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how the oil Pressure gauge and sensor works by Mike C


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#1 importwarrior

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Posted 17 June 2016 - 04:46 AM

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Mike C

The StarQuest oil pressure gauge works totally differently from that Miata setup.  The "slow response" is inherent to the design as well - nothing like the Miata's wad of grease to dampen the display.

Ever hear of a "bi-metallic strip?"  It's the same thing in old fashioned mechanical home thermostats - open one up and you'll see a coil of metal and a glass bulb attached to it.  That coil is really two different types of metal fused together (imagine gluing two sticks of Wrigley's gum to each other).  As we all know, metal expands and contracts with changes in temperature...but every metal expands and contracts at its own rate per degree of temperature change.  By fusing two different metals together, one wants to expand more than the other as that thermostat responds to temperature changes.  The only way each metal can expand as much as it wants to is if the strip bends a little... if they stayed flat, one metal would have to stretch more than it wants to as the other metal expands due to the temp change.

Inside the oil and coolant temp gauges on StarQuests you'll find bi-metallic strips wrapped in a special type of wire - basically a nichrome wire I believe.  (same stuff used to ignite model rocket motors)   Nichrome wire has a fair bit of resistance so it heats up when electric current flows through it.  Since it's wrapped around a bi-metallic strip, that heat makes the strip bend.  The end of the strip is attached to the gauge pointer to move the pointer.

Obviously heating stuff takes a little time - it can't be done instantly.  And when the electrical current is removed the bi-metallic strip will take a few seconds to cool down too.  Ergo the "slow response" that auto manufacturers like - it's less movement and thus less distraction to the average motorist.  The stock voltmeter is more like a d'Arsonval mechanism - i.e. the electric current through the voltmeter flows through a coil wrapped around a permanent magnet.  Current in that coil makes an electromagnetic field which reacts against the permanent maget's field leading to motion.  A plain old spring resists the motion... so more electrical current (from more voltage) makes a stronger electromagnetic field to pull harder agains the spring leading to a higher voltage indication.  No "thermal damping" involved which is why the StarQuest voltmeter responds quickly enough to show minor voltage fluctuations when the turn signals are ON.

The stock oil pressure sending unit is similar to the gauge by the way - another heating coil and bi-metalic strip.  Oil pressure pushes against a drum (just inside the largest diameter part of the sensor).  The center of the drum moves inwards (away from the oil inlet) due to oil pressure.  Part of the "guts" of the sensor are attached to the drum so they move in sync with oil pressure.  Under normal operation, electric current flows through the sensor (this current comes from the dash gauge and grounds through the sensor to the engine block) causing the coil to heat up and thus bend the bi-metallic strip.  The end of the strip is actually a small electric switch - when the switch bends enough the switch OPENS and cuts off the flow of electricity.  Thus the sensor starts cooling again.  The other contact of the switch is physically tied to the drum... so oil pressure affects how tightly the switch is closed.  Low oil pressure = switch isn't closed very tightly so only a little heat is needed to bend the bi-metallic strip enough to break the circuit.  High oil pressure pushes the switch closed hard - the bi-metallic strip has to heat up a lot hotter to move enough to open the circuit.  Folks comment that StarQuest oil pressure gauge readings vary a lot from car to car... one car might show about half-gauge pressure while another reads 75%... it's actually differences in oil pressure sensors.  The mechanism that attaches the bi-metallic strip to the rest of the sensor assembly actually has an adjustment (inside the body of the sensor... you can't reach it without major dis-assembly of the sensor) to adjust how firmly the switch contacts are closed when there is no oil pressure.  A sensor with a lot of "preload" on the switch will read higher oil pressures than one with little pre-load because it takes more time (more temperature accumulation) to bend the bi-metallic strip enough to overcome the preload.

The result of the StarQuest oil pressure sensor is basically an ON or OFF sequence with the ON time vs. OFF time varying due to oil pressure.  High oil pressure results in more ON time than low oil pressure... so higher oil pressure results in a longer time for electricity to actually flow through the sensor and thus lets more heat build up in the dash guage to move the needle further.  The ON and OFF times are anywhere from a few tenths of a second to several seconds - i.e. really slow compared to how rapidly oil pressure can change.  Very different from the aftermarket sensor mentioned for the Miata which outputs a varying resistance depending on oil pressure, similar to how the StarQuest fuel level sensor works.  So, to make a rapid response oil pressure display on a StarQuest one would have to replace both the sensor and the guage itself.

mike c.

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