Question about 1991 Chevrolet K1500

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1991 Chev 1500 pickup, 5L, 4 speed o/d automatic transmission shifts roughly, especially at lower gears. Is there still a modulator valve? If not what controls shift timing? Someone told me the trannies then used a governor and no modulator. Something I read said the cable from the throttle body controls shifting. Thanks, James

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  • James Moshonas Oct 13, 2010

    That was a good answer and certainly clarifies a lot of things for me. I will do the 4L60 adjustments as you provided. Figure 3 shows an object toward the rear which looks like the modulator on a Turbo-Hydramatic. Did they use the same photo as for the turbo or is there a module or a governor that also affects shifting?

    The shifter does often fall out of park if the engine is still running so the cable adjustment must be off. The throttle plate seems to stick when first touching the gas; most often when initially coming out of park. While driving it seems to be OK. I removed the throttle body and cleaned the oxide away from the choke plates and the inner body walls. So might this also be associated with the same cables? There are three cables on the throttle body and none have an adjustment there, that I have noticed.

    I am currently out of work and am trying to make this truck last so your help is immensely appreciated. It may have 341,000 kilometres but runs very well.

    Thanks, James

  • James Moshonas Oct 17, 2010

    Yes, I now understand the first answer and why I have to do what the mechanic suggested. This is great.




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Vacuum Modulator -(THM 400 only) REMOVAL & INSTALLATION NOTE: This applies to the THM 400 only.

  1. Raise and support the front end on jackstands.
  2. Disconnect the vacuum line at the modulator.
  3. Remove the screw and retaining clamp. Fig. 1: Vacuum modulator on the THM400 1991 Chev 1500 pickup, 5L, - 84907163.gif

  4. Remove the modulator. Be careful, some fluid may run out. To install:
  5. Install the modulator.
  6. Install the vacuum line. Be careful not to kink the vacuum line during installation. Replace any lost fluid.
The Governor is used in newer transmissions:

HYDRAULIC CONTROL SYSTEM The hydraulic pressure used to operate the servos comes from the main transmission oil pump. This fluid is channeled to the various servos through the shift valves. There is generally a manual shift valve which is operated by the transmission selector lever and an automatic shift valve for each automatic upshift the transmission provides. NOTE: Many new transmissions are electronically controlled. On these models, electrical solenoids are used to better control the hydraulic fluid. Usually, the solenoids are regulated by an electronic control module. There are two pressures which affect the operation of these valves. One is the governor pressure which is effected by vehicle speed. The other is the modulator pressure which is effected by intake manifold vacuum or throttle position. Governor pressure rises with an increase in vehicle speed, and modulator pressure rises as the throttle is opened wider. By responding to these two pressures, the shift valves cause the upshift points to be delayed with increased throttle opening to make the best use of the engine's power output. Most transmissions also make use of an auxiliary circuit for downshifting. This circuit may be actuated by the throttle linkage the vacuum line which actuates the modulator, by a cable or by a solenoid. It applies pressure to a special downshift surface on the shift valve or valves. The transmission modulator also governs the line pressure, used to actuate the servos. In this way, the clutches and bands will be actuated with a force matching the torque output of the engine.
At any rate, if the auto transmission shifts roughly, try to have it flushed out and refilled with new transmission fluid for $100 or so.

If this doesn't solve the problem, the transmission will have to be rebuilt to original specifications, or replaced with a transmission that doesn't have this problem (used or another rebuilt).

Posted on Oct 13, 2010

  • Duane Wong
    Duane Wong Oct 17, 2010

    If the transmission doesn't shift into park very well, then the linkage inside the pan is worn.
    I had one fixed on a 1980 suburban that wouldn't go into park consistently for about $125 a few years back at a transmission shop.


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The Vacuum Modulator applied only in the THM-400

Check this additional info, about...

