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MG MGB Technical - Locating electrical problem

After killing the alternator by jumping the battery with the polarity reversed, a few things other electrical things broke. I blew the two radio fuses and the bottom fuse in the fuse block.

After charging the battery and replacing those fuses, most things work fine again. The radio works, the gauges work, the car starts, the headlights work, etc.

But the ignition indicator light no longer goes on at all; and the brake lights, license plate lights, and side flashers don't work. I've checked the bulbs and they're all fine. The rear blinkers (hazard lights) and reverse lights do work.

I've been staring at the wiring diagrams from the MG manual and advanceautowire.com; and I can't locate where the likely problem is. I'm thinking there's an in line fuse that has blown or I fried a wire somewhere.

Does anyone know which wires I should be looking at, if there's a fuse along the line somewhere, or if there's some other possible fault?

Jeremy
1974.5 mgb roadster

J. Palgon

You most likely fried your alternator (regulator and/or diode pack) and that is why the ignition light is not working.
You most likely will have to go through each affected circuit starting at the starter and inspect, test, and clean every connection (use dielectric grease on all the connections to prevent corrosion in the future). Then you will have to do it for all the ground connections.
As a short cut test to see if you are getting power to the sockets of the affected lights and check the grounds. Test the brake light switch to see if power is getting there.
Kimberly

Good advice from Kimberley, don't expect to find a 'magic bullet' that will cure all of them, even though they seemed to happen at the same time.

Check for 12v both sides of the brake light switch with the pedal pushed and the ignition on. No 12v at the switch green move forward to the fusebox spades, the green fuse itself must be OK if the gauges and reverse lighst work. 12v on both move back to the bullet conenctors by the rear lights. 12v on green but not green/black means the switch is faulty.

The number plate lights are fed from the bullet connectors for the rear lights, so if the rear parking lights are working check these bullets. The number plate lights probably also have a wired ground if a rubber bumper (unlike the main light clusters) so check these as well. Ditto the side markers. If none of the parking lights work check the top two fuses, one fuse feeds each side.
Paul Hunt 2

That should be green/purple for the brake lights.
Paul Hunt 2

Kimberly and Paul,
Thanks for the input. After testing with a voltmeter, I found that the brake and taillights weren't getting power. I eneded up cleaning up all my connections--still nothing. Despite having replaced all the fuses to begin with, I replaced them yet again. Apparently, one of my replacement fuses was bad (although not visibly so) because this solved the problem.

However, I do have a few other electrical curiosities. I don't recall if the side indicators went on before, but they do not now. The bulbs are fine but all four indicators fail to light. The rest of the blinker system works fine, as does the hazard switch. I presume the side indicators are supposed to illuminate when the blinkers are on. Correct?

Also, now that I've cleaned up the switches and connections, the sealt belt light on the console goes on--but only when the headlights are on. Is this normal? And the light doesn't go off when I connect the seat belts. Should that light go off when the belts are fastened?

Jeremy
J. Palgon

Jeremy: Since you say none of the four indicators fail to light, I assume you mean the side marker lights on each corner of the car. They aren't supposed to blink with the sidnal lights. They come on when the light switch is in the first position, parking lights on, or in the headlights on position.

The seat belt light should come on when the ignition switch is on, in position II, when a gear is selected and either the driver or passenger seat belt isn't fastened. It should go out when the belts are fastened. The seat belt sequential control system is in the circuit, it isn't servicable and I have never seen any drawings, so isolating the problem may be difficult.

Clifton
Clifton Gordon

Clifton,
Right you are. The side marker lights do indeed go on with the headlights. I just forgot their normal state of operation. They work fine.

The seat belt light situation is a bit more curious. It doesn't go on unless I turn on the headlights. And it didn't go on previously because the contacts were a bit rusty. Since it's not important--I have no problem remembering to put on my seat belt (and it's the law in CA)--I'll probably either leave it or remove the bulb.

Thanks again everyone.

Jeremy
J. Palgon

Does the ignition light come on when you turn the car on, but not running? If not, it could be a symptom of something is wrong in the charging circuit. Or maybe a bad connection or a bad bulb.
Need to check where the power for the seat belt light is coming from. It is possible that someone may have plugged it into the dash lights.
Also be aware that on some of the models when you blow a diode the car may try to start when you pull up on the hand brake and the car is off.
Check your charging system to make sure it is working correctly. Not over charging your battery or sending electrons where they are not suppose to go.

Fuse boxes are known for corrosion. Make sure the back is clean and the connectors are clean. I have seen power failures where the fuse is good but the clips holding the fuse were corroded.

If you don't already have one, wiring diagrams are available at www.advanceautowire.com
Make a couple of copies, one for reference and one to write/make notes on while you are inspecting the electrical system. Some people have made enlarged copies and have had them laminated at places like Kinko's so they can use dry erase or similar markers on them.
Kimberly

Kimberly,
Well the charging system is definitely not working because I haven't replaced the alternator yet (so, no, the ignition light doesn't come on). The rebuilder who supplied my last alternator had offered me another one for $45 dollars including shipping. Quite a steal I'd say. Unfortunately, I haven't heard back from him on over a week. So I'm still in the market for an alternator. I'm hoping to avoid paying full price for a new one.

As I said in the original post, I do have a bunch of wiring diagrams. I have the ones in the MG manual and the ones from advanceautowire.com. After looking closely at the diagrams and checking through the work history (since my family was so wonderful as to have kept all records of the car since new); and it appears that the seat belt light was wired into the dash lights at some point. At the same time, the rheostat switch was bypassed and the seat belt buzzer disconnected.

I had discovered these last two things when I was cleaning up the electrical system a year ago. I think I somehow failed to notice the seat belt light the first go round.

So now it seems my electrical system is back to normal, except for needing an alternator. Anyone out there have a good or rebuilt alternator they'd be willing to part with?

Jeremy
J. Palgon

J. Palgon -

Check with your local chain parts store. A Bosche alternator is about $50-. Look for the Bosch alternator that fits a 1980 Ford Fiesta.
Kimberly

This thread was discussed between 30/05/2006 and 09/06/2006

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