I have a bank of three gauges on my dash, oil temp and oil pressure and water temp.
Each gauge has a Pos+ and Neg- wire for the small illuminating light, and three other terminals for Pos+ and Neg- and S for signal. They are wired as follows:
They've worked fine for 6 years and two weeks ago the oil pressure gauge (and only that gauge) went off the scale suddenly - hard to the right side, which is over 100PSI. I stopped and checked everything but the engine seemed fine, but blipping the throttle had it pinging the needle over to the right like a mad thing.
The circuit has the Oil pressure gauge in the middle as shown above, with the water temp and oil temp on either side, all sharing a common earth and power supply. The other two gauges when wired like this both work perfectly, rising/falling as appropriate to the state of the engine, but the middle oil pressure gauge goes off the scale.
Things I have done so far:
I changed the sender (transducer) unit on the oil line for a new one - needle still goes off the scale.
I ran a new separate wire from the sender to the gauge (so not using the old signal wire at all) - and needle still deflects off the scale.
I then ran a separate power and earth wire direct from the battery to the gauge and the needle still went off the scale.
I assumed the actual gauge on the dash must be defective so fitted a new one. Needle still went off the scale exactly the same as the old one!
I was surprised to note that I can also get the needle to go off the scale WITHOUT the sender wire attached - so only a power and earth connection, which seemed rather strange. I tried this in the house with the old gauge and new one too - attach a wire from Pos and Neg to a battery and the needle goes off the scale.
Now the odd thing - if I leave the earth wire disconnected it makes the gauge work 'normally' with no needle deflection - I tried it inside the house with a couple of wires and one of the gauges and a transducer with the Pos and Signal terminals connected onto a 9V radio battery. So obviously the sender resistance determines the needle activity.
However with the whole plot fitted on the van the gauge wont work, not with old wiring nor with new wiring, not in its original state with the earth connected, nor with the earth disconnected. It obviously DID work with all the wires connected previously as its been like this for 6 years.
I cannot understand why the existing wiring setup has suddenly made the gauge go awol, and if its some strange earth problem why its only affecting one of the three gauges. Today I disconnected the earths going to chassis and gearbox and scrubbed them up and coated with vaseline and retightened them. But no use, I've still got the problem.
SO to recap:
attaching all the wires gets needle off the scale, with the gauge (new one and old one) on the van or off the van.
attaching only Pos and Signal wires on the gauge OFF the van gets it working (it appears) as normal.
attaching only Pos and Signal wires with gauge and transducer in their correct locations on the vehicle gets nothing.
Now the only variable in all of this that I am aware of that has made its presence felt recently is that I have developed a diesel injection pump leak - oil line and transducer & wiring is directly below the diesel pump and some diesel is getting onto the sender end of the system. Could this be the culprit? (Diesel pump is being replaced shortly)
I am perplexed!
(PS have had some good advice on a UK forum but its not led me anywhere near a solution!)
Each gauge has a Pos+ and Neg- wire for the small illuminating light, and three other terminals for Pos+ and Neg- and S for signal. They are wired as follows:
They've worked fine for 6 years and two weeks ago the oil pressure gauge (and only that gauge) went off the scale suddenly - hard to the right side, which is over 100PSI. I stopped and checked everything but the engine seemed fine, but blipping the throttle had it pinging the needle over to the right like a mad thing.
The circuit has the Oil pressure gauge in the middle as shown above, with the water temp and oil temp on either side, all sharing a common earth and power supply. The other two gauges when wired like this both work perfectly, rising/falling as appropriate to the state of the engine, but the middle oil pressure gauge goes off the scale.
Things I have done so far:
I changed the sender (transducer) unit on the oil line for a new one - needle still goes off the scale.
I ran a new separate wire from the sender to the gauge (so not using the old signal wire at all) - and needle still deflects off the scale.
I then ran a separate power and earth wire direct from the battery to the gauge and the needle still went off the scale.
I assumed the actual gauge on the dash must be defective so fitted a new one. Needle still went off the scale exactly the same as the old one!
I was surprised to note that I can also get the needle to go off the scale WITHOUT the sender wire attached - so only a power and earth connection, which seemed rather strange. I tried this in the house with the old gauge and new one too - attach a wire from Pos and Neg to a battery and the needle goes off the scale.
Now the odd thing - if I leave the earth wire disconnected it makes the gauge work 'normally' with no needle deflection - I tried it inside the house with a couple of wires and one of the gauges and a transducer with the Pos and Signal terminals connected onto a 9V radio battery. So obviously the sender resistance determines the needle activity.
However with the whole plot fitted on the van the gauge wont work, not with old wiring nor with new wiring, not in its original state with the earth connected, nor with the earth disconnected. It obviously DID work with all the wires connected previously as its been like this for 6 years.
I cannot understand why the existing wiring setup has suddenly made the gauge go awol, and if its some strange earth problem why its only affecting one of the three gauges. Today I disconnected the earths going to chassis and gearbox and scrubbed them up and coated with vaseline and retightened them. But no use, I've still got the problem.
SO to recap:
attaching all the wires gets needle off the scale, with the gauge (new one and old one) on the van or off the van.
attaching only Pos and Signal wires on the gauge OFF the van gets it working (it appears) as normal.
attaching only Pos and Signal wires with gauge and transducer in their correct locations on the vehicle gets nothing.
Now the only variable in all of this that I am aware of that has made its presence felt recently is that I have developed a diesel injection pump leak - oil line and transducer & wiring is directly below the diesel pump and some diesel is getting onto the sender end of the system. Could this be the culprit? (Diesel pump is being replaced shortly)
I am perplexed!
(PS have had some good advice on a UK forum but its not led me anywhere near a solution!)
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