[Mgs] Fuel smell in boot/trunk

[PaulHunt73]

The MGA and MGB Jaeger gauges use a completely different method of voltage 
regulation to the MGB Smiths gauges, and the system voltage can vary 
significantly on both types - albeit a little less with alternators. 
Effectively the earlier system uses a Wheatstone Bridge, which is where with 
three known resistances you can arrange for a voltmeter to display the 
resistance of a fourth unknown resistance, and this is independent of supply 
voltage.  The three known resistances are in the fuel gauge (two being 
magnetic coils), and the unknown is the tank sender of course.

MGB Smiths gauges use a thermal voltage stabiliser which outputs an average 
of 10v over time regardless of system voltage, and thermal gauges, both 
using heating coils wrapped round bi-metal strips.  Despite being called a 
'stabiliser' the factory stabiliser is actually switching system voltage on 
and off about once per second, once the ignition has been on for a few 
seconds.  At high system voltages you get a relatively short 'on' time 
compared to the 'off' time, and at low system voltage you get a longer 'on' 
time, always averaging about 10v.  The tank sender simply varies the current 
through the gauge heating coils.  Because the gauges are thermal they are 
very slow acting which means unless you look very carefully you can't see 
the pointer moving as the voltage goes on and off.

PaulH.


----- Original Message ----- 
> From looking at the wiring diagram of the MGA I see that it doesn't have a 
> voltage stabilizer to feed the gas gauge & sender;  it depends on the 
> output voltage (V) of a terminal on the control box.