoid.
Dealers advertise starter motors for 68 to 80 cars. The ballast resistor starter can be used on all cars but the ones without the IGN terminal can't as there's no power supply to the coil when the starter is running on ballast resistor cars.
The IGN terminal can be confused with the terminal that takes the power supply to the solenoid from the relay, so care is needed here.
Both Charles and I thought Rick Astley had glossed over the subject in his book on MGB electrics. However I can't take credit for this information as when looking at the section on starting systems I found it on page 80.