71 240z oil pressure gauge not working

[zbill]

I have checked power & ground to gauges,w/temp works,yel/blk wire is

good from gauge to sending unit,swapped out sending unit. With key on

gauge sweeps to 140lbs,with engine started gauge slowly drops to below

zero and stays there. Is it the gauge?? and if so,can I change only the oil press side?? Appreciate any advise on this problem,Thanks,Aloha from Honolulu

[beermanpete]

It sounds like a faulty sender to me. Try a different sender.

[psdenno]

Sending units are often the problem with low or inaccurate readings. However, you mentioned that you swapped out the sending unit. Did you install a new one or a previously used one? Did the gauge react the same with the original sender?

Dennis

[zbill]

Replaced sender (new),How can I test the gauge??

[beermanpete]

When the wire to the sender is disconnected and the key is on the gauge should sit at 0. If you connect the wire to ground through a small light bulb (test light or meter dial light bulb) the gauge should move up some. The exact reading will depend on the wattage of the bulb.

A solid connection to ground will move the gauge past full scale very quickly. If you do this it would best to have a helper who is watching the gauge while you carefully brush the wire to ground to see if the gauge responds. A solid ground for too long might damage the meter.

You could verify the sender is working by using an ohmmeter to measure the resistance. When the engine is stopped the resistance should be high, probably about 100 ohms. When the engine is running the resistance should fall as the oil pressure rises. I suspect the resistance will be about 20 ohms or so when the oil pressure is high.

[het976]

Just an FYI...

I had the same issue many years ago (c1985)....bought a new sending unit and still got low readings...then bought a cheap mechanical gauge and verified there was oil pressure, assumed it was the gauge and drove it many years with no oil pressure gauge. Sometime later I found a used gauge put it in...still no oil pressure, turned out the new sending unit I bought years back was bad too...

So, you might get a cheap mechanical oil pressure gauge and see if you really have pressure then go from there.

Edited January 20, 2012 by het976

[philbar73]

I had the same problem; the pressure would be good at the start then drop way off when warm. The sending unit was the original, it was 39 years old. I put in a new sending and the pressure looks good, about 20% of the gauge at idle, a little over half way at 2000 RPM's. If you take off the oil fill cap when its running and you see oil splashing around you probably have pressure. I would agree with the other that suggest changing the sending unit.

[zbill]

Ok got home a little early tonight,using simple test light between neg on bat and probe in

end of yel/blk to sender,key on,gauge will read to about 3/4,and at about 1/3 the test light

starts to pulsate on/off,will not go to 140lbs. Then when grounding yel/blk sending wire,gauge will go to 140lbs but either way it is slow steady climb. My voltmeter is not working well to test resistance of sender,Im either going to look for another new sending unit or borrow another meter. I really appreciate all your input and help,Thanks alot!!

I'll let you know as soon as I can get back to it.

[zbill]

Ok got home this afternoon with 2nd new sender (different brand),gauge still goes straight

up to 140 with key on!!,Substituded new wire from sender (yel/blk),subed ground direct from n/bat to ground on gauge and subed pos from bat to gauge.Sitll the same!!!! MUST

BE GAUGE!!!! Anyone got a know good gauge for sale?? LMK,Thanks Z Bill

[pjoe]

This probably has nothing to with your issue but FWIW...I spent a week trying to figure out why my oil pressure guage wasnt working and it turned out to be a bad tachometer. My tachometer wasnt working properly either. My oil pressure guage was completely dead so it was a little differnt than your situation. My car is a 71 too.

Brian

[beermanpete]

Perhaps the wire that connects to the sender has a bare spot on it somewhere that touches the chassis when the wire is conencted to the sender but not when you remove it to make the tests. Check the wire for bare spots. Maybe you could move the wire around whiel the key is in to see if you can make some affect to the gauge's reading.

[zbill]

I though of that and thats why I substituded the yel/blk wire completely,whats wierd is that the gauge goes striaght to 140 when just the ignition is turned on and then to zero

as soon as engine is running and stays there. The only thing I have not tried to sub is the

gauge (don't have another one). Still looking for one. Thanks