electric oil pressure gauge

[Marty Levenson]

When i bought a rebuilt A4 a few years ago it came with an electric SW oil pressure gauge and a sensor. Tried it out recently and it didn't work...stayed at zero. Wondering how to figure out which is broken: meter or sensor?

I have a perfectly good direct read oil meter, so thinking I will sell whichever piece works.

Thanks!
Marty

[joe_db]

A standard oil pressure sensor is 240 ohms at 0 pressure and 33 ohms at full pressure. Use an ohmmeter to see if the sensor is good.

[Al Schober]

I measured resistances of my oil pressure set from AC Spark Plug Div. Gage unit has three terminals, and an ammeter should show continuity between all 3 pairs. Resistance varies from 85 to 180 ohms depending on the pair. The sender should show a low resistance continuity at zero pressure, increasing resistance with pressure.
In my experience, it's usually the sender that goes bad. It has moving parts, including a wiper that moves across a wound coil. Dirt under the wiper will cause it to read open, while a worn spot on the wound coil will cause erratic readings.

[joe_db]

That sounds "non-standard". USA standard one terminal senders are 240-33 ohms connector to ground and more pressure/temperature/fuel equals less resistance = more needle movement.

Originally Posted by Al Schober

I measured resistances of my oil pressure set from AC Spark Plug Div. Gage unit has three terminals, and an ammeter should show continuity between all 3 pairs. Resistance varies from 85 to 180 ohms depending on the pair. The sender should show a low resistance continuity at zero pressure, increasing resistance with pressure.
In my experience, it's usually the sender that goes bad. It has moving parts, including a wiper that moves across a wound coil. Dirt under the wiper will cause it to read open, while a worn spot on the wound coil will cause erratic readings.

[JOHN COOKSON]

Originally Posted by joe_db

. USA standard one terminal senders are 240-33 ohms connector to ground and more pressure/temperature/fuel equals less resistance = more needle movement.

AKA conductance. Conductance is the reciprocal of resistance. The higher the conductance the more easily the material conducts electricity and the higher the meter will read.

TRUE GRIT

[JOHN COOKSON]

Originally Posted by Marty Levenson

and it didn't work...stayed at zero. Wondering how to figure out which is broken: meter or sensor?
Thanks!
Marty

Sending unit, wiring, gauge
Here's how to test all three components and figure out which one doesn't work.
Sending unit: See post #2
Wiring: isconnect the wire sending unit-> gauge at the sending unit and gauge ends. Connect a jumper wire at one end of this wire and run the jumper somewhere near the other end. Use the ohm meter on both ends. Should be zero ohms.
Gauge: Disconnect the wire from the sending unit at the gauge. Power up the gauge*. It should peg low. Next connect a short wire from the sending unit terminal on the gauge to ground. Power up the gauge*. It should peg high.
*When you power up the gauge you will also be powering up the coil. If the coil is powered up for more than a minute or two when the engine isn't running and the points or EI happen to be closed the coil can become heat damaged.

TRUE GRIT

[ndutton]

If the wiring is determined to be functioning I'd replace the gauge and sender as a set regardless of which one is the problem. Different gauges use different resistance ranges so if the sender is determined to be the problem you'll have to be sure to replace it with one compatible with the existing gauge. Same-same with the gauge, the replacement must be compatible with the existing sender.

Changing them both at the same time eliminates the troubleshooting and the risk of incompatibility. Buy both from the same source on the same invoice and be done with it.

I practice what I preach. My last instrument panel project included new gauges. They read in the same range as the old gauges but I was concerned about compatibility and therefore accuracy matching new gauges with old senders. New senders were ordered from the same place as the gauges and concerns went away.

My 2¢