MG-Cars.net

Welcome to our resource for MG Car Information.

Recommendations

Parts

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 TD TF 1500 - Ignition warning light on MGTD

Why is there a resistance winding in series with the Ig warning lamp, and why is it a winding rather than a solid resistor? My guess is that the resistance functions in place of a fuse, in that should an inadvertant short occur across the bulb contacts, wires would burn without something to limit the current , and the 70 ohms of resistance would limit short circuit current to less that 0.2 amps. I doubt it had to do with the bulb rating as it would be cheaper to use a 12 volt bulb rather than a 2 volt bulb and resistor, and it would be cheaper and more perminent to use a resistor rather than a fuse, although additional cost and complexity would be involved with proper mounting of a separate resistor. Maybe the socket and its resistive winding originated on some other application and MG simply adapted it. This has always been an unanswered question and I wonder if anyone out there really knows the answer?
Bill Cole
Bill Cole

Bill - You have to look at things in context of the day in which these cars were built, plus the fact that the MG factory never threw thing out - instead they used them until they were all used up and then went to components of the day (that day being 1950 - 1953 in the case of the TD). Those light sockets with the resistance windings on them go clear back to the 30s when resistors where very large, regardless of power ratings. I would guess that in those days, a 2 volt bulb in series with 70 ohms of resistance wire made sense to the builders, particularly when you consider that at 0.2 amps across 70 ohms produces 2.8 watts of power - a rather large resistor, even today. The bulb rating did in fact, change during the manufacture of the TD and they started using 12 volt bulbs. I have substituted resistors on some of the sockets in our car and even increased the series resistance in the case of the fuel warning light s it wouldn't be so bright as to be distracting at night.

Yes, they did some strange things when building our cars, or any cars of that vintage. I would suggest that owners don't get themselves too wrapped up in trying to figure out why they did things so awkwardly in our estimation, with the advantage of today's technological knowledge (for instance - why did they use points that burn out over time to trigger the fuel pump, when a Hall effect or optical circuit would have been a much better choice?). Enjoy the car and all of it's quirkiness. Cheers - Dave
David DuBois

"Enjoy the car and all of it's quirkiness"

Hear, Hear. I couldn't have said it better.

warmly,
dave
Dave Braun

In conjunction with the Daves' answers, the light unit/resistor is self-contained. I agree that the period separate resistor would be a big bulky thing that had to go somewhere in line.
George Butz

Dave D. -- Remember that the fuel pumps were designed in the 1930's. They didn't have any thing like Hall cell's (that were practical) and photosensitive transistors then. Contacts were all there was!

These other methods of turning the coil ON did not start coming until after the transistor was revealed in the early 1950's.

Cheers,

Bob
bobj50

Thanks to Dave D, Dave B, George and Bob who responded,and its not an issue of getting too wrapped up in things, but mearly trying to answer a question I have been puzzled by for years. So, accepting the idea that a 2.8 watt resistor probably was too big to hang somewhere, why did they use a resistor at all. Were the 12 volt bulbs of the day too bright or were they concerned about burning up the wiring should a short occur in place of the bulb (someone shorting a test probe across the contacts)? The resistance would protact against that.
Bill
Bill Cole

'or were they concerned about burning up the wiring should a short occur'
You know....looking at the back of the dashboard (as I have been for two nights), I somehow don't think that was a concern....?
gblawson(gordon)

Gordon.
Rather than laying upside down just remove the steering column mounting bolt-lower the column to the seat.Remove the six screws holding the dash panel . I use an old blanket to rest the panel on and then you can work out in the open. The wiring is normally long enough to do this without stretching or pulling wires.
Sandy
conrad sanders

Sounds like a good idea... more then I want to do for just the bulb, but I would like to get back there and straighten out some of the 'interwoven' wires... quite a bit has been done over the years and I would like to wrap a few and sort out some others!
gblawson(gordon)

Now I find myself missing the instrumentation of my old light measurements laboratory. I'm not that apt to jump on the Abingdon folks as just 'using up parts'. I'd love to be able to measure the light output of the 2.5 volt lamp at various currents. The function of the ignition warning lamp is to give an indication of the difference between the output voltage of the generator and the voltage of the battery. It's not just to show you that the ignition switch is turned on.
Bud Krueger

Dave D. -- Remember that the fuel pumps were designed in the 1930's. They didn't have any thing like Hall cell's (that were practical) and photosensitive transistors then. Contacts were all there was!

These other methods of turning the coil ON did not start coming until after the transistor was revealed in the early 1950's.

Cheers,

Bob
R. K. Jeffers

This thread was discussed between 04/10/2008 and 06/10/2008

MG TD TF 1500 index

This thread is from the archive. The Live MG TD TF 1500 BBS is active now.