Ttalk Tech
 TD Stuff by Bud
Ignition & Petrol Warning Lights

The TD's Ignition and Low Petrol Warning lamps are certain to come up quite often on TD forums.  The reason for this is due to the existence of some resistance wire wrapped around the sockets of original TD indicators.  This is further brought about by the use of a variety of bulbs in these sockets, e.g., 12 volt bulbs and 2.5 volt bulbs.  Hopefully, this page will help to answer some of the questions.  The approach that I have taken is to make a series of measurements on my 1952 TD.  A digital multimeter (DMM) and Ohm's Law provide virtually all of the answers.  For those who may not be familiar with it, Ohm's Law is the vary basic relationship of voltage, current and resistance in electrical circuits, V=I*R. When dealing with power (watts) it is common to see P (watts)=V (volts) * I (amps), or P=V^2/R, or P=I^2*R.

I have reason to believe that the ignition and fuel lamp sockets in my 52TD are still original.  The wiring in car was in very poor shape when I began the restoration in 1988, but it appeared that none of the components on the instrument panel had ever been replaced.  The only components that I had to replace were the Inspection Sockets.  When I removed the components from the panel in preparation for repainting it I noted that the Ignition and Petrol Light sockets were covered with a shiny black wrapper  of some sort.   My 'restoration' philosophy has been to retain functional components whenever possible, so the sockets, lenses and bulbs were reinstalled just as they were found, even though the green lens has faded almost to being transparent.

The bulb in my Petrol Warning Light is labeled "EDISWAN 2.5V .2A FNG".  I measured the resistance of the bulb at about 1.2 ohms.  This is a 'cold' reading.  From ohm's law, the operating value for a 2.5V/.2A bulb is about 12.5 ohms (R=V/I).  The bulb in the Ignition Warning Light merely says '12V'.

Wiring descriptions and terminology used in the following discussion are taken from wiring diagrams found in Section N of The M.G. Midget (Series "TD) and (Series "TF") Workshop Manual, particularly the diagram on page N.23.

The Petrol Warning Light is wired between the A4 terminal of the fuse box and the Petrol Tank Unit where it is closed to earth when the petrol gets low.  I removed the appropriate green wire from the A4 terminal and reconnected it through a digital ammeter.  I then went under the car and removed the green w/black wire from the sending unit at the tank and connected it to ground.  The reading on the ammeter was 152mA, i.e., .152 A.  The battery voltage was 12.22 volts, so ohm's law says that the circuit resistance is about 12.22/.152 ~ 80.4 ohms.

Next step was to disconnect the battery and convert the DMM into a digital ohmmeter.  I connected the meter to the green line that connects to the A4 terminal and then probed the innards of the Petrol Warning Light socket.  There was continuity (0 ohms) between the green wire and the center terminal of the lamp socket and no connection to the outer terminal of the socket, i.e., the internal  spring.  I then connected the meter to the (disconnected) end of the cable at the Petrol Tank Unit.  Probing the lamp socket showed no connection to the inner terminal of the socket, but a measurement of 70.2 ohms was found to the outer (spring) bulb contact.  So, the resistance of the socket wiring is about 70 ohms.  Add another 10 ohms for the warm light bulb and the operating current of about 150mA is just about what one might expect.

How about the Ignition Warning Light?  The only printing on the bulb in my Ignition Warning Light says "12V".  The table on page N.18 of the Workshop Manual says that the bulb should be a #987, 12V, 2.2W.  Applying ohm's law (R=E2/P) will give us the bulb's operating resistance, in this case (12*12/2.2) ~ 65 ohms.

A similar test setup was used.  Please note that the Ignition Warning Light is connected between the A3 terminal of the fusebox and the 'D' terminal of the Control Box, the smaller of the two yellow wires.  I removed the yellow wire from terminal 'D' and connected it through the DMM (as an ammeter) and then turned on the key (after reconnecting the battery).  The reading on the ammeter was 108 mA (.108A).  Again, ohm's law says that the circuit resistance, R= V/I, or 12.22V/.108A ~ 113 ohms.

The battery was then disconnected and the DMM was converted into an ohmmeter.  The yellow lead from the D terminal  was found to go directly to the center pin of the Ignition Warning Light socket (bulb removed).  I then measured the resistance from terminal A3 of the fusebox to the outer (spring) portion of the socket and found 70.5 ohms!  Sounds familiar.

Subtracting this 70.5 ohms from the circuit resistance of 113 ohms indicates a lamp resistance of about 43 ohms.  This would mean that the lamp is operating at about .5 watts, since P=I*I*R, (.108*.108*43~.5).

So, it seems that an unmolested, original, warning lamp socket probably has about 70 ohms of resistance wire wrapped around it.  The Ignition Warning Lamp should last forever since it's operating at less than half of its designed voltage at its highest stress level.  Using non-resistor sockets and 12v bulbs can work fine, but will be much brighter (and hotter).  Unlike some later MG models, the Ignition Warning Light is not a necessary component of the charging system in a TD.  There is also an indication that the 12v warning bulbs are the same as the ones used for instrument illumination.  To check this out I took one of my instrument illumination bulbs, labeled "12V 2.2W" and repeated the current draw tests with this bulb.  The current through the Petrol Warning Light dropped from 145 mA to 100.2 mA.  There was a noticeable decrease in brightness.  The bulb was then swapped in the Ignition Warning Light and the current was seen to drop from 108.8mA to 100.6mA.  Again, there was a noticeable decrease in brightness, but it was still quite useable.  If one wants to emulate the wired socket with an unwired one and a discreet resistor it would just be a matter of obtaining a suitable resistor and wiring it in.  The actual value of the resistor isn't critical.  Standard resistor values in the neighborhood are 68 ohms and 75 ohms.  But, just make certain that it's rating is at least 2 watts.

BTW, the Brown & Gammons catalog lists a bulb #GLB987 for both the warning lamps and "dash & instrument illumination". Think I'll start looking for some miniature screw base 2.5 volt lamps.  The G.E. Lighting Catalog lists a #14 bulb specified as a 2.5v, 1 watt, miniature screw base, G-3.5 lamp for 2 D-cell flashlights.  It sounds good, but it doesn't work.  I bought a package of #14 bulbs from Radio Shack and tried one in my Ignition Light.  It was barely visible.  The bulb package says that it's a 300mA lamp.  I'm looking for a source of 2.5v, .2A, G-3.5, miniature screw base bulbs.

A Unipart package, labeled 'Rover recommended', Auto Bulb, GLB 987 lists the bulb as 12 V 2.2W.  The bulbs sold by Abingdon Spares are called '987'.
Front and rear images on new Ignition Warning Lamp (note the resistance wire around the socket)
Images courtesy of Ethan Lyman:

For lamp socket mounting information see WarningLamps.htm

last edited May 4, 2004