Welcome to our resource for MG Car Information.



MG parts spares and accessories are available for MG T Series (TA, MG TB, MG TC, MG TD, MG TF), Magnette, MGA, Twin cam, MGB, MGBGT, MGC, MGC GT, MG Midget, Sprite and other MG models from British car spares company LBCarCo.

MG MGB Technical - Ignition lamp glow--dynamo--con't

As some of you know, I've been experiencing an ever slight glow from my warning light when any two or three appliances were on. After checking generator function, replacing voltage regulator and cleaning all "white circuit" connectors, the problem was still ongoing. It was the ignition switch. Finally crawed under the dash and removed it and hot wired the car. No glow whatsoever!! Even impressed my son that I could hot wire a car! I have a used switch in much better condition and it is temporarily installed. Changing the lock barrel was a little tricky but figured that out after a while--getting my original barrel in the new switch was a bit harder as the retaining pin would not receede by hand pressure. The solution--a good whack with a rubber malllet!

Here's the catch--with the replacement switch installed, and all the appliances on, the glow is back!! But it is reduced from the trace amount of glow I had prior by about 75%. So now, it's very, very dim. I'm ready to call it fixed but I wonder if I could ask a favor for my peace of mind:

Will those of you with generator cars pull the ignition light out of the tach, start the car, turn on wipers, fan, turn signals and step on the brake lights. Do you see any glow (even the most remote, faint orange) of the filament in the bulb. RPM's at 2500 or so. The degree of glow I'm looking for can not be seen through the red jewel--the lamp must be out so you can see the filament.


Paul Hanley

If I get time between replacing the thrust washers in the back axle tomorrow I will do this , may just measure the volts between WL and the SW on the coil and also to the brown on the starter solenoid. Thats if I am not up to getting under the dash .
S Best

A much useful measurement than each person's assessment of glow is to measure the voltage between the brown/yellow at the dynamo or alternator and the white at the connectors by the steering column. It is this voltage that causes the warning lamp to glow, and ideally it should be 0v i.e. no glow, both sides being at about 13v to 14v. With the white lower than the brown/yellow one or more bad connections back through the connectors - white - ignition switch - brown - solenoid - alternator or dynamo control box is indicated. Testing at these points also will show where. With the brown/yellow lower than the white on alternator equipped cars an alternator problem is indicated, particularly if it also lower than the brown at the alternator. The lamp will also glow if the alternator output is very low due to a fault.

In 35 years of driving I have never been aware of a glow from the warning light, on dynamo or alternator equipped cars, at any time including night with lots of load on. It is inevitable that there should be *some* voltage across the lamp, but if it is so dim that you cannot see it through the red lens in darkness then IMHO you are chasing something that doesn't need fixing.
Paul Hunt

Bad news I'm afraid, once the cut out goes in , the ign lamp on my car is completly off .I was inside a garage (door open of course ) and pulled the lamp out and shielded it with my hand . As the cut out pulled in it just went out and stayed out unitil I let the revs back down .
S Best

Thanks for the test, Mr. Best. Appreciate it! Paul, I think you're right. There is no real problem here. My concerns are for the upcomming driving season when I will undertake a 6000+ mile journey through deserts and over 11,000 foot altitudes. No time for a breakdown, eh? Especially since my route will take me to parts where I'm a lower member of the food chain!! (lions and tigers and bears, oh my!) Just going through all the systems with a fine comb before putting her to bed for the winter. As long as I can run headlamps, wipers and fan with no glow, I feel I'm good to go which is where I am now--only with the addition of brakes and turn signals will the filament turn orange. Can't imagine too many driving situation where I'd need everything running for so long as to discharge the batteries.

Thanks again for the hand holding!

Paul Hanley

I think your results of glow v load should reassure you . It is a fact that a dynamo cannot sustain all loads and recharge the battery at the same time , we had a demostration of that in Denmark last year when a TD in the party had to be run with the headlights off as it just ran out of battery ampere hours. FWIW our B is of an age that prevented it claiming this relief , however I had no problems running 100% of the time with headlights on ( although not much traffic helps here , no time idling with the ign light on ).If without much load the light is out you are in good shape .
S Best

The soft glow of the warning light is not necessarily an indication that the battery is not being charged, only that there is a volt-drop, probably due to current, between the two sensing points. However if the lamp glows brightly it probably *does* mean the battery isn't being charged, and that is its function :o)

As long as the voltage at the battery is at least 13v it is charging. If you are really concerned a voltmeter should be confirmation of charge or lack of it.
Paul Hunt

The best reassurace you can give yourself is to drive the car during the winter. If there are any weaknesses they are most likely to show up then. If you can put a reasonable number of miles on with no problems you are more likely to feel confident at tackling your long journey.
ian thomson

This thread was discussed between 26/11/2004 and 03/12/2004

MG MGB Technical index

This thread is from the archive. The Live MG MGB Technical BBS is active now.