Test it. A voltmeter on the light-green/green wire from the stabiliser 'I'
terminal to the fuel and temp gauges should show 12v when you first switch the
ignition on for several seconds, then start pulsing on and off about once per
second. However unless the voltage is reading less than 12v when it's on then
it's going to be tricky to check that the on and off periods are averaging
about 10v as they should, and not something significantly lower. If you are
inclined you can make a test rig consisting of a 10uF capacitor, a 100k
resistor, and a diode. Connect them in series so that current can flow from
the light-green/green wire (I took mine off the fuel gauge which is the only
stabilised gauge) through the diode and the resistor to the capacitor, which
is connected to earth on the car. Observe polarity on the capacitor if it is
marked with + or -. Connect your meter across the capacitor and turn on the
ignition. The voltage should slowly rise, and it may reach battery voltage
before the stabiliser starts pulsing on and off. Once it starts pulsing, the
voltage will slowly settle at the average, which should be about 10v (mine
measures 9.5v). The component values aren't critical, the smaller the
capacitor the quicker the rising and settling times will be, but it will waver
more about the average rather than give steady reading. Ditto the resistor.
The diode is to stop the capacitor discharging back through the stabiliser
heater winding, and without it the meter reading will waver more. I've been
wondering about a test rig for years, and your question was the prompt to send
me out to the garage and have a go.
The black wires on those gauges are only for the night-time illumination,
nothing to do with their operation. The sender should have a black earth wire
from the uninsulated terminal on the sender to the earthing point in the boot
on one of the number plate fixing bolts, which also serves the fuel pump and
reversing lights. However with both gauges suddenly reading low I doubt it is
that.
PaulH.
----- Original Message -----
So I am not sure the next step to figure out this annoyance.
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