Oil pressure gauge

[srp]

My oil pressure gauge needle moves to full scale when I turn on the ignition switch, and drops back when the engine is started. It then drops back a touch more when the engine is revved. Any ideas? It's a Perkins 4-108, with a Perkins panel. There seems to be a sensor of some sort on the block, stb side, and another on the bottom of the oil filter housing, but couldn't trace which went to the panel today (wires seem to change colour mid-way through the sheathing!)

 

[robbieg]

If it is an electric one....

..may be a faulty sensor. I would try checking the connections and if no joy with the wiring swap it out and try a new sensor before worrying about the engine having a problem.

 

[srp]

Yes, it's an electric gauge. I'm wondering if the sensor has been wired back to front, since the reading should obviously be zero when first switching the ignition on, and would possibly increase a little when increasing the revs from tickover. So, if I swop the terminals on the sensor, will I damage anything?

 

[VicS]

I reckon you have mismatched gauge and sensor One or other has been replaced with the wrong type

One of your sensors may be for a warning light. ??

 

[srp]

You may be right Vic, although the panel is a Perkins one. I think the oil warning light sensor is on the filter housing, but is not connected to anything on the panel (or I have a faulty/unconnected buzzer!).

The other problem is that the wiring is a total mess, and the previous owner was clearly a first rate bodger. I am slowly sorting it all out, but am still uncovering faulty connections, badly crimped, not crimped at all, etc etc. Am going out to the boat again tomorrow with a box of electrical goodies, a set of crimping pliers and a multi-meter to try and at least make good the worst parts.

Would a multi meter across the sender terminals will give me a clue about its' polarity and whether the wires can be reversed.

 

[theoldsalt]

The sender may have only one terminal (positive as the electrical system is usually zero volts earth) with the other via engine block (battery zero). The sender varies the positive voltage with the engine oil pressure.
If the sender has two terminals then a multimeter will identify the polarity



.

 

[little_roundtop]

My oil pressure gauge needle moves to full scale when I turn on the ignition switch, and drops back when the engine is started. It then drops back a touch more when the engine is revved. Any ideas? It's a Perkins 4-108, with a Perkins panel. There seems to be a sensor of some sort on the block, stb side, and another on the bottom of the oil filter housing, but couldn't trace which went to the panel today (wires seem to change colour mid-way through the sheathing!)

I've had a similar problem when fitting a new oil pressure gauge and sender. It seems there are two "standards", European and American. The difference is in the range of resistance values the sender and gauge use (at least that's true for Faria instruments). They have a table of the resistance values for both "standards" here. In my case I'd managed to buy an American gauge and a European sender. The effect was exactly as you describe.

 

[wotayottie]

Yes, it's an electric gauge. I'm wondering if the sensor has been wired back to front, since the reading should obviously be zero when first switching the ignition on, and would possibly increase a little when increasing the revs from tickover. So, if I swop the terminals on the sensor, will I damage anything?

Maybe its one of those bling type guages beloved of Clio lad - you know the way that all their guages flip to max when first turned on. You can check whether this is the case by looking for below water line neons and checking the size of your exhaust outlet.

The problem with using guages to monitor this sort of thing is that it gives you something else to worry about. Does the engine work OK ? Yes it does - so ignore the guage until it starts doing something different

 

[dmayes]

Oil pressure

Don't ignore oil pressure warnings, engines take time to overheat and smell awful and you can still get them back working but low oil pressure can destroy an engine in minutes without any warning signs except a big clunk as the engine seizes up. Engine sender units are usually variable resistors or switches so polarity does not matter to the sender only the way it is wired, if you have a sender with two connectors then one connector will be either earth or 12v so making the guage go to empty or full depending on which you have connected. Get busy with a meter, check the two connectors on the sender for voltage. Check the guage for voltage and maybe use a wandering lead to see FSD on the guage (12v then earth).

 

[srp]

Done some more investigation - the oil pressure sender measures 246Ω with the engine not running, and 72Ω at tickover, which according to the table linked to in LittleRoundtops reply indicates it is a US spec sender (and also indicates that I have around 55psi which is rather good news).

The gauge is a VDO with the model number N02 124 125 - I cannot find the specs for this anywhere but the symptoms would indicate that it is not matched to the sender and may therefore be a Euro spec rather than US spec.

Alternatively, I may be able to swop the terminals on the gauge, but that would need some careful checking before I risk a possible short circuit - the existing wiring looks like it has been done by a chimp on acid.

Thanks to all for the help.