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Oil Pressure Sending Unit: Converting from Light to Gauge

Discussion in 'Electrical and Interior' started by FL85GrandPrix, Mar 20, 2017.

  1. FL85GrandPrix

    FL85GrandPrix Apprentice

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    As title states, my '85 GP came from the factory with idiot lights. I am converting to the OEM option Rally gauges that actually show oil pressure and water temp. The water temp sender looks straight forward. It's on the driver side, forward part of the cylinder head. 1 green wire. Just swap it out with the gauge version. The oil sender is confusing me though. Currently it has a 2 wire switch on the back of the block near the distributor. the connector has 4 wires plugging into it. On one blade it looks like two black wires. The other blade has a blue and tan or maybe faded orange wire. The oil pressure senders "for gauges" only have 1 wire blade. So what's the trick here to convert my 2 blade, 4 wire idiot light sender to a 1 wire gauge sender? I was expecting a 1 wire sender, and found a 2 blade sender with 4 wires hooked to it.
     
  2. MrSony

    MrSony Royal Smart Person

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    If you have an electric choke, most likely one of those wires tee's off into the wire for the choke. GM used the signal from the oil pressure switch to operate the electric choke. No oil pressure, engine is off. Oil pressure, engine is on and choke is on.
     
  3. Texas82GP

    Texas82GP Comic Book Super Hero
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    Mr. Sony is correct. The hot to the choke is routed through the idiot light oil switch near the distributor (SBC) so the choke is only powered when there is oil pressure. If it were me, I would mount the sender for the gauge in the port near the oil filter (SBC). I would then identify the wire that provides the signal from the original oil pressure switch to the light. I would remove that wire from the connector at the original oil pressure switch, and extend it down to the new sender for the gauge. The pigtail for the sender may still be available. Check RockAuto.

    Here is a great resource for wiring diagrams:
    http://www.maliburacing.com/wiring.html
     
  4. FL85GrandPrix

    FL85GrandPrix Apprentice

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    Good info guys!! Thank you. My question is, how did cars come from the factory that had the oil press gauge? Wonder what the "OEM" setup was. Yes I have the electric choke. But being FL, not sure i would ever need that.
     
  5. Bonnewagon

    Bonnewagon Comic Book Super Hero

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    Both the oil and coolant senders are different for gauges than idiot lights. Idiot lights used a simple pressure/temp grounded circuit. Gauges used a variable resistance . Some got complicated like yours. Here is how it should be wired. This is from Jared's link. [​IMG]
     
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  6. Texas82GP

    Texas82GP Comic Book Super Hero
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    It's going to take a guy with an unmodified setup like ed1948 to comment on how they were setup stock. My car was a diesel, but it had a gauge. The wiring is very different from the gas engine harness though. I suspect that the stock SBC setup was as I described above, with the switch in the port near the distributor for the light and with the sender in the port near the oil filter for the gauge. Having the sender near the oil filter can be a problem for mechanical clutch linkage but since none of the cars past about 81 had manual transmissions, this isn't an issue. I overcame it with brass fittings, since I will be running mechanical clutch linkage.
    20170126_204225.jpg
     
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  7. FL85GrandPrix

    FL85GrandPrix Apprentice

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    Bonnewagon- Thank you for the drawing. Very helpful. Where is jared's link? It looks like this is the wiring for the "optional gauges". So there should be a page for the idiot lights. With both of those I would be good to go. Thank you all for the help.
     
  8. axisg

    axisg Royal Smart Person

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    OP. Its in the bottom of post 3 linking to maliburacing
    - 2 separate sensors with 2 separate pickup locations. The one behind the distributor is indeed linked in with the choke. 1 line in, 2 lines out to the light and the choke. The other is down by the Oil Filter as on Jareds picture above. Not all the blocks were machined for this, if yours is not then you need to tee off behind the distributor.

    Jared - please be cautious with those brass fittings. I had one let go years ago when I tee-d off behind the distributor and it broke making a huge mess and leaving me stranded. A couple of friends have mounted the sensor on the wheelwell tucked under the Master Cylinder using braided line with a 90deg fitting to clear the clutch linkage

    https://www.amsperformance.com/cart/AMS-Remote-Oil-Pressure-Sending-Unit-Line.html
    I am sure Jegs or Summit has something for much less dough
     
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  9. pontiacgp

    pontiacgp Rocket Powered Basset Hound

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    have no worries, Jared's engine will never start with heads that have no exhaust ports.....:D
     
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  10. Bonnewagon

    Bonnewagon Comic Book Super Hero

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    I could not find the diagram for the idiot light. But I do know that all it is, is 12 volts to an indicator bulb in the dash, and the bulb ground goes to the sender. 5 psi oil pressure opens the circuit and the light goes out. No pressure and the switch closes and the bulb lights. With the key in the "ON" position, motor not running, the bulb lights and that is known as "bulb test" mode. Hey Jared, is that a second gen Camaro I see hiding in the shadows there? NICE!
     
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