QuestionHi Roland,
My 1988 Chrysler New Yorker (w/o turbo) constantly shows the message "low differential oil pressure" on the message center. The differential is full of oil and I've replaced the sensor, but still get the message. The only way to stop it is to disconnect the lead to the sensor. Any ideas? Thanks for your help.
Sincerely,
Mark
AnswerHi Mark,
The '88 manual shows the circuit but has no explanatory text about this particular warning circuit's logic. It appears that the sensor only comes into play when you put the transmission into reverse*, because the power to the sensor is attached to the same switch that turns on the back-up lamp. I believe the function is that the sensor will break that power source if the pressure in the transmission is sufficient to open its switch. Disconnecting the plug to the sensor is the same as opening the sensor switch (e.g. it sends the message that the pressure is OK.) In my first response to you, I suggested that you may have too low pressure in the hydraulic system of your transmission, if my understanding is correct. What I would suggest is that you put an ohmmeter or continuity tester across the terminals of that sensor and notice what happens to the reading with the engine off and then with the engine running and the transmission placed in reverse. It should change from infinite to 0 ohms (or the reverse, depending upon the logic of the circuit). If you see no change at all, then that would corroborate my theory. If it does change, then the problem is with the electrical wiring from the sensor through the 'engine node' of the EVIC system through the digital data bus wire to the message center. But if the issue was with the electronics I would believe that there would be other false messages problems.
So take a reading on the sensor and see if it 'notices' whether the transmission is not spinning (e.g. engine is off) vs. when it is under power and set in Reverse. If not, then I would suspect that either the sensor is inaccurate or the hydraulic pressure at the port of the transmission to which the sensor is plugged-in is below specifications required by the sensor. It may be at the reverse clutch port of the trans which is supposed to read 165-235 pounds when the trans is in reverse is below that value. Were that the case, then you may want to change the filter inside the trans or make other pressure measurements to assess if the internal pump of the trans is wearing out. Have you experienced anyother shifting issues?
Interestingly, the '89 manual describes the circuit differently, but I won't go into details. The proof of the pudding is the measurement at the sensor itself as I described.
Roland
*Check me out on this: the warning should not appear when you first start the engine, but rather when you move the shift lever to reverse, if my analysis is correct.
PS The '89 manual describes the circuit a bit differently, and says that it is sensitive only in the forward drive positions. So you would perhaps have to move the selector to the D or OD position to do the test that I described and compare to the 'off' reading, as before. Perhaps you can describe for me which port the sensor is positioned at. There are 6 ports in a row from front to rear of the trans.
The '89 manual says that to check the circuit, disconnect the sensor and short across the two terminals of the harness plug. Then start the engine and wait for 15 seconds. If the warning doesn't appear, then the wiring is possibly 'open'. If warning does display, then measure the transmission pressure at the ports according to the table in the chapter about the transmission. So there is another approach to evaluation. I would presume, though the manual doesn't say so, that if the pressure tests check out, then that leave the sensor switch as the problem.
Roland
