jza80

iTrader: (32)

The oil pressure gauge has been flaky since I bought my FD, sometimes reading what I think is normal, sometimes reading very low (usually at idle), and sometimes reading 0 and bouncing around. It seems like if the gauge was reading true I would have a blown up engine lol so I think the problem is not with actual oil pressure. Usually it seems to work correctly, even when it flakes it seems to recover eventually. Also, the flaky behavior only seems to happen one time in a drive so if it happens at all, once it recovers it seems to work okay for the rest of the drive.

I have cleaned the contact terminal on the pressure sending unit, and the charging harness with its connector is brand new yet the behavior continues so the problem is not the connection at the sender like I thought/hoped. What I am wondering is... how reliable the sending unit is... and if the failure pattern is indicative of a bad sender...or, could I have some other problem like a fault in the cluster?

It's like $100 or so for a new sending unit #N3A114820, so I thought I'd ask the community for some advice before shelling out the duckets. I do want to get this fixed, though, I hate niggling faults...

Thanks in advance for any advice!

DaveW

I've had that happen twice. I removed and cleaned out the sender with carb cleaner and compressed air both times, and it fixed it for an extended period of time (at least 3 years). The small orifice in the sender gets plugged from baked-on oil, etc. and causes the malfunction.

jza80

iTrader: (32)

Good information!

I will give this a try. Thanks!

DaleClark

iTrader: (55)

It's worth a try cleaning it out but my experience has been when it is reading low over time it's the contacts inside that are shot. Unfortunately you can't take it apart and clean it or anything.

It's worth getting a new one. I put a new one on like 10 years ago and it's still doing a perfect job.

The oil pressure gauge isn't as important on a rotary as a piston engine, but it's nice to have the gauge working so you car doesn't look broken .

Dale

DaveW

The 1st time it happened to me, not knowing better, I bought a new one. A few years later, the "new" one started flaking out. So it's actually happened 3 times, but I have successfully cleaned this one twice. In my case, every time the problem developed over a few days, not over months. And by then I had read on this forum that others had success cleaning it. YMMV

And the sender is not a low pressure on-off switch with "contacts." It sends a signal more or less proportional to pressure. So it, IMO, is likely a strain-gauge or solid-state type pressure transducer (read the post below - I had it mostly wrong...). Those, in my experience, don't have contacts that go bad. They either yield internally, giving a too-high signal, the diaphragm gets perforated or cracked (giving a low or zero reading), or some internal wiring gives up. Or, as in my case, the inlet orifice gets blocked.

DaleClark

iTrader: (55)

I took a bad one apart once (YEARS ago) and I remember it had a sweeping arm going over a board, kind of like the fuel level sender. Spots on the board were worn or not-good looking if I recall.

Dale

mdp

The oil pressure sensor on our cars is a rather crude affair consisting of a mechanical diaphragm, a wire wound variable resistor, return spring and a spring tensioned wiper arm. The engine oil pressurizes the mechanical diaphragm chamber pushing the diaphragm up. This moves the wiper arm across the resistance wire changing the the resistance output. Our cars have a unique resistance output, from about 50 ohms near zero psi (it is an open circuit at 0 psi) and 10 ohms at 120 psi. This makes it very difficult to find a aftermarket replacement (more about that later).


From my experience the gauge has five failure modes:

1. The connector breaks or loses contact due to dirt and oil getting inside.
2. The orifice leading to the diaphragm clogs.
3. The diaphragm cracks.
4. The resistance wire wears out from the wiper arm rubbing against it
5. The return spring or the wiper arm tension spring weakens or breaks.

Now about an after market replacement, the closest thing that I have found is this sensor. Unfortunately, its output is close enough to work, but would be slightly inaccurate. The other drawback to this sensor is it isn't that much of a cost savings. If you are really adventurous you can modify the insturment cluster gauge to use just about any resistance sensor you want. Here is the link on how to preform the surgery necessary, it's for a Miata pressure gauge but ours is of the same build, just different windings and resistors.

Another thing, have you noticed how slow to react our pressure gauges are? Blip the throttle and the oil pressure doesn't change because it can't react quickly enough. Well, the Miata guys have a fix for that. It seems that Mazda put a big blob of grease/wax like substance on the needle shaft to dampen its movement. Check out this site on how to remove the wax and your gauge will respond just as quickly as the tachometer, very cool.

As for me I'm on my 6th pressure sensor after 23 years of ownership, I bought eight sensors from Ray back in the day when they were $85. So, I have about 6 more years until I have to worry about another sender.

DaveW

Thanks Dale and MDP for educating me.

jza80

iTrader: (32)

Just thought I'd update this with a resolution - I wound up purchasing a new oil pressure sender after not having any luck with cleaning.

This solved the problem completely, and I have to say the gauge moves much more smoothly now throughout the range of operation (and does not stick at 0, either of course). Very happy with the restored performance, and it was great that the new part is still available from Mazda...