Shift Linkage

  1. Raise and support the front end on jackstands. Block the rear wheels.
1991 Chev 1500 pickup, 5L, - 9169b7f.jpg

Fig. 1: Shift linkage-THM 400 and THM 700R4 transmissions


Fig. 2: Shift positions-THM 400 and THM 700R4 transmissions

Fig. 3: Shift linkage-4L60E and 4L80E transmissions


Fig. 4: Shift positions-4L60E and 4L80E transmissions

  1. Loosen the shift lever bolt or nut at the transmission lever so that the lever is free to move on the rod.
  2. Set the column shift lever to the neutral gate notch, by rotating it until the shift lever drops into the Neutral gate. Do not use the indicator pointer as a reference to position the shift lever, as this will not be accurate.
  3. Set the transmission lever in the neutral position by moving it clockwise to the park detent, then counterclockwise 2 detents to neutral.
  4. Hold the rod tightly in the swivel and tighten the nut or bolt to 17 ft. lbs. (23 Nm) on all but the 4L80E transmission. On the 4L80E transmission, tighten the bolt to 20 ft. lbs. (30 Nm).
  5. Lower the vehicle.
  6. Move the column shifter to park and check that the engine starts. Check the adjustment by moving the selector to each gear position.

Shift Cable
This procedure applies to 4L60-E and 4L80-E transmissions.
  1. Place the transmission in park and apply the parking brake.
  2. Raise and support the vehicle on jackstands.
  3. On the shift cable end slide the black retaining clip forward just enough to allow the white lock button to be out.
Do not push the white lock button completely out.
  1. Push the white lock button out just far enough to free the metal core adjust body inside the core.
  2. Inspect the core adjust body for dirt or debris that may restrict its travel.
  3. If there are any restrictions found the shift cable end should be washed with soap and water. If after cleaning the cable the travel of the core adjust body is still restricted the shift cable assembly must be replaced.
  4. Lower the vehicle, turn the ignition switch ON and move the transmission lever from park to one-to-ten times.
  5. Place the shift lever in park and turn the ignition switch OFF . Raise and support the vehicle on jackstands.
  6. Make sure the transmission is in mechanical park by rotating the control lever clockwise until it stops.
  7. Push the white lock button in to secure the core adjuster body adjuster and slide the black retainer clip rearward until it covers the white lock button and locks in place over the shift cable end.
  8. Lower the vehicle and turn the ignition switch ON .
  9. Move the shifter through the gear ranges and ensure that the light comes on under the shift column letter when the shifter is positioned under that letter.
  10. Ensure that the engine starts only when the transmission is placed in park or neutral. Adjust the park/neutral switch if necessary. Refer to the park/neutral switch adjustment procedure in this section.
  11. Turn the ignition switch to the LOCK position and ensure that the key can be removed in the park position only.
  12. Release the parking brake, start the engine and check for proper transmission shift operation.

Throttle Valve Cable
This procedure applies to the THM 700R4 transmission.
The adjustment is made at the engine end of the cable with the engine off, by rotating the throttle lever by hand. DO NOT use the accelerator pedal to rotate the throttle lever.
  1. Remove the air cleaner.
  2. Depress and hold down the metal adjusting tab at the end of the cable.
  3. Move the slider until it stops against the fitting.
  4. Release the adjusting tab.
  5. Rotate the throttle lever to the full extent of it travel.
  6. The slider must move towards the lever when the lever is at full travel. Make sure that the cable moves freely.
The cable may appear to function properly with the engine cold. Recheck it with the engine hot.
  1. Road test the truck.

Neutral Start/Back-Up Switch
The procedure is for switches that are mounted on the transmission only.
This adjustment is performed with the switch removed from the vehicle. Neutral position tool J 41364-A or its equivalent is required for this procedure.
  1. Position tool J 41364-A or its equivalent onto the park/neutral switch. make sure that the slots on the switch are lined up with the lower tabs on the tool.
  2. Rotate the tool until the upper locator pin on the tool is lined with the slot on the top of the switch.
Do not remove the tool from the switch.
  1. Install the switch and remove the tool.

This applies to the THM 400 only.
  1. Raise and support the front end on jackstands.
  2. Disconnect the vacuum line at the modulator.
  3. Remove the screw and retaining clamp.

Fig. 1: Vacuum modulator on the THM400

  1. Remove the modulator. Be careful, some fluid may run out.
To install:
  1. Install the modulator.
  2. Install the vacuum line. Be careful not to kink the vacuum line during installation. Replace any lost fluid.
Hope helps, remmeber keep us uodated.

Posted on Oct 13, 2010


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