delco1946 Posted August 16 Share Posted August 16 Why would my oil pressure gauge suddenly be reading low. Yes, I know it always reads low but now it’s reading lower than usual and is no longer responsive when I rev the engine. It also isn’t reading as high upon startup when cold. Any tips to diagnose this? It seems like either of the gauge is now malfunctioning although none of my other gauges are, or else it’s reading is accurate and which case something is wrong with my engine now. Link to comment Share on other sites More sharing options...
viven44 Posted August 16 Share Posted August 16 I have done a couple of quick tests in the past. I am pretty sure I learned it from discussion on the forum Connect the oil sending unit to the temperature sending unit wire. You can use an alligator clip wire... -- If the temp gauge behaves like the oil gauge, then your sending unit or something else needs work. -- If the temp gauge behaves fine like your oil gauge used to, the gauge needs work... ----- If you identify the gauge needing work, confirm by connecting the oil sending unit wire to ground. It should swing all the way past 'High'. If it doesn't REPLACE THE GAUGE as the factory manual might say. If you have actual oil pressure issues, you will hear it first I am really thinking you just have high contact resistance, so the gauge is 'lazy'... the wiring probably needs to be cleaned up ... sometimes those contacts can corrode on the post of the sending unit. I have always just made it a habit to plumb real gauges so I can accurately benchmark oil pressure right from the start... Link to comment Share on other sites More sharing options...
BigBrother-84 Posted August 17 Share Posted August 17 I have done a couple of quick tests in the past. I am pretty sure I learned it from discussion on the forum Connect the oil sending unit to the temperature sending unit wire. You can use an alligator clip wire... -- If the temp gauge behaves like the oil gauge, then your sending unit or something else needs work. -- If the temp gauge behaves fine like your oil gauge used to, the gauge needs work... ----- If you identify the gauge needing work, confirm by connecting the oil sending unit wire to ground. It should swing all the way past 'High'. If it doesn't REPLACE THE GAUGE as the factory manual might say. If you have actual oil pressure issues, you will hear it first I am really thinking you just have high contact resistance, so the gauge is 'lazy'... the wiring probably needs to be cleaned up ... sometimes those contacts can corrode on the post of the sending unit. I have always just made it a habit to plumb real gauges so I can accurately benchmark oil pressure right from the start... I recently noticed similar behavior for Big Brother's oil pressure gauge. Lower than usual at cold start, and lower-lower when hot. Never drops out of the "normal" range, but flirting with the lower line. I'll give a try to Vivek suggestion. Link to comment Share on other sites More sharing options...
viven44 Posted August 17 Share Posted August 17 I recently noticed similar behavior for Big Brother's oil pressure gauge. Lower than usual at cold start, and lower-lower when hot. Never drops out of the "normal" range, but flirting with the lower line. I'll give a try to Vivek suggestion. If you are going down this route to test the gauge, you can also use a resistor between the oil pressure sending unit wire and battery ground for a more precise calibration check Per the factory manual -- Use of 73 ohm resistor should put the gauge at L graduation mark -- Use of 22 ohm resistor should put the gauge at mid-scale I have one of these for random uses like this https://www.amazon.com/dp/B08FD1XVL6/?coliid=I1Y5TVN6EEZO8M&colid=3IGQF00LDI4AO&psc=1&ref_=cm_sw_r_cp_ud_lstpd_9G5AXQMXMTH3V7CEZBV6 Link to comment Share on other sites More sharing options...
85lebaront2 Posted August 17 Share Posted August 17 You may have what I experienced on a 390 FE engine in my 1977 F150. The cup plug that retains the relief valve spring, cracked, the tab on the pickup kept the remainder of the plug restrained, but the back side of the plug behind the spring is connected to the inlet side of the pump via a roughly 3/16" hole. Since the pump body is at or slightly above the max oil level, it sucks in air rather then just liquid oil. Link to comment Share on other sites More sharing options...
