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MG MGB GT V8 Factory Originals Technical - Ignition Switch

I was hoping someone could help me out with trouble shooting my ignition switch, I believe it is shorted between the white and green (W/G) and brown (N)when I am in the "on" position. In this position without going to the start position the starter wants to engage. The W/G is the only wire that is always shorted to the N lead in every position of the key switch on my car, acc, on, and start, with the exception of the off position. I don't think it should be shorted in all positions but I could be wrong.

Does anyone have access to a switch for a 1978 B which I believe is the same from 1974-1980 that they could ohm out for me?
Michael S. Domanowski

There's a diode in the break warning light cct. which causes this to happen when it short ccts. It's behind the dash somewhere. Removing it will cure the fault, but you won't get any break failure warning or confidence test when you first switch on. I don't know exactly where the diode is because a P.O had removed it from my car. Barrie E
Barrie Egerton

Barry that is good news to me if indeed that is the problem, it should be easier to remove the diode than replace the ignition switch. This problem started cause I had a bad relay(s) (ignition and/or starter) and when I replaced one I hooked the negative to hot somehow. Then this problem occured after I replaced the melted ground wire from the relay and both relays.
Again thanks for the tip.
Michael S. Domanowski

White/green should be live in the first (accessories) and second (run) 'on' positions of the ignition switch and should only power a radio or similar. The white (ignition) is powered in the second and third (start) 'on' positions of the switch, and the white/red (crank) is only powered in the third 'on' position, when the white/green is disconnected. The switch also has a pink wire which is grounded whenever the key is in the ignition, and a slate or grey which has 12v when the ignition is off.

The diode problem that Barrie refers doesn't cause the starter to crank when the key is turned to the 1st or 2nd positions, only the correct 3rd position, but it does cause the starter to continue cranking even when the key is released, unless the handbrake is dropped. This doesn't sound like the problem Michael has, more like cross-connected wiring given his subsequent post.
Paul Hunt

While we are on the subject of ignition switch, after a long restoration mine is finaly ready to roll, everything seems to be working OK?, except that the wires coming out of the ignition switch are getting HOT to the touch, also wires going to the lower fuse on the fuse box are HOT, anybody could tell me what cuold be the problem?

Well Paul according to what you are saying my switch seems bad as it should not be turning the starter when it is in the run (on) position but it is. My switch seems to be functioning incorrectly in that the white/green is hot in all 3 positions and the white is only hot in the on (run) position but open in the start position.

Another symptom is if I disconnect either brown wire from the fuse block the starter will not attempt to start in the run(on) position but won't give any juice to the starter in the start position either.

I'm really thinking I have to order a new switch. Someone else mentioned to me something about a remote start switch to use as a tester in place of my standard ignition switch, anyone know anthing about these?

And thanks for all the feedback.
Michael S. Domanowski

Romney - wires getting hot indicates too much current flowing i.e. a short if they get hot for a long distance, or a switch or fuse with high-resistance connections if it is only getting hot a short distance from the switch or fuse. The ignition switch should be powering the 2nd fuse up - white (or white/brown on late cars) to green - and not the bottom fuse which should be brown to purple and powered all the time. If it is the white and green that is getting hot then it seems likely that something on the green circuit is partially shorting and drawing too much current, but not enough to blow the fuse. Disconnect the green fuse and see if the wires at the switch i.e. the brown and white are still getting hot. The car should still start and run like this but you will have no instruments, turn signals etc.

Michael - if the starter starts and stops with the ignition switch unplugged and the brown linked to and disconnected from the white/red, and the car starts and runs with the white and the white/red connected to the brown (and the white/red disconnected when it is running) and the engine stops when the white is disconnected from the brown then it does seem that the switch is at fault.
Paul Hunt

I'm sorry to report that after fitting a new ignition switch my problem remains. In the ďONĒ position (not start) the starter will engage. Iíve been through the wiring diagram a dozen times and canít figure out what is causing this. Is there anyone out there with any ideas? 1978 MGB V8 with 62 Olds engine and mini starter from D&D.

Could it be the starter solenoid?

How do I test that?
Michael S. Domanowski

I've had a similarly curious problem with my starter ever since I've gone to the later model Ford starter.Maybe this relates to other later starters as well.I've had intermittent signs of a bad solenoid since day one,the ole click click click.I also noticed that the wires to the ignition switch would get very warm if I held the key in the start position after the click (looking for a bad connection).After much hair pulling and a new solenoid I finally discovered that the starter would work fine if I ran a 14 gauge wire to the S terminal.Switch to 16 gauge,click click.Of course the ignition switch and all the other original wiring appears to be 16 or even 18 gauge.I solved the problem by mounting a basic Ford relay (fortunately hidden down on the tranny) that is now wired to the ignition switch that in turn has 12 gauge wire to the starter solenoid.A solenoid to activate the solenoid so to speak.My guess is that the new starter solenoid pulls a heavier draw and therefore needs the heavier gauge wire.Whatever the theory,I know it works fine now.Hope to see a lot of you in Mich...

Michael, it sounds like your start & ignition relays have crossed wiring. The start relay should have a black (ground/earth) & a white /red wire at the coil of the relay & a brown & white/brown wire at the contacts of the relay. The white/brown wire should go to the starter solenoid. The ignition relay also has a white/brown wire which goes to the fuse block. If these wires are crossed, the starter solenoid will be energised at "switch on". If this is not the case, then the white/red wire from the ign. switch must be getting 12V at "switch on" instead of "start" Good luck. Barrie E
Barrie Egerton

A friend of mine had a problem with his '77 B which was the fault of the diode for the brake warning/emergency brake light. The diode was shorted and every time he pulled the emergency brake his starter would engage...a feedback of some kind to the starter relay. Have you had your emergency brake pulled when your problem occurs? Replacing the diode fixed the problem. By the way there are two diodes under the dash.

