Oil pressure gauge stopped reading correctly

[Texasroadrunner]

I recently started having trouble with my oil pressure gauge on my '72. It's always run 45 to 60 psi. But now it often goes from 60 psi to suddenly dropping to about 15 psi and staying there, no matter what the RPM is. When I start up a few days later, sometimes it's normal but sometimes not. I did replace the sender with a new Beck Arnly unit but that didn't change a thing. When I connect the wire to the sender to ground, the gauge slowly goes to max. So, the gauge is working right. The Ohms measured from both senders, new and old, are about the same. 90 ohms with no pressure and about 30 ohms with the engine running.
I did find something strange. When the engine is off and the key is on, the voltage from the gauge wire to the sender reads12.5 volts. When the engine is running, it shows all kinds of random values and jumps around from zero to 12v or higher and very erratically. I've been looking into the schematics and none of them show where the +12V voltage is routed from for the gauges. Not much help. The water temp gauge that it's paired with works just fine. Next, I'm going to pull the gauge and see if the internal voltage regulator they talk about in the FSM is at fault. Has anyone ever had a problem like this? Thanks for any help.

 

[moggy240]

I have not had that specific problem but in the past I have fitted a mechanical guage via a tee piece off the oil pressure mount while still using the electric one to check it was working ok

 

[Texasroadrunner]

I have not had that specific problem but in the past I have fitted a mechanical guage via a tee piece off the oil pressure mount while still using the electric one to check it was working ok

My next attempt is a mechanical pressure gauge. But I don't have a good place to put it on the console or dash. So a temporary set up will at least give me peace of mind that the engine does have good oil pressure. Finding a 1/8 JIS pipe thread fitting for the hole in the block (like the sender) seems to be the hard part. It's not an 1/8 npt hole, the thread pitch is very close but the OD is off and will only engage 2 threads and that will leak.

 

[jonbills]

Its a 1/8 BPT parallel thread I'm pretty sure.

 

[Texasroadrunner]

By parallel you mean not a tapered thread? JIS is the Japanese Industrial Standard which is identical to and interchangeable with the BSPT (tapered) threaded connectors. I could tap the hole in my block for 1/8 NPT but I don't want any chips getting into the oil gallery. I hope to find a BSPT or JIS fitting locally so I can run an 1/8" tube for a mechanical gauge. I really want to know what my oil pressure actually is.

 

[jonbills]

Sorry, you're right and I'm wrong, it is tapered.
These fittings, and adapters between bsp and npt are quite common here so I'm sure I could get what you want and. post to you.

 

[Texasroadrunner]

I found a 304 stainless steel one on Ebay last night. Now I can have the stock sender and an outlet for a mechanical gauge.
https://www.ebay.com/itm/313833264707

 

[jonbills]

I tried one of those with my stock pressure sender and an oil temp sender, but they interfered - the temp sender was too long to go in the side hole, and the pressure sender was too long to go in the side hole. I ended up with a Y pipe like this one:

Hydraulic Equal ' Y ' F x M x F BSP : BZP Pipe Fittings | eBay

PLATED BRASS (BZP). All our BZP hose tail adaptors are one-piece (non-swivel) fittings. BSP THREADED PIPE FITTINGS. BSP threaded plated brass fittings and hose tail adaptors. BSP THREAD SIZING. BSP thread sizing is NOT straightforward.

www.ebay.co.uk

 

[Texasroadrunner]

I'm just looking at the oil pressure. Not concerned with the oil temp yet. I think this one will work okay. Thanks for the part info. Amazing what's out there.

 

[Minksport]

Hi, I have this which I recently picked up with some other Datsun S30 parts from Rob G. Is this what your looking for or are you now sorted?
Regards, Mark

 

[Texasroadrunner]

Hi, I have this which I recently picked up with some other Datsun S30 parts from Rob G. Is this what your looking for or are you now sorted?
Regards, Mark

Already ordered one from EBay, It arrives today. But thanks.

 

[Farmer42]

Coming back to your original point, I'm pretty sure that the sender unit is an earthing switch and the wire is an earth wire from the gauge. The sender works on the same principle as the fuel sender in that the pressure provides a stronger earth which moves the gauge needle. It sounds like either the wire has a resistance issue somewhere or the oil pump is giving irregular pressure. If you have had grit or crud in the oil it can score the chambers in the pump and that can affect the pressure intermittently. Try running a new wire from the back of the gauge directly to the sender. If it gives a good reading when the engine is running then you have a break or resistance in the original wiring. It's 40-odd years old so it is possible! If it still happens then I would look at the pump.

 

[Texasroadrunner]

Coming back to your original point, I'm pretty sure that the sender unit is an earthing switch and the wire is an earth wire from the gauge. The sender works on the same principle as the fuel sender in that the pressure provides a stronger earth which moves the gauge needle. It sounds like either the wire has a resistance issue somewhere or the oil pump is giving irregular pressure. If you have had grit or crud in the oil it can score the chambers in the pump and that can affect the pressure intermittently. Try running a new wire from the back of the gauge directly to the sender. If it gives a good reading when the engine is running then you have a break or resistance in the original wiring. It's 40-odd years old so it is possible! If it still happens then I would look at the pump.

My first step is to add a mechanical gauge to rule out any oil pump problems. Then I'll inspect all wires to the stock oil gauge, making sure it's getting +12v all the time. Then I'll trace the wire from the gauge to the sender. I have to find something. It's odd how the gauge will drop and hang at 15 psi at any rpm hot or cold. There's a gremlin running around and I'll catch him. Most of the wiring has been replaced along with a modern fuse block. So I suspect one of the original connectors.

 

[Texasroadrunner]

I'm now writing the final discovery and solution to my bizarre oil pressure reading. I bought a mechanical gauge, tee adaptor and new sender to test everything out. After connection of the new parts, at start up, the mechanical gauge was reading pressure. I had also connected the original sender on the tee. To my amazement, the dash oil pressure gauge started to climb. Slower than the mechanical one but withing 10 seconds they both read the same 60 psi. At idle they both read 40 psi while the engine was warming up. You may ask what is different? Answer: when I pulled the new sender off of the block, I noticed a piece of spooge (California dirt biker term for crap) hanging from the inlet hole. I pulled it out and took a closer look with a loop magnifier. Hmmm, looks like old Teflon tape mixed with black stuff had clogged the sender's hole. This chunk wasn't in the original sender when I removed it, so it was probably a floater in the oil passage that randomly found the sender. I am overjoyed. This means no electrical gremlin to dig for and no Datsun oil pressure gauge to replace. To celebrate, I put in a fresh set of NGK BPR6ES plugs gapped at .050" to match the MSD ignition's beastly voltage. My '72 starts and revs instantly. Wow, look at that oil pressure. End of story.