Help with diagnosing non-working gauge

[Wdeertz]

My n2k nmea transmission oil pressure gauge recently stopped working. Recently it was showing up but giving me erratic readings and now it doesn't even show up on the n2k network. With the engine off the sensor registers about 1440 ohms and with the engine idling it registers about 700-800 ohms so I believe the actual sensor is working. (see photo #1). I traced the wires from the sensor to the device shown in the 2nd photo. The wires show continuity to this point. I believe this second device converts the analog resistance to J1939 signals which are passed to the JD ECU for display.

Anyone know how to test this second device? I would hate to replace if this wasn't the source of my issue. I should point out that the transmission oil temperature is being fed into this second device and the oil temperature is properly displaying. What is the likelihood of this device properly passing the temperature but not the pressure? Open to any other suggestions on diagnosing my problem. Thanks

 

[STB]

Those resistance values don't seem correct to me. See the attached table. Make sure you are measuring directly at the sender andbthat you are getting a good connection to the terminal. And the grounding threads going into the engine or as close as you can get. Fully disconnect it from everything, if needed.

The pinout attached ahoukd help you check power, etc, on the i/o module.

You can wire a resistor in place of the sender to bypass it.

Full docs are here:
https://www.murcal.com/

See what you find and report back?

 

[Wdeertz]

STB, thanks for the info. I came across that document also when I searched on that part number but disregarded it as my transmission manual shows a normal operating pressure of 333-349psi and that table only shows up to 100psi. When measuring those resistances with my multimeter I had the wires disconnected and made good solid connection to the probes.

Thanks for the wiring diagram for the can device, a google search didn’t turn this up for me. With the wiring diagram I’ll hook up a spare j1939 to n2k device I have laying around to the 2 CAN wires and see if I get oil pressure. If so then I can rule out that this device is the issue and it’s a broken wire somewhere along the line. I’ve tested continuity from the sensor all the way to where the wire exits my engine room and goes to the pilothouse. Beyond this point it becomes more difficult to trace. Will let you know what I find out. Thanks

 

[STB]

It should still have the same resistive range from 33 through 240 Ohms. They do that to enable things to be compatible downstream and for the current range to be powerful enough to overcome wiring with narrow enough error bars.

Attached is the table for your specific version.

 

[Wdeertz]

It should still have the same resistive range from 33 through 240 Ohms. They do that to enable things to be compatible downstream and for the current range to be powerful enough to overcome wiring with narrow enough error bars.

Attached is the table for your specific version.

STB, thank you, thank you, thank you. I put a 68 ohm resistor across the pressure sensor probes and the gauge showed 325psi. I think this confirms that the sensor is bad. I guess the high ohm of the in place sensor caused the Cansend device to sense as open circuit which is why it didn’t even register on the n2k network. I’ve ordered a new sensor. Thanks again for your help.

 

[STB]

STB, thank you, thank you, thank you. I put a 68 ohm resistor across the pressure sensor probes and the gauge showed 325psi. I think this confirms that the sensor is bad. I guess the high ohm of the in place sensor caused the Cansend device to sense as open circuit which is why it didn’t even register on the n2k network. I’ve ordered a new sensor. Thanks again for your help.

Bingo. Good work!