Hard shift from first to second gear

Hi and thanks for using 2carpros.com

I have to be honest, I don't feel a vacuum leak or u-joint would cause a hard shift. Often times it is related to hydraulic pressure in the transmission or an electrical component such as a faulty shift solenoid. Especially since it only happens from first to second. First, make sure the fluid is clean and full. If you haven't serviced it in a long time, you may have a filter starting to plug lowering fluid pressure. That could cause a hard shift.

Here is a link that discusses different transmission problems. I thought I would add it just in case there was something there you may recognize as a symptom you are experiencing.
https://www.2carpros.com/articles/automatic-transmission-problems

Next, I want you to check for trouble codes. Your truck has a OBD1 diagnostic system. Here is a link that shows how to find codes. Do this and let me know if there are codes.

https://www.2carpros.com/articles/buick-cadillac-chevy-gmc-oldsmobile-pontiac-gm-1983-1995-obd1-code-definitions-and-retrieval-method

Let me know what you find or if you have other questions.

Take care,
Joe

Hi again.

The knock sensor is mounted on one of three locations. It will be on the engine block. I attached a picture of the three locations. Now, the knock sensor is designed to recognize engine detonation. Basically, if there is valve ping, it changes the ignition timing to prevent the ping. Honestly, I do not feel that has anything to do with the transmission. The same with the o2 sensor.

Since the fluid is clean and full, I have a feeling it either has to do with line pressure or the 1-2 shift solenoid is bad. Picture 2 indicates where it is located on a 4L60E Automatic trans.

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Here are the directions for checking line pressure on a 4L60E transmission, which I think you have.
Pictures 3 and 4 correlate with this test.

LINE PRESSURE CHECK
1. Prior to beginning line pressure check, check the following:

a. Transmission fluid level.

B. TV cable adjustment.

C. Outside manual linkage.

D. Engine tune.

2. Connect a tachometer to engine and install oil pressure gauge as shown.
3. When checking minimum TV line pressure, apply brakes, then take line pressure readings in gear range and engine RPM indicated in chart.

4. When checking full TV line pressure, tie or hold TV cable to its full extent of travel and apply brakes. Take line pressure readings in gear range and engine RPM indicated in chart.

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If you have the 4L80E trans, here are the directions for testing line pressure. Please note the time limitation for the actual test listed in the directions. It requires a scanner and these ones for the OBD1 systems are getting harder to find. Picture 5 correlates with these directions.

LINE PRESSURE CHECK
WARNING:Brakes must be applied at all times during this test to prevent any unexpected vehicle motion. Total test running time should not exceed two minutes, or transmission damage could occur.

1. Connect Tech 1 scan tool or equivalent to vehicle ALDL connector.

2. Connect suitable oil pressure gauge to line pressure tap.

3. Start engine and ensure transmission is at normal operating temperature and fluid level is full.

4. Check for trouble codes and repair if necessary.

5. With gear selector in Park, access "Override Force Motor" test on the scan tool, then increase force motor current in 0.1 amp increments (allowing five seconds for pressure to stabilize) and note pressure reading. Compare results with pressure chart. Line pressure will pulse either high or low every 10 seconds to keep force motor plunger free. This is normal and will not harm transmission.

6. If pressure readings differ greatly from pressure chart or if pressure is not within 35 - 177 psi in drive, park or neutral ranges and 67 - 324 psi in reverse range, refer to "Troubleshooting."

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The last picture I attached is the location of the 1-2 shift solenoid for the 4L80E transmission.

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Because your truck is a 1/2 ton V6, I feel it has the smaller trans. However, I added info on both. There is one other thing I would like to add. Way back in the day, (1996) there was a technical service bulletin (TSB) that was related to replacement of the shift solenoids. If I recall, it dealt with a rough shift. However, I don't think it was only related to the 1-2 shift. I did a lot of digging and found the TSB. I thought I would add it in case you wanted to read through it. It was related to the 4L80E transmission, which I question you have. The last picture I attached correlates with this TSB.

