Decided that it was time to bite the bullet and stop guessing. I have
reason to believe that the ignition and fuel lamp sockets in my 52TD are
still original. I wasn't about to remove the panel to remove the
sockets, so I isolated them schematically and made some measurements.
First, let me correct an earlier posting of mine. The bulb in my fuel
warning lamp is labeled "EDISWAN 2.5V .2A FNG", rather than the '2W'
that I reported earlier. I measured the resistance of the bulb at about
1.2 ohms. This is a 'cold' reading. The operating value for a 2.5V/.2A
bulb is about 12.5 ohms, thanks to Dr. Ohm.
The fuel warning lamp 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 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 our friend Dr. Ohm 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 between the green line that
goes to the A4 terminal and then probed the innards of the fuel lamp
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 spring. I then connected the meter to
the (disconnected) end of the cable at the fuel sending 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)
terminal. 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 right in the ballpark.
How about the Ignition Warning Light? The only printing on the bulb in
my lamp says "12V". The table on page N.18 of the Workshop Manual says
that the bulb should be a #987, 12V, 2.2W. As Jeszs pointed out (E^2/P)
will give us the bulb's operating resistance, in this case (12*12/2.2) ~
65 ohms.
Similar test setup. Note that the Ignition Warning Lamp 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 is V/I, or 12.22V/.108A ~ 113 ohms.
Disconnected the battery and turned the DMM back into an ohmmeter.
Learned that the yellow lead from the D terminal goes directly to the
center pin of the Ignition Lamp 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 impedance of 113 ohms
indicates a lamp resistance of about 43 ohms. That's probably about
right for this lamp.
So, I'd say that an unmolested, original, indicator 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 it's designed voltage at it's highest stress level. Using
non-resistor sockets and 12v bulbs can work fine. The Ignition Warning
Light is not a necessary component of the charging system. The Petrol
Warning SHOULD be bright (IMHO) so as not to be ignored. Also, please
note that the 12v bulbs are the same as the ones used for instrument
illumination
Gee, that was fun! Takes me back to pre-retirement. I spent about
forty years in the world of light standards.
Bud Krueger
52TD
Jeszs V. Benajes wrote:
>Hello Bob,
>
>The resistor to be connected in series depends on the value of the internal
>resistance of the bulb. But in the case of a 6 Volt bulb to be hooked at a
>12 Volt source, the theoretical answer is very easy: the resistor should
>have the same resistance as the bulb.
>
>The actual resistance of the bulb can be either measured or calculated:
>Internal resistance (in Ohms): nominal voltage squared (6V x 6V) divided by
>nominal power output (in Watts).
>
>Of course that is all theoretical, applying Ohm's law.
>
>Regards.
>
>Jesus
>
>53 TD
>
>
>-----Mensaje original-----
>De: owner-mg-t@autox.team.net [mailto:owner-mg-t@autox.team.net] En nombre
>de Bob Howard
>Enviado el: domingo, 28 de marzo de 2004 14:11
>Para: ddubois@sinclair.net
>CC: mg-t@autox.team.net
>Asunto: Re: switch winding (again)
>
>Dave,
> For my TD, I jumped one socket's mile of wire and bought a replacement
>socket for the other; use 12v bulbs in each. As you said, the 12v is
>mighty bright. The 12v bulb in the new socket, that has some resistance
>wire wound around it though I can't tell how much and didn't measure
>resistance, is dimmer though still visible night and day.
> How does one figure the resistance needed to use a bulb other than
>12v, for instance, using a more-common 6v in a socket that had its
>original mile of wire rust away?
>Bob
>
>
>On Sat, 27 Mar 2004 11:00:41 -0800 Dave & Liz DuBois
><ddubois@sinclair.net> writes:
>
>
>>Bud,
>>
>>The fuel warning lights were originally 2.5 volts and they had a
>>resistor in the for of wrapped nichrome wire wound around the
>>outside of the socket. I added an additional resistor on mine to cut
>>
>>
>down the
>
>
>>brightness of the light, which I found a pain at night. I don't see
>>any reason that the resistance winding couldn't be bypassed and a 12
>>volt bulb used in place of the 2.5 volt bulb. The 2.5 volt bulbs are
>>
>>
>a
>
>
>>bit hard to find, although Moss or Abingdon should carry them as they
>>are available, they just have to be special ordered.
>>
>>Good luck,
>>Dave
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