2002 Nissan Maxima - transmission shifting & performance issues - P0720 & P1574

Question

I have a 2002 Nissan Maxima which which started exhibiting a number of transmission issues shortly before and during a 2,600 mile road trip across the country, and is still experiencing these problems; the issues displayed are as follows:

• Two DTCs are stored in the PCM - P1574 "(ACSD) Vehicle Speed Sensor" and P0720 "AT Vehicle Speed Sensor."
• The "AT CHECK" warning lamp will flash several times for approximately 10 seconds when first starting the vehicle or turning the key to ignition "ON."
• The cruise control is barely operational; after shutting off the car and restarting it, it will work for about 20 minutes before disengaging and flashing the cruise control indicator. Oftentimes it will simply not work at all.
• At cruising speeds 70 MPH - 90 MPH the vehicle will sometimes experience a light "shudder," as though the whole vehicle is shaking forward & backwards.
• At speeds between 90 MPH - 110 MPH the transmission will, at random, occasionally neutralize for a half second, then downshift into 3rd or 2nd gear (not all the time), and then return to OD; this occurs particularly under heavy throttle, during passing or when ascending on a long incline.
• At low speeds, particularly after an extended period of highway driving, the transmission will not shift out of 1st gear.

Based on the research I have done in the factory service manual, it appears that the primary culprits could be as follows: the revolution sensor, located on the driver side of the vehicle and accessible through the wheel well; an issue with the sensor harness or connectors; a TCM failure; or mechanical failure of the parking pawl lock / idler gear, the revolution of which the preceding sensor detects (doesn't seem likely to me - the parking pawl engages fine).

It should be noted that the speedometer is still fully operational and displaying the correct vehicle speed.

Thus I have two questions: besides what is listed above, are there any other systems or components which I should inspect for diagnosis which could set the above DTCs and cause the above issues? Additionally, I had no choice but to drive the vehicle in this condition approximately 2,600 miles across the country due to an emergency situation - could I have caused any mechanical damage to the transmission, what form would this damage take, and how could I go about ascertaining whether any has occurred?

Answer 1

Kudos on a well-written question.

Your deduction on the cause of the problem seems rational. The symptoms indicate that the transmission's control electronics are having a hard time figuring out how fast the car is going and therefore can't decide on the proper gear to use.

I have no personal knowledge of the revolution sensor on your vehicle or how it works, but if it's a Hall effect sensor or a variable reluctance sensor, a magnet is involved. If the transmission is worn and shedding metallic particles, this type of sensor, when exposed to those particles, can give intermittently faulty readings because the magnetic particles in effect short-circuit the magnetic field lines that trigger the sensor, causing the output to fall to some intermediate level between a digital 1 and 0.

If you have a scope and you can monitor the sensor output while the vehicle runs with the drive wheels off the ground, you'll have some idea if the sensor output is within spec or if it's intermittent. (If you do this, your ABS light will come on, but no matter.)

If you can remove the sensor and examine it, you'll know if it's covered with magnetic particles. If it is, cleaning it and re-installing it may solve the problem temporarily, but that would indicate that your trans has wear particles in it so the sensor could become covered in particles again.

You asked about harm? Well downshifting into 2nd at 90 - 110 MPH may not be advisable, but since the car didn't blow up right then and there, perhaps no harm was done.

We love to hear follow-up reports here when repairs are made or vexing problems are solved, so it would be great to hear back with an edit to your question or a comment when you have a final repair. We're also here for additional questions as they pop up.