On 6/2/05, Neil Paisnel <[Email address removed]> wrote:
hat
Neil,
If the 60 has an oil pressure gauge like the 80, voltage is not an
accurate indicator of the health of the system, but better than
nothing.
I have my instrument cluster stripped to pieces on the workbench and
am investigating how the gauge works. Well, the gauge is a very crude
instrument based on a bimetallic strip with a length of wire wrapped
around it, acting as a heater element. Deflection movement is
translated from linear to rotary with a very short fulcrum. The
tolerances are very loose and would give different readings form gauge
to gauge. The heater element is also subject to variations in battery
voltage and ambient temperature.
The gauge's heather element is connected to the battery and the
pressure sender acts as a variable resistor connected to ground. When
oil pressure is low, sender resistance is high. Current flowing
through the heather element is low enough and does not cause
deflection of the bimetalic strip - the pointer is at the bottom of
the scale. Higher pressure lowers sender resistance, hence more
current flows through the gauge. That deflects the bimetalic strip
more and moves the pointer up the scale.
I noticed that recently the pointer in my gauge dropped lower than 0
when ignition key was off. When the engine was running it showed
pressure lower than normal. Readjusting the bimetalic strip rectified
the problem. The same may have happened in your case, too. The FSM
does not provide any current values allowing for gauge calibration, so
measuring actual pressure with a calibrated gauge is the only way to
make sure the oil pump is working properly.
And the reason why current rather than voltage is used to measure oil
pressure is possibly to even out pulsations in the system but still it
is a very rudimentary method of indicating pressure.
--
Rgds,
Roman (London, UK)
'92 HDJ80