working oil pressure gauge
 

I've searched all forums but have yet to find an adequate solution for those that want a working oil pressure guage in their RX-8 vehicles. My understanding so far is that only a new sending unit is required to make the current guage a working one, but such a unit doesn't seem to exist at this point. As an owner of a '91 MX-5 I do appreciate seeing when the engine oil is warm via a working gauge.

Does anyone have any information regarding a potential solution?

Working.....
 

A japanese member posted pictures of his pressure and temp sender setup, it fits between the oil filter and its mount, then runs to conventional aftermarket gauges. Looks like it is made for the RX-7.
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doc

An oil pressure gauge that moves thru a range is not going to tell you when the oil is warm. It won't give you any info on oil temp. It would only show you variations in oil pressure.

I am waiting for Mazda to provide some sort of PCM flash that I can pay for, to give me real acting temp & oil pressure gauges.

It should be in the programming, I hope.

A working oil pressure gauge will tell you when the oil is warm, but not it's temperature. When cold the reading on the pressure gauge is always pretty far to the right of center, even when engine is at idle. When warm the gauge reads at or near the center line when engine is at idle, ans swings right when each gear is engaged.

Variations in oil pressure DO indicate oil temperature, because oil viscosity (and thus pressure from the oil pump) changes significantly with temperature. For example, max oil pressure observed in a Miata at 5K rpm: 90 psi cold, 50 psi hot. The only difference is the temperature - when your oil pressure at 5K rpm is around 50 psi, then your oil is warmed up to operating temperature.

Regards,
Gordon

i contacted Mazda USA through their web site re: an alternative to the current "on/off" oil pressure gauge. They stated that nothing else was available. So- unless someone can come up with a woking solution that can be widely adapted, we're stuck with the current gauge.

I'm going to resurrect this thread to see if there've been any developments in rigging a sending unit to the stock "oil pressure guage" to make it a real one.

I'm pretty disappointed that Mazda lists an "Oil pressure gauge" in the RX-8 features and specs on the MazdaUSA site, and then it turns out it could best be called an oil pressure indicator if they wanted to at least play word games and not lie outright. Everything else about the car and the company seems so genuine. I don't get it.

This won't happen as the actual oil pressure sensor on the engine is just a switch. To get a real "oil pressure" reading it seems you have to do what's already been done and get a pressure sending unit and aftermarket guages.

Ok, so confirming as I thought ... the current oil pressure gauge is merely an "idiot light with a needle"?

Guys-maybe parent Ford can help us. If someone in the forum works for Ford they may have access to Ford SVT sending units and gauges? I have SVT Focus and came stock with oil temp and oil pressure gauges and temp gradually goes up and oil pressure goes up and down with rpm when shifting so I know is working.

Thanks,

Bob

I know it has been verified that the sending unit is really a switch. How about the gauge mechanism itself? Is there anything about the gauge hardware that would prevent it from working properly if hooked up to a proper signal? I personallyhate the idea of retaining the stock "gauge" and then adding a second "real gauge."

jds

That's exactly my thinking on the second gauge option. I want to put it the way it should have been from the factory.

I've already done something along these lines in my last car. They all shipped wired wrong from the factory so that the 12 volts that was supposed to test the Temp light went to the gauge instead. This pegged the gauge hard every time you started the engine. Eventually, the needle would start getting stuck pegged. I had to take the gauge pod off, cut two wires, make one loose end safe, and splice the remaining three. Once we have the right part, I don't see fixing the 8's oil gauge being any harder.

Doesn't the oil pressure guage/idiot light with a needle sweep with the 20 brake pump test for L & M flash? If so then there is SOME signal that is capable of making the needle move back and forth.

I don't know for sure what the nature of the signal to move the needle would be, but a first guess is a voltage increase/decrease.

Correspondingly, I don't know how the typical pressure senders work, but another first guess is maybe by varying resistance with pressure variance?

(I'm not a mechanic, I'm a software engineer so forgive my guesses/assumptions.)

If the above assumptions are correct, it should be possible to put in a sender that reports pressure, and appropriately scale the signal into the guage input. Might also have to send a bogus or approximately in range pressure signal to the ECU if it monitors this.

But those are some big assumptions... :)

There is no connection between the oil pressure switch and the gauge. The oil pressure switch provides a signal to the ECU (either "pressure" or "no pressure"), and the ECU determines what signal to send to the gauge. You could change the binary switch for an analogue sender, but that won't make any difference unless you reprogram the ECU logic so it knows how to interpret that different signal to display varying pressure readings on the gauge.

In other words - it could be done by Mazda, but it can't be done by anybody who isn't capable of cracking the ECU code and reprogramming the ECU. If anyone can... while you're at it, could you lean out the A/F mixture at high rpm and get me my 9 hp back? ;)

Regards,
Gordon

Sounds like a nice side project for Racing Beat's ECU test bench! On the other hand, depending on how the ECU interfaces with the gauge cluster, it might be possible to disconnect the oil pressure gauge from the ECU and feed it from a new sending unit that is plumbed in somewhere. Of course, if they did something fancy like interface to the entire gauge cluster via a serial bus or something, then we're SOL. Surely they didn't over-engineer the gauge cluster to that extreme....

jds

That's an excellent idea. I'm going to e-mail Racing Beat when I get home, and tell them about this thread and try to interest them in the project. I hope they'll see us as a bunch of potential customers for a kit.

Ok, I finally got around to sending that message to RB. I'll post any developments.