1996 DeVille 4T80E Transaxle Disengaged as if Shifted Into Neutral – HELP!

[Corvette6769]

I was driving my 1996 DeVille Concours on level grade with cruise control on at 60 mph when, all of the sudden, without warning or any sound, the transmission disengaged and engine raced as if it were shifted into neutral.

Regardless of shift selector position P R N D 3 2 1, the transmission still acts like it is in neutral (not even trying to engage) and the car will not move under its own power (rolls freely in neutral).

It does not even try to engage whatsoever (no noise or slipping).

In Park the car cannot be pushed, so I know neither axle is broken or stripped splines.

Fluid looks/smells fine and has no leaks. I did notice that the fluid level does not change when engine is started.

When the shifter is moved from park, that the doors do not lock as usual.

I read the onboard diagnostic codes and find PC1810 Current (P1810 - TFP Valve Position Switch Circuit).

What would cause this?

[Cadillac Jim]

The 1997 FSM, pages 7-141 through 146, cover DTC P1810. The TFP is the Transmission Fluid Pressure manual valve position switch. There are actually five of them and the combination of which ones are closed tells the PCM what gear the transmission is selected. It's inside the transmission. The wires on the wiring harness re the PNK, DK BLU, and RED wires, which are on terminals N, R and P, respectively on the wiring harness connector at the transmission.

You should see the Service Engine Soon and the Service Transmission warning lights.

The first thing that the PCM says to do is to look at the wiring harness and connector, particularly at the connector. The FSM also says that low fluid will set this DTC.

If DTC P1520 (Transmission Range Switch Circuit) is set, fix that first. If there *are* any other DTCs, let us know.

Moving the shifter will lock and unlock the doors only if the key is on.

[Corvette6769]

Thank you for your reply.

The only DTC showing current are PC1810, IP2711, and RS0062.

DTC does show PC1520 History.

Moving the shifter in and out of Park with engine running has always in the past locked and unlocked the doors, but ever since this problem showed up, in and out of Park does not lock/unlock the doors anymore.

Looking in book 2 of my factory Cadillac service manual (GMP/96-E/K-2) on page 7A-144 Automatic Transaxle - 4T80-E pertaining to "DCT P1810 TFP Valve Position Switch Circuit Malfunction", in addition to "DTC P1810 is a Type A DTC" I see the following and wonder the meaning of the terms "disables" and "inhibits" mean in this context;

Action Taken When the DTC Sets

• The PCM illuminates the Malfunction Indicator Lamp (MIL).
• SERVICE ENGINE SOON, SERVICE TRANSMISSION displays on the Driver Information Center (DIC).
• The PCM substitutes the value of the previous TFP Val. Position Sw. if it is consistent with the Park/Neutral status.
• The PCM commands default line pressure.
• The PCM disables steady state adapts.
• The PCM disables garage shift adapts.
• The PCM inhibits Torque Converter Clutch.
• The PCM inhibits torque management.
• The PCM inhibits garage shift pressure calculation.

[Corvette6769]

Full list of codes:

PC0603 History P0603 - Control Module Long Term Memory Reset - P0603 Internal Control Module Keep Alive Memory (KAM) Error
PC0717 History P0717 - Input Speed Sensor Circuit Low Input - P0717 Input/Turbine Speed Sensor Circuit No Signal (Transmission)
PC1406 History P1406 - EGR Valve Pintle Position Circuit
PC1520 History P1520 - Transmission Range Switch Circuit
PC1579 History P1579 - P/N to D/R at High Throttle Angle - Power Reduction Mode Active
PC1599 History P1599 - Engine Stall or Near Stall Detected
PC1644 History P1644 - Traction Control Delivered Torque Output Circuit
PC1810 Current P1810 - TFP Valve Position Switch Circuit
IP2711 Current PASS-KEY open/shorted key pellet before good key.
No ACM Codes
No SDM Codes
TC0073 History
RS0062 Current Left Rear Position Sensor Fault
No PZM Codes
No IRC Codes
No RFA Codes

[winterset]

Go into the car start it up, and go into diagnostic mode (OFF & WARMER)

At PCM? codes choose yes(up fan)

Look at your hvac display.

When you press the brake pedal, "HI" will illuminate

When the car is in park or neutral "DE FOG" is illuminated

As soon as the car is put in any gear, "DE FOG" is turned off

This is how my working 96 deville reports. Yours should be the same if your shifter is working correctly with the pcm.

[airmike]

I would definitely be checking that wiring harness/connecter.

[KHE]

Is the fluid at the correct level?

If the fluid level is correct and the wiring connector is verified and the problem still exists, it might be a good idea to drop the pan - there might be debris that is causing the TFP valve to stick?

[Corvette6769]

This still has me stumped. Anyone have any other ideas?

[winterset]

Did you try checking if the shifter is working? Using the cars diagnostic setting, and checking the icons on the hvac is the best and easiest check. I mentioned it above, but didn't hear back results.

[Cadillac Jim]

The fact that the doors don't lock/unlock as the shifter goes into and out of Park, with the key on, is a ringer that the switch isn't working. This can be as simple as a loose connector ir actuation cable. If the dash P - R - N - D - 2 - 1 lights aren't following the shifter, then the switch on the shifter isn't moving, isn't working, or it's connector is off or a wire is broken. The code means that the shifter switch and transmission gear signal don't agree.