Shifting problem when starting off from a start

Check for broken motor mount, trans linkage is mounted at some point to the engine and/or trans. When engine moves your linkage adjustment will change.

Hi marco check your fuses especialy the stop light fuse do your reae brake light work

IDLE Sentsor

Check the oil level in the transmission for one. run a fault code check as well

possable bad brake lite switch. also check fuses.

there is a limp mode but I doubt if that is the problem
It indicates that the torque converter clutch servo/solenoid is faulty
run the fault codes to confirm if that is it or a faulty TCM
check wiring around the transmission

The clutch is your problem as it is not lifting enough to disengage when the engine is running .It may be an adjustment (if cable clutch) or if its hydraulic check for leaks ,also check to see how much the slave cylinder is moving when somebody presses the pedal (engine not running).If the clutch fork is moving enough then you are going to need a new clutch.

There is a switch called the neutral lockout switch that has probably failed and needs to be replaced. This switch keeps you from being able to start the car in park. Try putting the transmission in neutral and see if it will start.

Use like reference guide for GM Cadillac 1967-1989 Repair Guide Neutral Safety Switch - REMOVAL & INSTALLATION

All Except 1985-89 Eldorado and Seville
See Figure 1
The neutral safety switch prevents the engine from being started in any transmission position except Neutral or Park. The switch is located on the upper side of the steering column under the instrument panel on column shift cars, and inside the shift console on floor shift models.

1. Remove the console for access on floor shift models.
2. Disconnect the electrical connectors.
3. Remove the neutral switch.

To install:

1. Place 1967-70 column shift lever models in Drive, and 1971 and later models in neutral. Locate the lever tang against the transmission selector plate on column shift models.
2. Align the slot in the contact support with the hole in the switch. Insert a 3 / 32 in. (2.38mm) diameter pin in place. The switch is now aligned in Drive position.

1973 and later neutral safety switches have a shear-pin installed to aid in proper switch alignment so that insertion of a pin is unnecessary. Moving the shift lever from Neutral shears the pin.

1. Place the contact support drive slot over the drive tang. Install the switch mounting screws.
2. Remove the aligning pin. Connect the electrical wiring, and replace the console.
3. Set the parking brake and hold your foot on the service brake pedal. Check to see that the engine will start only in Park or Neutral.

Fig. 1: Installation of a common neutral safety switch



ADJUSTMENT
See Figure 2
1974 and later cars do not have a neutral safety switch. Instead, these cars have an interlock between the lock and the transmission selector, which is non-adjustable.

1. Place the shift lever in Neutral.
2. Move the switch until you can insert a gauge pin, 3 / 32 in. (2.38mm) diameter, into the hole in the switch and through to the alignment hole.
3. Loosen the switch securing screws. Remove the console first, if necessary.
4. Tighten the screws and remove the pin.
5. Step on the brake pedal and check to see that the engine will only start in Neutral or Park.
   

Fig. 2: Adjustment of a common neutral safety switch


1985-89 Eldorado and Seville
See Figure 3

All neutral safety/back-up light switches come with a small plastic alignment pin installed. Leave this pin in place.

1. Place the shifter assembly in the Neutral position.
2. Remove the shifter lever-to-switch nut and the lever.
3. Disconnect the electrical connector from the neutral safety/back-up light switch.
4. Remove the neutral safety/back-up light switch-to-transaxle bolts and the switch from the vehicle.

To install:

1. Position the shifter shaft in the Neutral position.

If using an old switch or the plastic pin (new switch) is broken, install a3/32in. (2.38mm) diameter pin gauge (drill bit) into the neutral safety/back-up light switch; the switch is then locked into its Neutral position.

1. Align the flats of the shifter shaft and the neutral safety/back-up light, then, align the switch-to-tang on the transaxle. Torque the switch-to-transaxle bolts to 22 ft. lbs. (30 Nm). Remove the pin gauge.
2. To complete installation, reverse the removal procedures. Make sure that the engine starts only in the Park and Neutral positions.
   

Fig. 3: Neutral start and back-up lamp switch - 1985-89 Eldorado and Seville

Hope help with this (remember rated and comment this). Good luck.

Stuck in Park?
Check to see if the stop lights are working - shift interlock should be the same circuit Check stop light fuse
Try turning the key to 1st position (Unlock)
Shift to neutral to start vehicle and then shift to Drive