Where is the transmission control solenoid on a 1998 cavalier located

The 1-2 shift solenoid valve on a Saturn Vue 3.6 is typically located in the transmission. The exact location of the solenoid valve can vary depending on the year and model of the vehicle, but it is commonly found inside the transmission's valve body. The valve body is the area of the transmission that houses the shift valves and controls the flow of transmission fluid to control the gear shifts.
It's important to note that accessing and replacing a shift solenoid valve on a transmission can be a difficult and complex task that requires special tools and knowledge. It is not recommended to attempt this repair unless you have experience working on transmissions and have the proper tools and equipment. It's also important to consult the Saturn Vue 3.6 service manual or a professional mechanic or transmission specialist to have the proper steps and safety procedures to follow.

What are you talking about auto shift control solenoid ? Which transmission does yours have ?
Automatic Transaxle - 3T40 Automatic Transaxle - 4T40-E
1-2 Shift Solenoid Valve
In transmission valve body
Transmission Component Location
Automatic Transmission Internal Connector End Views
2-3 Shift Solenoid Valve
In transmission valve body
Is there a DTC - diagnostic trouble code stored ? Which gear or gears are not working ?

It sounds like the kick down or the solenoid Inside the tranny

Here is what the service manual says about the EPC solenoid, the processor looks at allot of different inputs to command the solenoids, such as Throttle Position Sensor, Vehicle Speed Sensors, Transmission Range Sensor and RPM signal, The PCM controls the shift patterns for the transmission. Electronic Pressure Control (EPC) Solenoid
The electronic pressure control (EPC) solenoid is located on the main control valve body and is controlled by the PCM. The EPC controls line pressure used to apply and release clutches and bands in the transmission.
The 2-3 shift solenoid is located on the main control valve body and is switched on and off by electrical signals from the PCM. The 2-3 shift solenoid activates the 2-3 shift valve which controls the automatic shifting between second and third gear. Second gear is established when the 2-3 shift solenoid is switched on. The 2-3 shift solenoid is switched off when vehicle speed increases and requires a second to third gear upshift.
The 3-4 shift solenoid is located on the main control valve body and is switched on and off by electrical signals from the PCM. The 3-4 shift solenoid activates the 3-4 shift valve which controls the automatic shifting between third and fourth gear. Third gear is established when the 3-4 solenoid is switched off. The 3-4 shift solenoid is switched on when vehicle speed increases and requires a third to fourth gear upshift.

