Oil pressure gauge reads zero - testing sender

[zambant]

My thornycroft T80 has suddenly developed a fault, it tells me I have no pressure when the engine is running - I suspect is either the oil pressure gauge or sender - or worst still the oil pump has failed

Looking at the schematic the gauge has +12v going into it the other side of the gauge goes to the sender and the other side of the sender goes to ground so they are connected in series.

I have a complete mental block - how can I test the sender and the gauge please. I can use a meter on volts and ohms just

Not going to start it until I sort it out

Thanks

John

 

[pvb]

Lots of advice on t'interweb on testing, eg http://answers.yahoo.com/question/index?qid=20100329174051AAtofhh and http://www.autozone.com/autozone/re...il-Pressure-Sending-Unit/_/P-0900c15280037cfe

 

[theoldsalt]

I also have a T80.

I believe that if you remove the wire from the sender and short it to earth it should result in a full scale deflection of the pressure gauge. The sender is a resistance that varies with pressure. That will test the gauge.

To test the sender it would be necessary to fit a mechanical guage either in place of the sender or in parallel with it. It is not necessary to run the engine; just turn it over on the starter with the stop pulled out to prevent it firing (turn off the raw water seacock first to prevent filling the engine with sea water through the exhaust and run the starter for only a few seconds to see if oil pressure builds up)

Some owners fit an additional mechanical oil pressure gauge permenantly in the engine bay as they do not like to rely solely on the electric gauge.

Update edit. Having read further it may be that the gauge will read full scale when the wire is removed from the sender. Either way this will test the gauge.

Edit : apologies for mis-spelling "gauge" (guage) - now corrected.

 

[Norman_E]

If it ran without any oil pressure for more than a few minutes I would have expected you to have heard some nasty noises followed by it seizing. Unless the drive to the oil pump has sheared an oil pump will usually produce some oil pressure, particularly with a cold engine, even if very badly worn.

 

[mattlad]

Short the wire that goes to the sender unit down to earth and see what the gauge says. If the gauge goes up you need a new sender unit. If you have no reading you will need to use a gauge tester. That should determine whether it is a broken wire or a gauge fault. If all those are okay then it may be an engine fault. If you look down the oil filler, or remove the rocker cover you will soon be able to see if you have oil at the top end of the engine.

 

[srp]

Just measure the resistance at the sender terminals. A Euro sender will be around 10 ohms at 0psi, going up to 180 ohms at full pressure (depends whether it is a 5 bar or 10 bar sender). A US sender works the opposite way round, with 240 ohms at 0 psi, dropping to about 30 ohms at full pressure. If you also do the other tests mentioned above you can work out if the sender, gauge or wiring is at fault. I'm assuming you don't have a spare mechanical gauge lying about.

New senders, plus technical info at http://www.asap-supplies.com/marine/oil-pressure-senders

 

[zambant]

Thanks all

 

[zambant]

Short the wire that goes to the sender unit down to earth and see what the gauge says. If the gauge goes up you need a new sender unit. If you have no reading you will need to use a gauge tester. That should determine whether it is a broken wire or a gauge fault. If all those are okay then it may be an engine fault. If you look down the oil filler, or remove the rocker cover you will soon be able to see if you have oil at the top end of the engine.

Ok so I have found and fixed the broken wire and the gauge works fine, checked dipstick and oil level is fine - so after the engine had run for a couple of mins I stopped it, removed the oil filler cap and stuck my finger in the top of the engine but could find very little oil - is that normal?
Thanks

 

[theoldsalt]

Zambant,

So as I understand the situation, the gauge is OK but still no oil pressure indicated on the gauge; so it must be either a faulty sender or no oil pressure.

Next thing to check is the sender before worrying that the oil pump has failed.

I have never stuck a finger in the top of the engine to detect oil so I don't know how much to expect. I would suspect very little at low revs.

 

[zambant]

Sorry - confusion here - sender and guage ok and showing reading when engine runs - so thats ok now.

After the engine had run for a couple of mins I stopped it, removed the oil filler cap and stuck my finger in the top of the engine but could find very little oil - is that normal?

Thanks

 

[CAPTAIN FANTASTIC]

Also, check the oil filter for any metallic particles; indication of something has broken.