GM Buick/Oldsmobile/Pontiac/Full-Size 1975-1990 Repair Guide

Gasoline Engine Adjustments



Turbo Hydra-Matic 250

Only the 250 series has an externally adjustable band; the procedure is covered below.

The intermediate band must be adjusted with every required fluid change or whenever there is slippage.

  1. Position the shift lever in NEUTRAL.
  3. Loosen the locknut on the right side of the transmission. Tighten the adjusting screw to 30 inch lbs. (3 Nm).


See Figure 1

  1. Loosen the clamp spring screw on the shift linkage clamp. This is the screw which will slide up and down the rod coming down from the steering column once it is loosened.
  3. Set the lever on the transmission into NEUTRAL by moving it counterclockwise to the L1 detent, then clockwise the correct number of detent positions to NEUTRAL. With three speed automatics, move the lever three positions to NEUTRAL ; with four speed overdrive units, move it four positions to NEUTRAL.
  5. Place the transmission selector lever (in the vehicle) in NEUTRAL as determined by the stop in the steering column. DO NOT Use the indicator pointer for reference.
  7. Hold the clamp (into which the clamp spring screw is threaded) flush against the equalizer shaft lever and then tighten the shift linkage screw just finger tight. You must not exert any force in either direction on the rod or equalizer lever as you tighten the screw. Once the screw is hand tight and its position is fixed, torque it to 21 ft. lbs. (29 Nm).
  9. Check that the key cannot be removed and that the steering wheel is not locked with the key in Run and the transmission in REVERSE . Check that the key can be removed and the transmission linkage is locked when the key is in Lock and the transmission is in PARK . Be sure the vehicle will start only in PARK and NEUTRAL . If it starts in any gear, the neutral start switch must be adjusted. Start the engine and check for proper shifting in all ranges.

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Fig. Fig. 1: Exploded view of a shift linkage assembly; all models similar


See Figures 2 and 3

200-4R and 700-4R Models
  1. Make sure the ignition switch is off. Depress and hold the metal readjustment tab all the way down. This is located on the engine end of the Throttle Valve cable. Move the slider until it is located directly against the fitting, and then release the adjustment tab.
  3. Rotate the throttle lever to give wide open throttle position. Watch the slider as you do this; it must move toward the lever as you rotate the throttle. If the slider works properly, this will complete the adjustment.

After the adjustment, make sure the cable works freely after the engine has warmed to operating temperature.

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Fig. Fig. 2: Throttle valve cable adjustment; 200-4R and 700-4R models

All Other Models
  1. With a small screwdriver, pry gently on the bottom of the snap lock to release the detent cable.
  3. Push the throttle lever to the wide open throttle position (engine off) and hold. Push the snaplock tab downward until flush with the cable.

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Fig. Fig. 3: Throttle valve cable adjustment; snap lock type


See Figure 4

Turbo Hydra-Matic 400

All the General Motors divisions covered in this guide use the same detent switch on their THM 400 equipped models. The switch is mounted on the accelerator pedal bracket and is for all intents and purposes, self-adjusting. If a new switch is installed, a preliminary adjustment should be performed according to the accompanying illustration.

  1. Perform the adjustment with the engine off.
  3. Push the plunger of the downshift switch forward until it is flush with the switch housing.
  5. Push the accelerator pedal to the wide open throttle position to set the switch.

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Fig. Fig. 4: Detent switch adjustment