The 6.5L GM turbo diesel engine, a workhorse in Chevrolet and GMC trucks and SUVs of the mid-1990s, utilized an early form of onboard diagnostics known as OBD-I. While 1998 models and later adopted the more advanced OBD-II system, understanding the OBD-I system for these earlier 6.5 diesels is crucial for diagnosing engine issues. This guide provides a detailed look at the OBD-I diagnostic system used in 1993-1995 6.5L GM turbo diesels, helping you interpret those “Service Engine Soon” light flashes and troubleshoot potential problems. If you’re working on a 1998 GMC 6.5 turbo diesel, please note that it will use the OBD-II system, which is different from what is described here. This article focuses exclusively on the OBD-I system found in 1993-1995 models.
Understanding the OBD-I Diagnostic System on 6.5L GM Diesels
For owners and mechanics working on 1993 to 1995 Chevrolet and GMC vehicles equipped with the 6.5L turbo diesel engine, familiarity with the OBD-I diagnostic system is essential. Introduced in 1993, this first-generation onboard diagnostic system was a significant step forward, although it differs markedly from the OBD-II systems that became standard in later years, including the 1998 models you might be researching.
OBD-I’s primary function is to alert drivers to potential malfunctions, primarily within the engine and transmission systems. When a fault is detected, the system stores a Diagnostic Trouble Code (DTC). In the OBD-I system, these DTCs are simple numeric codes, each corresponding to a specific issue. It’s important to understand that a single engine problem can trigger multiple DTCs, each offering a piece of the diagnostic puzzle. These codes, in conjunction with other diagnostic procedures, empower technicians and DIY mechanics to accurately pinpoint the source of vehicle problems.
On 6.5 diesel vehicles from this era, DTCs can be accessed using a dedicated diagnostic scan tool or, conveniently, by initiating an on-board diagnostic sequence directly within the vehicle, without needing any specialized equipment. Crucially, the “Service Engine Soon” light on your dashboard is your first indicator. If this light illuminates, it signifies that at least one DTC is currently stored in the system and ready to be retrieved. The on-board diagnostic sequence for these 6.5 diesels is straightforward, making it accessible for anyone to check for error codes.
Locating the OBD-I Connector on Your 6.5 GM Diesel
To access the OBD-I system, you’ll need to find the diagnostic connector. For 6.5 GM diesel vehicles of the 1993-1995 model years, this is a 12-pin ALDL (Assembly Line Diagnostic Link) connector. Its standard location is underneath the dashboard, to the left of the steering column.
Figure 1 – Location of the OBD I diagnostic connector under the dashboard of a 1993-1995 GM truck.
However, if you don’t immediately see the connector in this location, it’s worth checking behind the dash. Over time, the mounting tabs for the connector can break, causing it to become detached and tucked away with the wiring harness behind the lower dash panel. A quick search in this area should reveal the 12-pin connector.
OBD-I Connector Pin Configuration
Once you’ve located the ALDL connector, you’ll need to understand its pinout to initiate the diagnostic sequence. The key is to connect terminals “A” and “B”. This process, often called “jumping” the terminals, can be done with any conductive material. A short length of small-gauge copper wire, with its ends stripped, is ideal for creating this connection.
Figure 2 – Pinout diagram illustrating the terminal layout of the 12-pin GM OBD I connector used in 6.5 diesel vehicles.
Reading Diagnostic Trouble Codes (DTCs) on a 6.5 Diesel
Reading DTCs on a 6.5 diesel equipped with OBD-I is a unique process that relies on interpreting flashes from the “Service Engine Soon” (SES) light. After you initiate the diagnostic sequence by jumping terminals A and B, the SES light will begin to flash in a pattern that represents the stored DTCs. This system is somewhat reminiscent of Morse code, where the number of flashes indicates a numerical value within the code.
Short pauses between flashes signify that the system is still within the same digit of a code, while a longer pause indicates the completion of a digit and the start of the next.
Let’s break down how to interpret the flashes:
- Short Pause (-): Separates flashes within a digit.
- Long Pause (—): Separates digits within a code.
- SES: Represents a brief illumination of the Service Engine Soon light.
Example Code Interpretations:
- SES—SES-SES: Indicates one flash, then two flashes, representing the DTC 12.
- SES-SES—SES-SES-SES: Indicates two flashes, then three flashes, representing the DTC 23.
- SES-SES-SES—SES-SES-SES-SES-SES: Indicates three flashes, then five flashes, representing the DTC 35.
Figure 3 – The “Service Engine Soon” light on a 6.5 diesel instrument cluster, your key to reading OBD-I diagnostic trouble codes.
