Roman...Oil Pressure



I too have had the instrument cluster apart, and the 60 gauge is the same,
heater wire around a bi-metallic strip. I was lent a gauge from a scrap
vehicle, and it was supposed to be working, but the wire had overheated at
some point and melted some of the plastic inside, the bi-metal strip bent
the wrong way and the needle was solid, but it still showed a healthy 65ohm
resistance of the coil. This is the book figure. The display side of the
gauge just has a H and L mark, a single arc between the two, and one
possible two division marks, all in white on black background. This system
too connects one pole to the battery , the other to the sender.
I had considered "re-calibrating" my gauge by carefully bending the strip,
to show a correct reading....but would I be just masking the problem of a
true low oil pressure situation? I have measured the oil pressure with a
capillary gauge and it is showing what I would consider to be a good to high
reading (80 psi cold on tickover, 60 when warm, and up to 100 at 2500rpm).
But why would the gauge, from working normally one day to suddenly showing a
lower reading, suddenly go out of calibration? I still have this nagging
doubt that the problem is internal mechanical. I have the new gauge
arriving soon, so hopefully this will solve and electrical problem.
As for the sender on the 60, there were two types, a smaller stud
(3/4inch)with a male spade terminal fitting, and a larger round unit about
inch and half across. This is the type I have. A new one arrived the other
day, with fitting instructions stating that it must be installed so that the
arrow marked on it and the drop of red paint are at the top. This unit
does not seem to work "variable resistance as oil pressure changes" system.
I have not tested it yet, but the Toyota FSM give the test procedure for
this type of sender as follows: Connect battery voltage through a 3.4 watt
bulb to the sender, as oil pressure rises, the bulb will flash, the speed of
flashing changing as oil pressure changes. SO it would seem that it is
pulsing current through the gauge heating element. Unfortunately the book
does not give any figures stating pulses per minute at stated oil pressures.
I am still waiting for delivery of the new gauge, it was ordered Friday of
last week, still not arrived, but hopefully this next week!!


On 6/4/05, Neil Paisnel <[Email address removed]> wrote:
g a
Sorry, Neil. You've lost me. You say the oil pressur has been measured
with another, trusted gauge and is within specs. So it is your gauge,
not oil pressure, that is the issue, isn't it? If so, recalibrating
the gauge is all you need, as it stopped showing the true pressure.
Why did it go wrong? There are quite a few of reasons, including
gremlins, but if you put in a new gauge and it works, your problem has
been solved. If not, it's back to square one - measuring oil pressure
with a calibrated gauge, then re-checking the sender.
The sender reacts to the rotation of the pump, which has six or eight
teeth (can't remember
correctly). When the bulb flickers it just shows that oil is being
pumped. The number of pulses is not essential (# of teeth times pump
timing gear ratio times engine revs). Because pumping action is not
linear, a heater elements is used to average the peaks and troughs.
It's the current generated by these pulses that makes the gauge work.
If there's additional resistance (other than the sender itself)
between the gauge and ground, the gauge will also read incorrectly.
Hope it helps. Please keep me posted.
Roman (London, UK)
'92 HDJ80
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