Safety Warning
DIY auto repair can cause serious injury, fire, or vehicle damage. These guides are for informational purposes only. Always follow OEM torque specs, wear PPE, and consult a certified mechanic if you are unsure. You are solely responsible for your safety.
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HOMESERVICE MANUALSCHEVROLET1997FORWARD CONTROL 5.7 R, AUTOMATICREPAIR AND DIAGNOSIS (SINGLE PAGE)TRANSMISSIONAUTOMATIC TRANSAUTOMATIC TRANSMISSION DIAGNOSIS - 4L80-E (2-DIGIT CODES)DIAGNOSTIC TESTSDTC 59: TRANSMISSION FLUID TEMPERATURE (TFT) SENSOR CIRCUIT HIGHDIAGNOSTIC PROCEDURES
1997 Chevrolet Forward Control 5.7 R, Automatic
Diagnostic Procedures
1997 Chevrolet Forward Control 5.7 R, AutomaticSECTION Diagnostic Procedures
- Ensure fluid level is correct. Connect scan tool to DLC. Turn ignition switch to ON position. DO NOT start engine. Using scan tool, record freeze frame and failure records for reference. Data will be lost when DTCs are cleared later in this test.
- If scan tool displays a TFT sensor voltage more than 4.92 volts, go to next step. If scan tool displays a TFT sensor voltage less than 4.92 volts, see DIAGNOSTIC AIDS.
- If scan tool displays a TFT sensor voltage less than 5.1 volts, go to next step. If scan tool displays a TFT sensor voltage more than 5.1 volts, check for short to voltage in TFT sensor signal circuit. Repair as necessary, then go to step 7). If circuit is okay, check PCM/TCM for faulty or intermittent connections. Repair as necessary, then go to step 7). If connections are okay, replace PCM/TCM, then go to step 7).
- Turn ignition off. Disconnect transmission 20-pin connector. Install Jumper Harness (J-39775) on PCM/TCM side of 20-pin connector. Using a DVOM, measure voltage between ground and terminal "L" of transmission 20-pin connector. See Figure. Turn ignition on. If voltage is 4.9-5.0 volts, go to next step. If voltage is not 4.9-5.0 volts, check for high resistance or open in TFT sensor signal circuit. Repair as necessary, then go to step 7). If circuit is okay, check PCM/TCM for faulty or intermittent connections. Repair as necessary, then go to step 7). If connections are okay, replace PCM/TCM, then go tostep 7).
- Turn ignition off. Connect fused jumper wire between terminals "L" and "M" of transmission 20-pin connector. See Figure. Turn ignition on. If TFT sensor voltage is less than .2 volt, go to next step. If TFT sensor voltage is more than .2 volt, check for open in TFT sensor ground circuit. Repair as necessary, then go to step 7). If circuit is okay, check PCM/TCM for faulty or intermittent connections. Repair as necessary, then go to step 7). If connections are okay, replace PCM/TCM, then go to step 7).
- Turn ignition off. Install Jumper Harness (J-39775) on transmission side of 20-pin connector. Using a DVOM, measure resistance between terminals "L" and "M" of transmission 20-pin connector. See Figure. If resistance is 16 k/ohms at 50°F (10°C) to 133 ohms at 230°F (110°C), see DIAGNOSTIC AIDS. If resistance is not 16 k/ohms at 50°F (10°C) to 133 ohms at 230°F (110°C), remove transmission oil pan. Check transmission wiring harness for open in TFT sensor signal and ground circuits. Repair or replace transmission wiring harness as necessary, then go to next step. If wiring harness is okay, see DIAGNOSTIC AIDS.
- After repair is complete, select DTC on scan tool. Select "Clear Info" function. Select "Specific DTC" and enter DTC 59. Ensure TFT sensor indicates a voltage less than 4.92 volts for 2 seconds. If DTC 59 is not present, repair is complete. If DTC 59 is still present, repeat test.
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When to See a Mechanic
Stop DIY work and contact a certified mechanic immediately if any of the following apply:
- • You smell fuel, burning insulation, or see smoke.
- • Brakes feel soft, pull hard to one side, or make grinding noises.
- • The engine overheats, stalls repeatedly, or misfires under load.
- • You are missing required tools, torque specs, or safe lifting equipment.
- • You are not confident in the next step or safety outcome.