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.

[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

[SteveJ]

If you are up to removing the gauge from the dash, it's not to difficult to test. I have only opened up the fuel gauge/ammeter, so I can't say for sure about all of the wiring, but if you open up the meter, you can confirm the following.

If the oil pressure gauge works like the fuel gauge, you have the Yellow/Red wire providing 12 VDC+. The gauge probably works on a voltage divider with one part of the gauge wired to black and the signal wire for the gauge wired to Yellow/White. (It says Yellow/White in the FSM, but it may be Yellow/Black in reality.) Get a 0-100 Ohm potentiometer and 2 6 VDC lantern batteries and wire them up to the gauge as shown in the picture. Vary the resistance on the pot and look for movement in the gauge.

By the way, you didn't use teflon tape or some other sealant like that on the threads, did you? You should only use a sealant that has enough metal in it to be conductive. Also, it's a good idea to have an ohmmeter or multimeter rather than a test light. A meter will give you a better quantitative answer.

[zbill]

After replacing 2 senders,substituding all the wires (power,ground & sender),trying another
known good gauge,it turned out that the 2 "NEW" senders and old one were all bad. Ordered 3rd new one from Motor Sports screwed & pluged it in tonight & BAM with the key
on it moves to zero & after engine starts it starts to read oil pressure (what a novelty!!)
So thanks Beerman,Steve & Others for your help!! Mahalo Plenty

[het976]

Wow, I thought I was unlucky with two bad sending units. Too bad you had to go through all that extra work, but great it is up and running.

[beermanpete]

Phew! I am glad you finally solved the problem. It sure is tough to solve problems when you get so many new parts that all turn out to be bad. I hope you were able to return all the bad parts for a refund.

[mlc240z]

Old adage is "New doesn't necessarily mean good".
Sucks you had to go thru all that to find out, but glad you solved it.

[Zedyone_kenobi]

Wait, sorry to bring back an older thread, but the sending unit cannot have teflon tape on it?

[SteveJ]

Wait, sorry to bring back an older thread, but the sending unit cannot have teflon tape on it?

The tape will insulate the sender. It needs a path to ground via the engine.

[Zedyone_kenobi]

Looking at the wiring diagram, I see that you are right!!

I cannot believe I never thought about that. May explain my sluggish oil pressure gage.

[makaofox]

Agree sorry bring up an old thread. I dont even have the oil pressure sending unit connected and the gauge reads full sweep no matter what. I obviously have some oil pressure since its rebuilt but not sure if its my guage or not. Someone in the earlier posts stated they had a bad tach and that contributed to the issue. I have a bad tach, it reads but very inaccurately for whatever reason. It worked at one point then got fried. So just curious as to what the issue is? Whether it is the guage or a faulty ground. I want to start by taking it out the dash and cleaning the connection.

[beermanpete]

Agree sorry bring up an old thread. I dont even have the oil pressure sending unit connected and the gauge reads full sweep no matter what. I obviously have some oil pressure since its rebuilt but not sure if its my guage or not. Someone in the earlier posts stated they had a bad tach and that contributed to the issue. I have a bad tach, it reads but very inaccurately for whatever reason. It worked at one point then got fried. So just curious as to what the issue is? Whether it is the guage or a faulty ground. I want to start by taking it out the dash and cleaning the connection.

It sounds like the wire that connects to the sending unit has shorted to ground somewhere. Trace the wire back from the engine to the meter and look for a bare spot in the insulation (cut, slice, pinch, etc.). The holes where the wires pass through the firewall is a likely place for a problem if the grommets are rotted away.