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MG MGB Technical - Ignition light glowing

Ok Wizards of the electrical, here is one for you. 1969 MGB GT. New wire harness and then installed two relays for halogen headlights. Ignition light would now only reduce its brightness to about half once the car started. I replaced the regulator to no avail and then took it in to be checked. He said I had the regulator pack hooked up wrong and he put in new bearings while it was apart. Well I hooked it up and get 14 volts to the battery but the light still glows at half strength. What gives? Also blinker lights are intollerable slow!.
Need some enlightenment.
Thanks.
Geoff Kimler

Slow turn signals indicate (ho ho) low system voltage i.e. less than 14v regardless of what is at the battery. The ignition warning light acts like a pair of balance scales - it doesn't glow when the ignition is off and the engine is stopped because it has 0v both sides. It doesn't glow when the engine is running and everything is working properly because there is 14v both sides. For it to be glowing one side has to have more (or less) voltage than the other. 12v on the white but less than 12v on the brown/yellow (between alt and light) indicates a bad alt, as does more than 14v on the brown/yellow. 14v on the brown/yellow but less than 14v on the white indicates bad connections in the brown - ignition switch - white circuit which will also affect the turn signals. It is possible to get a bad connection at the starter solenoid which results in a full 14v alt output at the battery but not at the remainder of the cars electrics, you need to take some comparative voltage measurements.
Paul Hunt

Thanks, Paul. This helps. Now I have to talk the local Pixie into getting behind the dash and taking some measurements for me!
Geoff

Could you replace the ignition light with one of those dual colour LEDs? The sort where it is two LEDs in parallel but with opposite polarity. That way it would glow a different colour depending on which side of the circuit was bad. I've thought about replacing panel lamps with LEDs before but for the instruments the light is too directional to work well without a diffuser. The bulbs radiate all around whereas the LEDs have a narrow viewing angle in comparasion. But for the high beam and ignition light they would work great.

Just a thought. I've been fiddling with my fuel tank today and I think the petrol fumes are making me go funny :)

Simon
Simon Jansen

Geoff - the white from the warning light joins two others at the ignition switch, if you can get to that through the radio hole.

Simon - I wonder how many times that feature would be useful ... Given the current through the warning light primes the alternator maybe that through a LED wouldn't be enough.
Paul Hunt

Yep, didn't say it was a useful modification :)

Simon
Simon Jansen

I understand Paul Hunt's suggestion. But what i dont understand is why my ignition light comes on at about half after i shut the key off and remove it. the engine also stays running durring this. the only way i can shut the light and engine off is to manually trigger the electric fuel pump safty switch inder the dash. that aught to be puzzleing to figure out.
Thanks Brett
Brett

Brett - if your car has the anti-runon valve and ignition relay i.e. approx 77 on you almost certainly have a problem in the valve or its plumbing. Briefly, there is a wiring design error on these cars that means that it is only the anti-runon valve cutting off the fuel supply to the jets that stops the engine, the ignition continues to fire even when turned off. If a problem occurs with the valve or emissions plumbing it will no longer do it. Full story at http://www.mgb-stuff.org.uk/wn_fuelframe.htm and click on 'Running-on'.
Paul Hunt

Geoff, got a volt meter? With the engine runnning measure the voltage at both the battery leads and the ignition lead on the alternator. Should be 12 - 14V depending upon engine speed. Now measure the voltage at the ignition switch. Probably reads lower due to a bad connection in the switch or between the switch and battery. Tha lamp is glowing because the alternator is powering the system through the lamp.

Brett - do you have the correct lamp in the ignition indicator. Once the engine is running the alternator powers itself. Slight chance that a higer watage bulb in the indicator could poer the ignition. More probable is seperate source of power from a fault relay as Paul suggested.

Simon - Guarantee, a LED will no provide enough current to turn on the alternator.
Cheers one and all
Doug Gordon

Yes, Doug. I just purchased a new for Harbor Freight, made in China. The one for Sears had become so far out of calibration I was getting 34 volts out of the alternator!
I intend to trace through the system to see if I can follow 14 volts throught out.
Thanks to one and all.
Geoff

This thread was discussed between 21/02/2003 and 25/02/2003

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