TD Stuff by Bud
Ignition & Petrol Warning Lights
The TD's Ignition and Low Petrol
Warning Lamp 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.
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