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Discussion Starter · #1 · (Edited)
OK, this one had me stumped for a while. Thought I understood the theops (theory of operation) of this circuit, but evidently missed one small detail. :eek: Should've known better but, after tearing gauge apart and checking it electrically, I couldn't find anything wrong with it. :eek:

READ THIS!! It may save you some time!

Symptom: OP gauge pegs pointer as soon as ignition key is turned on.

Cause: OP sender G (gauge) terminal is OPEN (infinite ohms to ground).

Background: OP gauge has two coils inside it arranged in a "V" at 90 degrees with the pointer pivot in the center of the "V". The coils are connected together electrically at the center of the "V" and this common point is also connected to the "G" terminal (variable resistance) of the OP sender. The other end of the "zero" coil is the +12V terminal, while the other end of the "max psi" coil is connected to GND.

With no oil pressure, the OP sender G terminal is at minimum resistance (~10 ohms) and the majority of the current flows through the "zero" coil and the OP sender to ground, which keeps the pointer at zero when the ignition key is turned on.

Once the engine is started and oil pressure builds, the resistance of the OP sender G terminal increases in direct proportion to the oil pressure, causing more current to flow through the "max" coil, deflecting the needle to the right. VOILA, you're reading oil pressure!

HOWEVER, if the OP sender's resistance exceeds ~180 ohms (5 BAR on gauge), ALL current flows through the "max" coil and the pointer pegs. There HAS TO BE some resistance (~10-180 ohms) on the OP gauge's G terminal, else it will peg the pointer.
 

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Not long ago I set up a small little test station for just that same curcuit. I was seeing what looked to be very odd readings at the guage so instead of changing guages and wiring and sensors in the car I just redid it outside the car. In doing this I found some very odd things but also confirmed some oiling issues. One thing I found was that if you ever crossed the 2 wires coming from the sensor you would see the pegged guage otto speak of before starting it and then would go to nothing when it was fired up. You would also see the idiot light dim with increased pressure. This was hard to see in the car but easy with it in my hands. It seems there are two switching types in the sensor, one that is more on off and another that is variable resistance based on oil pressure. I don't think I ever posted anything about my idiot light getting dim.
 

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from my personal findings, when the oil pressure guage pegs out when you turn the key on, its usually a busted wire between the sending unit and the guage, and sometimes the sending unit itself. i'm thinking one wire changes its voltage, power, or what not to operate the needle in the guage, and the other wire just activates the idiot light for "no pressure" or "any pressure"
 

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i'm trying to get my engine running on blocks at my school right now and i was going to prime the engine by just turning it over with the starter since the ignition and carb aren't hooked up yet, but when i took out the oil pressure sending unit it was a different size from the gauge he had and he didn't have an adapter to make it fit, so i think i'm going to grab the guage from my dash and use that instead, but i can't figure out which wire on the oil pressure sending unit is ground, the spade connector or the screw down type terminal?

i think thats all i need to know to hook it up, unless there is more than i think from the guage, all i think it needs is the +12V and the 2 wires from the sending unit, or is there something else?
 

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Brendan, the two wires from the oil pressure sensor go to the gauge in the dash. One is for the lite in the gauge, the other is to make the needle move in the gauge. The sensor is grounded when it is screwed in the side of the block. What makes it a bit testy, to know which wire goes where on the sensor, is, both wires are the same color. There was a quick and dirty check somewhere recently to help hook up those wires at the sensor. I believe it was, if the wires are hooked up wrong, with the key on, the gauge would peg out and the lite would not be on. Do a search, you may find it. HTH.
 

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alright, as long as i know that it doesn't do damage if they are hooked up backwards then i can just guess and change them if its wrong, thanks
 
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