Gary Lewis Posted August 17 Share Posted August 17 If you are going down this route to test the gauge, you can also use a resistor between the oil pressure sending unit wire and battery ground for a more precise calibration check Per the factory manual -- Use of 73 ohm resistor should put the gauge at L graduation mark -- Use of 22 ohm resistor should put the gauge at mid-scale I have one of these for random uses like this https://www.amazon.com/dp/B08FD1XVL6/?coliid=I1Y5TVN6EEZO8M&colid=3IGQF00LDI4AO&psc=1&ref_=cm_sw_r_cp_ud_lstpd_9G5AXQMXMTH3V7CEZBV6 Just a word of warning, you'll burn the resistor up very quickly on the mid-scale test, and it'll get quite hot on the L test. The formula for current is I = E/R and for wattage is I*I*R, meaning current squared times resistance. Here are some specs: The chopper/ICVR puts out roughly 6v And our page on Documentation/Electrical/Gauges says "... each has roughly 12 ohms of resistance." So if you calculate the current to put the scale at the L mark: 5V/(73+12) = .06 amp. And power dissipated in the resistor will be .06 x .06 x 73 = .26 W, which is all that resistor is rated for. But to go mid-scale the current will be: 5V/(22+12) = .15 amp. So power dissipated in the resistor is .15 x .15 x 22 = .5 watt, which is twice what that resistor is rated for. And since you are working with heat to move the gauge (they are actually thermometers with a heating element below them) you'll need to run the circuit for several seconds to heat the gauge up. And the resistors will get very hot - if not burn up. DAHIK. Link to comment Share on other sites More sharing options...
viven44 Posted August 17 Share Posted August 17 If you are going down this route to test the gauge, you can also use a resistor between the oil pressure sending unit wire and battery ground for a more precise calibration check Per the factory manual -- Use of 73 ohm resistor should put the gauge at L graduation mark -- Use of 22 ohm resistor should put the gauge at mid-scale I have one of these for random uses like this https://www.amazon.com/dp/B08FD1XVL6/?coliid=I1Y5TVN6EEZO8M&colid=3IGQF00LDI4AO&psc=1&ref_=cm_sw_r_cp_ud_lstpd_9G5AXQMXMTH3V7CEZBV6 Just a word of warning, you'll burn the resistor up very quickly on the mid-scale test, and it'll get quite hot on the L test. The formula for current is I = E/R and for wattage is I*I*R, meaning current squared times resistance. Here are some specs: The chopper/ICVR puts out roughly 6v And our page on Documentation/Electrical/Gauges says "... each has roughly 12 ohms of resistance." So if you calculate the current to put the scale at the L mark: 5V/(73+12) = .06 amp. And power dissipated in the resistor will be .06 x .06 x 73 = .26 W, which is all that resistor is rated for. But to go mid-scale the current will be: 5V/(22+12) = .15 amp. So power dissipated in the resistor is .15 x .15 x 22 = .5 watt, which is twice what that resistor is rated for. And since you are working with heat to move the gauge (they are actually thermometers with a heating element below them) you'll need to run the circuit for several seconds to heat the gauge up. And the resistors will get very hot - if not burn up. DAHIK. Thanks for those calculations Gary. I better have a 0.5W or 1W set handy. Temp rise also leads to increase in resistance itself so affects validity of results so I agree. The right tool for the job! I have used it for fuel sending unit approximation before, I assume the fuel gauge has similar challenges in using a resistor? Link to comment Share on other sites More sharing options...
Gary Lewis Posted August 17 Share Posted August 17 Thanks for those calculations Gary. I better have a 0.5W or 1W set handy. Temp rise also leads to increase in resistance itself so affects validity of results so I agree. The right tool for the job! I have used it for fuel sending unit approximation before, I assume the fuel gauge has similar challenges in using a resistor? Full scale takes 10 - 12 ohms and the current is 5/(12+12) = .21 amps. So power is .21 x .21 x 12 = .53 watts. So if you are going to have the ability to measure full scale you may want 1 watt resistors. Link to comment Share on other sites More sharing options...
viven44 Posted August 17 Share Posted August 17 Full scale takes 10 - 12 ohms and the current is 5/(12+12) = .21 amps. So power is .21 x .21 x 12 = .53 watts. So if you are going to have the ability to measure full scale you may want 1 watt resistors. I have used it for fuel sending unit approximation before, I assume the fuel gauge has similar challenges in using a resistor? Link to comment Share on other sites More sharing options...
Gary Lewis Posted August 17 Share Posted August 17 I have used it for fuel sending unit approximation before, I assume the fuel gauge has similar challenges in using a resistor? All of the gauges, save for the ammeter, are the same. You may have gotten by with it, but when I tested to create that writeup on the gauges I smoked a few resistors. I probably ought to rewrite that page to include those calculations so people will understand the amount of power they'll be playing with. But right now I'm writing up a 3D page... Link to comment Share on other sites More sharing options...
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