Check Engine Light Codes and Meanings
Long blinks Short blinks Problem
1 1 Camshaft position sensor
1 2 Mass air flow sensor
1 3 Engine coolant temperature sensor
1 4 Speed sensor
1 11 EVAP control system (flow)
1 14 Fuel injection system right bank (too rich)
1 15 Fuel injection system right bank (too lean)
2 1 Ignition circuit
2 3 Closed throttle position switch
2 5 Idle speed control
2 9 Fuel Injection system left bank (too rich)
2 10 Fuel Injection system left bank (too lean)
3 1 ECU
3 2 Exhaust gas recirculation system flow problem
3 3 Front left O2 sensor
3 4 Knock sensor
3 5 Exhaust gas recirculation temperature sensor
3 6 EGRC-BPT valve
3 7 Closed loop control
3 8 Closed loop control
4 1 Air intake temperature sensor
4 2 Fuel tank temperature sensor
4 3 Throttle position sensor
4 7 Crankshaft position sensor
5 3 Front right O2 sensor
5 4 Automatic transmission control unit
6 3 Cylinder #6 misfire
6 4 Cylinder #5 misfire
6 5 Cylinder #4 misfire
6 6 Cylinder #3 misfire
6 7 Cylinder #2 misfire
6 8 Cylinder #1 misfire
7 1 Multiple cylinder misfire
7 2 Three way catalytic converter (right side)
7 3 Three way catalytic converter (left side)
7 4 Evaporative emission control system pressure sensor
7 5 Evaporative emission control system
7 7 Rear O2 sensor
8 1 Vacuum cut valve bypass valve
8 2 Crank position sensor
8 3 Absolute pressure sensor
8 4 Automatic transmission control unit
8 6 Evaporative emission canister purge control solenoid valve
8 7 Evaporative emission Canister purge control solenoid valve circuit
9 1 Front Right O2 sensor
9 3 Evaporative emission Canister vent control valve
9 5 Crankshaft position sensor
9 8 Engine coolant temperature sensor
10 1 Front left O2 sensor
10 3 Park and Neutral position switch
10 5 EGRC solenoid valve
10 8 Evaporative emission canister purge volume control valve
11 1 Inhibitor switch
11 2 Revolution sensor
11 3 Improper shifting to first gear (Automatic transmission)
11 4 Improper shifting to second gear (Automatic transmission)
11 5 Improper shifting to third gear (Automatic transmission)
11 6 Improper shifting to fourth gear (Automatic transmission)
11 7 Solenoid valve lock up function
12 1 Shift solenoid B (Automatic transmission)
12 3 Overrun clutch solenoid valve (Automatic transmission)
12 4 Torque converter clutch solenoid valve (Automatic transmission)
12 5 Line pressure solenoid valve (Automatic transmission)
12 6 Throttle position sensor (Automatic transmission)
12 7 Engine speed signal
12 8 Fluid temperature sensor
13 2 MAP/BARO Switch Solenoid Valve
13 5 Fuel pump control module
13 8 Cooling fan

Manual Transmission- 4.1 pints

Automatic- 3 speed THM125C--9 qts.
Automatic- 4 speed 4T40-E--12.9 qts

9.5 quarts
The check hole is located near the oil drain plug.

it should be around close to your battery or the alternator if not then it is by your starter depending on its position determines the difficulty of replacement! good luck!

You need to remove the filter.(It's the bid flat metal piece that you see with 1 or 2 screws holding on)

Below the link for Chrysler/Dodge auto-transmission manual
http://www.scribd.com/doc/7095618/545rfe-Transmission-Manual

Solenoid mounts outside the transmission, so you do not need to remove the transmission to install. It should be located between fuse box and air filter housing on the top of transaxle case and right next to transmission controle module.
I hope his step by step procedure will work for you. It is not hard at all to replace the solenoid. There are three 10mm bolts. You will also need to remove the input speed sensor. One thing to look out for is not bending the gasket when installing the new solenoid:

1. Disconnect battery negative cable.
2. Remove air cleaner assembly.
3. Remove electrical connectors for transmission control module (please refer the manual I send you and autozone.com website).
4. Remove the Transmission Control Module
5. Disconnect and remove the input speed sensor
6. Disconnect the transmission oil cooler lines. Cap off hoses and fittings to prevent foreign matter intrusion.
7. Disconnect the solenoid/pressure switch assembly connector.
8. Remove the three solenoid/pressure switch assembly-to-transaxle case bolts
9. Remove solenoid/pressure switch assembly and gasket

Keep everything clean to prevent foreign materials to go inside

1. Install solenoid/pressure switch assembly to case using a new gasket
2. Install bolts and torque to 13 N·m (110 in. lbs.)
3. Install 8-way connector and torque screw to 4 N·m (35 in. lbs.)
4. Uncap and install transmission oil cooler lines.
5. Install input speed sensor and torque to 27 N·m (20 ft. lbs.)
6. Install Transmission Control Module
7. Install air cleaner assembly.
8. Connect battery negative cable.

P.s. 1. You can fine some clips on YouTube.com at least for similar cars
2. Also at Autozone.com you can find the component location for your car and repair guide
3. Also there is a link for Mitchel1 online manual useb by US mechanic 1 week subscribtion is
11.99 , 1 month 16.99
http://www.2carpros.com/kpages/auto_repair_manuals.htm
Let me know if you need more info
Good Luck and happy fixing