Testing resistances in oil press sending units
04-17-04, 09:35 PM
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Testing resistances in oil press sending units
I set up an air pressure source with a regulator & gauge, and connected a used 1/2 gen oil pressure sending unit.
I then measured resistance at various pressures-and got a surprise!
PSI--------Ohms
0----------Infinite
20-----------40
30-----------39
40-----------38
50-----------38
60-----------39
70-----------38
Seems to me that this sender would read no pressure until something below 20PSI, but would read the same pressure thereafter?
Does anyone have the resistance specs for a sending unit?
Am I missing the boat by using air instead of oil?
I then measured resistance at various pressures-and got a surprise!
PSI--------Ohms
0----------Infinite
20-----------40
30-----------39
40-----------38
50-----------38
60-----------39
70-----------38
Seems to me that this sender would read no pressure until something below 20PSI, but would read the same pressure thereafter?
Does anyone have the resistance specs for a sending unit?
Am I missing the boat by using air instead of oil?
11-05-06, 06:31 PM
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Yeah, I did the same test my readings were a little different 42.8 ohms regardless of pressure over 25 psi or thereabouts. Using air, oil water makes no difference to the gauge. The fluid simply exerts pressure against a diaphragm that moves a lever inside the sending unit.
So curious as I am I dissassembled the oil pressure sending unit. It basically is a pressure switch. At lower than a certain pressure a resistor is in line with the sender feed voltage. this causes the gauge to read a nominal pressure. When the pressure reaches a certain level there are contacts that connect and allow the gauge to ground out and read a higher level. The position at which the switch makes contact is adjustable once the iol pressure sending unit is dissassembled. that is why the reading does not change with the pressure change.
Since the gauge in the dash has a thermal actuator (heating wire around a bi-metallic strip) when the motor slows or speeds up the needle position changes very slowly as the thermal actuator needs time to warm up and cool down. So the impression is that you are actually seeing a reading of the changing oil pressure with the changing RPM but that is not the case.
This has been a puzzle for several weeks for me as i have been trying to establish accurately what my oil pressure is. I installed a old Smiths mechanical gauge from my old CooperS and found I had 75 psi at 3000 rpm and around 48 at 1200 rpm. I was actually able to read about 5-6 psi on cranking rpm with this gauge.
I am trying to keep my car as original as possible but i would like to know precisely what my oil pressure leally is. So I will install a Brass T fitting into the oil pressure sending unit hole, mount the OEM sender in one branch of the T so my factory gauge continues to "function" as intended and install my mechanical Smith gauge in the other branch so I know what is really going on.
So curious as I am I dissassembled the oil pressure sending unit. It basically is a pressure switch. At lower than a certain pressure a resistor is in line with the sender feed voltage. this causes the gauge to read a nominal pressure. When the pressure reaches a certain level there are contacts that connect and allow the gauge to ground out and read a higher level. The position at which the switch makes contact is adjustable once the iol pressure sending unit is dissassembled. that is why the reading does not change with the pressure change.
Since the gauge in the dash has a thermal actuator (heating wire around a bi-metallic strip) when the motor slows or speeds up the needle position changes very slowly as the thermal actuator needs time to warm up and cool down. So the impression is that you are actually seeing a reading of the changing oil pressure with the changing RPM but that is not the case.
This has been a puzzle for several weeks for me as i have been trying to establish accurately what my oil pressure is. I installed a old Smiths mechanical gauge from my old CooperS and found I had 75 psi at 3000 rpm and around 48 at 1200 rpm. I was actually able to read about 5-6 psi on cranking rpm with this gauge.
I am trying to keep my car as original as possible but i would like to know precisely what my oil pressure leally is. So I will install a Brass T fitting into the oil pressure sending unit hole, mount the OEM sender in one branch of the T so my factory gauge continues to "function" as intended and install my mechanical Smith gauge in the other branch so I know what is really going on.