79 MGB V8
Bernie Posey

I had problems with a slowly turning over starter (when the ignition was on) when I was messing around with a ballast resistor set up on a non ballast type starter. I don't recall the exact details of my foolishness (I'm obviously repressing) but it was something to do with attaching the ballast by pass directly to the starter solinoid, or somthing else equally weird.
My solution was to get a 12V coil and ditch the ballast resistor, I've never missed it.

Do you still have the brake pressure failure switch in the car? If you do, turn the ignition to the "on" position (so the starter engages) and disconnect one of the wires from the brake pressure failure switch. If the starter disengages, it's the diode that is in the wiring harness behind the glovebox/center console.


This is like deja vu all over again. The diode problem causes the starter to keep cranking, once you have turned the key to 'start', unless the handbrake is down. If the starter starts to crank with the key just turned to the 'run' position and not to 'start' then it is simply (?) a wiring problem. In the run position you should have 12v on the brown, white, and if you have an accessories position the white/green. The white/red should only have 12v with the key in the crank position. If not then either the switch is faulty or wrongly wired, or the white/red is shorted to the white (somewhere else, disconnecting the white/red at the ignition switch will tell you which.

If there is no 12v on the white/red but the starter still cranks then there must be a cross-connection between the starter and ignition relays. Both the relays should have a brown wire on one side of the contacts and a black wire on one side of the winding. The starter relay has a white/red wire on the other side of the winding and the ignition relay a white. The problem comes about on the other side of the contacts. Most cars without an ignition relay have a white/brown wire, but on cars with an ignition relay this changed to brown/white, and the wire on the other sode of the contacts of the ignition relay was white/brown.

A third possibility is that you have bypassed the loom ballast resistance to the coil *and* got the brown/white and white/light-green wires reversed at the solenoid. The two spades are supposed to be different sizes but I have seen rebuilds where they are they same. Removing the wires and briefly touching the battery cable solenoid stud to first one then the other shows which is the operate terminal, the brown/white goes to that, the white/light-green goes to the other.
Paul Hunt

Well I've finally got the car operational again. It was the diode in the brake light circuit. I could not locate the diode so I cut one of the 2 white/red wires that lead to the start relay.

Much thanks to Barrie Egerton, Sydney Australia who originally suggested this as a problemas well as Justin and Bernie. Oh yeah does anyone know where under the dash that *(&^&^%$^%$# diode is?

Just so everyone knows if this diode shorts the starter WILL turn as soon as the key is put into the "ON" position if the brake lever is pulled up. It will behave normally if the lever is released.
Michael S. Domanowski

There are actually 2 diodes in the harness, but only one usually shows up on the wiring diagram. I think the second has something to do with the seat belt warning circuit, but am not sure. Easiest way to access the diodes is to remove the glove box. They are in the harness running along the cockpit side of the firewall just under the windshield, a little left of the glove box opening. There are several styles depending upon the year and what was in the parts bin. Most common is a black cylinder 1 1/2" long, 3/8" in diameter. Some are a rectangle 3/4" x 3/8" x 3/16" thick with terminals protruding from each end. You may have to twist the harness to find them, as they often wind up on the back side, out of sight.
Jim Stuart

Thanks Jim I'll hunt them down in my spare time (LOL) It seems everything I'm concerned with works OK without the diode being in the circuit let me know if it is necessary to have them functioning for anything important.

Another reason I had such a hard time diagnosing this problem is my "Haynes" MGB 1962 thru 1978 Owners Workshop Manual does not have the correct electrical diagram for my 1978 MGB.

My serial numbeer is actually SN GHN5UJ466015 G the book says the included diagram is valid for SN's GHD3360301 onwards which should include mine. This diagram is not the correct one for my car.

Can anyone tell me where I can obtain a correct diagram?
Michael S. Domanowski

That's the same manual I've got, same schematic. You know, with the starter cranking in run it really does sound like a couple of the wires are crossed. I assume if you pull the red/w wire off the starter relay the starter doesn't crank? Because if it does crank anyway you should check to make sure the two small wires on the starter solenoid are not switched. Switching these two wires will make the starter crank in run but perhaps not in start, depending on how the ignition switch is made. HTH

Jim Blackwood

My mistake, as you say it does start cranking as soon as you turn it to Run if the handbrake is up. On a UK car the diode is right at the top of the bulkhead behind the dash on the right-hand side, near the instrument voltage regulator. The only thing the diode does is to light the handbrake on/brake pressure imbalance warning lamp while cranking, but only if the handbrake isn't already illuminating it by being pulled up anyway!

There is a second diode shown in Haynes which was for the short-lived service interval warning system. There is also a connection from the white/red cranking wire to the time-delay electrickery box involved with the seat-belt warning system, which quite likely has a diode in it.

Haynes groups together several slightly different wiring diagrams into one to save space, with resultant loss of accuracy. For example the Workshop Manual has 19 just up to 74 (North America) and 77 (UK) whereas Haynes only has 9 for all years and all markets.
Paul Hunt

This thread was discussed between 10/07/2004 and 26/07/2004

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