Has anyone found an aftermarket transmitter that is a reliable replacement for the stock (not so reliable) unit. I have used a NAPA OP6622 but it reads high. 30-12 ohm range.

 

what are the pressure reading you are getting? Any idea what the stock pressure sender unit ohm range is? If the stock ohm range is higher than the new one, use an appropriatly sized resistor to get it right.

 

That would only work if the difference of the high and low ohm readings of the other sender were the same as stock. For instance if the stock one reads from 12-30 ohms (I have no idea) and you have an aftermarket that ran from 22-40 ohms a simple resistor would work (same as stock; difference between high and low of both is 18 ohms). If the other sender reads over a range different than the 18 ohms of our example you're screwed because then the readings are not proportional and a simple resistor can't correct that.

 

According to the FWM, Mazda specs the oil pressure sending unit's resistance as 43 ohms @ 30 PSI. I measured a used OEM sender's resistance vs pressure and plotted the results in the attached Excel spreadsheet. I found that the FD's sender has a distinctly non-linear curve.

Now remember that this is only based on one test and without benefit of a calibrated test gauge, but it is at least a place to start. Also, since the zero pressure resistance doesn't depend on the gauge's accuracy it's probably pretty close.

The spreadsheet also includes a 2nd line to simulate a potential sender with a linear resistance vs PSI. You can move it around by changing the values in the Linear Ohms column. As you can see a linear type sender will result in the varying amounts of error in the displayed PSI; the magnitude and location depending on the sender's actual resistance.

If others members have access to a few potential non-OEM replacement senders, they need to post results of similar tests so that we can narrow in on one or two likely candidates.