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Sir Sam · 02-27-2018, 03:09 PM

I will need to do some more testing in the future where I read through the OBDII port with a scanner to show what PSI to oil pressure sensor is seeing, and how that translates to the position on the TJ guage cluster. The TJ cluster is reading oil pressure through the PCI bus from the ECU, and then displaying that as an analog value. The question comes up as the the calibration, when it read "half" what PSI was that? When it went higher to 3/4 what PSI was that?

If you noticed the oil pressure gauge is the only one that does not have any units shown. This is because on the 2002+ TJ the oil pressure gauge is basically just an idiot light! The sensor on the newer TJ is a discrete, meaning it is either open or closed. In theory, when off or no pressure the switch is open circuit, and then when the engine is on and oil pressure is "good" it is closed. This way if your oil pressure drops below "good" or if it fails it should go open circuit. What the TJ ECU does is read this open or closed switch and send a value to the cluster. So the TJ cluster reading from the ECU only sees 0 PSI/bad, or X PSI/good. This means that on a stock TJ the oil pressure gauge either shows zero, or middle. Since the gauge is then basically just an idiot light they removed the units of measure.

However since the CRD ECU does read an analog pressure from the oil pressure sender the REAL psi is transmitted over the PCI bus, and since the TJ cluster is reading a real analog value it drives the gauge accordingly. The only thing left for me is to determine exactly what pressure the middle means, and exactly what pressure the high means.

I am working on a way to read the calibration from the processor onboard, which may allow me to change the scaling factor on the gauge. Likely that will be much further along, I'm just happy the Main functions, fuel level, coolant temp, RPM, and presumably speed all read correct.

In final form this will be wired into the normal dash wiring for the cluster for all functions such as turn signals, lights, 4wd indicator, etc, leaving the obdii port completely open like factory.

02-27-2018, 03:09 PM	#88
Sir Sam Working Mother Join Date: Mar 2004 Posts: 19,874	I will need to do some more testing in the future where I read through the OBDII port with a scanner to show what PSI to oil pressure sensor is seeing, and how that translates to the position on the TJ guage cluster. The TJ cluster is reading oil pressure through the PCI bus from the ECU, and then displaying that as an analog value. The question comes up as the the calibration, when it read "half" what PSI was that? When it went higher to 3/4 what PSI was that? If you noticed the oil pressure gauge is the only one that does not have any units shown. This is because on the 2002+ TJ the oil pressure gauge is basically just an idiot light! The sensor on the newer TJ is a discrete, meaning it is either open or closed. In theory, when off or no pressure the switch is open circuit, and then when the engine is on and oil pressure is "good" it is closed. This way if your oil pressure drops below "good" or if it fails it should go open circuit. What the TJ ECU does is read this open or closed switch and send a value to the cluster. So the TJ cluster reading from the ECU only sees 0 PSI/bad, or X PSI/good. This means that on a stock TJ the oil pressure gauge either shows zero, or middle. Since the gauge is then basically just an idiot light they removed the units of measure. However since the CRD ECU does read an analog pressure from the oil pressure sender the REAL psi is transmitted over the PCI bus, and since the TJ cluster is reading a real analog value it drives the gauge accordingly. The only thing left for me is to determine exactly what pressure the middle means, and exactly what pressure the high means. I am working on a way to read the calibration from the processor onboard, which may allow me to change the scaling factor on the gauge. Likely that will be much further along, I'm just happy the Main functions, fuel level, coolant temp, RPM, and presumably speed all read correct. In final form this will be wired into the normal dash wiring for the cluster for all functions such as turn signals, lights, 4wd indicator, etc, leaving the obdii port completely open like factory.