A/T - Shift Solenoid Update
Vehicle Powertrain Management Transmission Control Systems Actuators and Solenoids - Transmission and Drivetrain Actuators and Solenoids - A/T Shift Solenoid Technical Service Bulletins A/T - Shift Solenoid Update
A/T - SHIFT SOLENOID UPDATE
File In Section: 7 Transmission

Bulletin No: 37-71-48A

Date: November, 1996

INFORMATION

Subject:
Transmission Product Updates
(New Shift Solenoids)

Models:
1991-97 Chevrolet and GMC C/K, P, G Models
1991 Chevrolet and GMC R/V Models with HYDRA-MATIC 4L8O-E Transmission (RPO MT1)

This bulletin is being revised to update the part numbers and to include 1995-97 model years. Please discard Corporate Bulletin Number 377148 (Section 7 - Transmission).

Service Information

Starting August 14, 1996 (Julian date 226) all HYDRA-MATIC 4L80-E transmissions are being built with the new solenoid assemblies. The new solenoids are serviced for all 1991 through 1997 4L80-E applications.

Parts Information

Previous Part New Part Description 10478125 10478132 Valve assembly, 1-2 Shift Solenoid (Black) 10478126 10478133 Valve assembly, 2-3 Shift Solenoid (White)

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I hope this is helpful. I realize it is a lot of information. I would have already replaced the 1-2 solenoid, but again, I cannot be 100% sure that is the issue. That is why I am trying to give as much information as I can.

Let me know if any of this helps and if you have other questions.

Take care,
Joe

That sounds like an issue with the HVAC blend air door or mode door actuator. Look through this link and see if it correlates with what you are experiencing.

https://www.2carpros.com/articles/replace-blend-door-motor

Let me know.

Joe

That sounds like a blend air or mode door actuator making noise. It could be, however, related to the blower motor. It may be bad.

If you unplug the actuators, you will loose temperature control or air flow direction control. However, if it is the blend or mode door actuator, it should stop.

Let me know which you try and which makes a difference.

Take care,
Joe

Then it sounds like there is an issue with the blower motor. The fan could be hitting something or the motor itself may be bad. Here are the directions for removal and replacement. Just in case you want to dig into it. The attached picture correlates with these directions.

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REMOVE OR DISCONNECT
1. Negative battery cable.
2. Instrument panel compartment.
3. Front screw from right door sill plate.
4. Right hinge pillar trim panel.
5. Electrical connectors at ECM, as necessary.
6. ECM and mounting bracket.
7. Electrical connectors from blower motor, as necessary.
8. Courtesy lamp (if equipped).
9. Bolt from right lower dash support.
10. Blower motor cover.
11. Blower motor cooling tube.
12. Blower motor flange screws.
13. Blower motor.

INSPECT
- Blower motor terminals for distortion. Clean corrosion from terminals or replace motor as necessary.
- Flange of the blower motor for damage or distortion that could cause a leak. Repair as necessary.
- Blower fan for damage or distortion.

INSTALL OR CONNECT
1. Blower motor.
2. Blower motor flange screws.
3. Blower motor cooling tube.
4. Blower motor cover.
5. Bolt to right lower dash support.
6. Courtesy lamp (if equipped).
7. Electrical connectors to blower motor.
8. Mounting bracket and ECM.
9. Electrical connectors to ECM, as necessary.
10. Right hinge pillar trim panel.
11. Front screw into front door sill plate.
12. Instrument panel compartment.
13. Negative battery cable.
- Check circuit operation.

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If you have other questions, let me know. Also, I am basing this on unplugging the blower motor itself has stopped the noise. If that is incorrect, let me know.

Take care,
Joe

Chances are it is one of the actuators then.

Hi again.

The knock sensor is a component mounted on the engine block. Its purpose is to optimize engine performance via ignition timing. Honestly, that really has nothing to do with the transmission. I really feel it is one of the components I mentioned in an earlier post.

Hi and welcome back.

Which sensor are you referring to?

Joe