All stored DTCs will be displayed in numerical order and each code will repeat three times before the system moves on to the next stored code. The sequence always begins with DTC 12, which is a system check code confirming that the diagnostic system is functioning correctly. DTC 12 will flash three times at the start of each diagnostic sequence. If no other DTCs are stored in the system’s memory, DTC 12 will continue to repeat indefinitely.
Step-by-Step Guide to Enter Diagnostic Mode and Read Codes:
- Turn Ignition OFF: Ensure the ignition switch is in the “OFF” position.
- Jump Terminals A and B: Using a short piece of wire (16 AWG solid core wire stripped at both ends works well), connect terminals “A” and “B” of the OBD-I connector as shown in Figure 2.
- Turn Ignition to RUN: Switch the ignition switch to the “RUN” position. This will initiate the on-board diagnostic code retrieval sequence. Do not start the engine.
- Observe the SES Light: Carefully watch the “Service Engine Soon” light. It will first flash DTC 12 three times, followed by any other stored DTCs, also repeated three times each.
- Turn Ignition OFF and Disconnect Jumper: Once you have finished reading all the codes, turn the ignition switch back to the “OFF” position and remove the jumper wire from terminals “A” and “B”.
The diagnostic sequence will continue to loop as long as the ignition is in the “RUN” position and terminals A and B remain connected. If you miss a code or need to re-verify a DTC, simply allow the sequence to cycle again – it will always restart with DTC 12. If the SES light does not illuminate at all, double-check that terminals A and B are properly connected. Other potential issues could be a burnt-out SES light bulb or wiring problems in the OBD-I connector circuit.
Key Points to Remember When Reading 6.5 Diesel OBD-I Codes:
- Count Flashes: Keep counting the SES light flashes as long as the pauses between them are short. A long pause signals the end of a digit.
- Code Repetition: Each DTC is displayed three times before proceeding to the next code.
- DTC 12 as Start and Loop: DTC 12 is always displayed three times at the beginning of each sequence and the entire process loops continuously until the ignition is turned off.
- Two-Digit Codes: All OBD-I DTCs for the 6.5 diesel are two-digit codes, ranging from 12 to 99, with no single-digit codes or the number zero. Some numbers within this range are skipped.
- Numerical Order: Codes are always presented in ascending numerical order.
6.5 Diesel OBD-I Trouble Code List
Below is a comprehensive list of OBD-I Diagnostic Trouble Codes for the 1993-1995 6.5L GM Turbo Diesel engine. This list will help you understand the meaning of the codes you retrieve from your vehicle’s diagnostic system.
- 12 – Diagnostic system functioning properly (check code)
- 13 – Engine shutoff solenoid circuit fault
- 14 – Engine coolant temp sensor circuit low (high temp)
- 15 – Engine coolant temp sensor circuit high (low temp)
- 16 – Vehicle speed sensor buffer fault
- 17 – High resolution circuit fault
- 18 – High resolution circuit fault
- 19 – Crankshaft position reference fault
- 21 – Accelerator pedal position 1 circuit high
- 22 – Accelerator pedal position 1 circuit low
- 23 – Accelerator pedal position 1 circuit out-of-range
- 24 – Vehicle speed sensor circuit low
- 25 – Accelerator pedal position 2 circuit high
- 26 – Accelerator pedal position 2 circuit low
- 27 – Accelerator pedal position circuit out-of-range
- 28 – Transmission range pressure switch circuit fault
- 29 – Glow plug relay fault
- 31 – EGR control pressure/barometric sensor circuit low
- 32 – EGR circuit fault
- 33 – EGR control pressure/barometric sensor circuit high
- 34 – Injection timing stepper motor fault
- 35 – Injection pulse width error, short duration (pulse width shorter than desired)
- 36 – Injection pulse width error, long duration (pulse width longer than desired)
- 37 – Torque converter clutch (TCC) brake switch fault, stuck “ON”
- 38 – Torque converter clutch (TCC) brake switch fault, stuck “OFF”
- 39 – Torque converter clutch (TCC) stuck “OFF”
- 41 – Brake switch circuit fault
- 42 – Fuel temperature circuit low (temp high)
- 43 – Fuel temperature circuit high (temp low)
- 44 – EGR pulse width error
- 45 – EGR vent error
- 46 – Malfunction indicator lamp (MIL) circuit fault
- 47 – Intake air temperature sensor circuit low (temp high)
- 48 – Intake air temperature sensor circuit high (temp low)
- 49 – Service throttle soon lamp circuit fault
- 51 – P.R.O.M. error
- 52 – System voltage high, long
- 53 – System voltage high
- 54 – PCM fuel circuit error
- 56 – Injection pump calibration resistor error
- 57 – PCM 5 volt shorted
- 58 – Transmission fluid temperature circuit low (high temp)
- 59 – Transmission fluid temperature circuit high (low temp)
- 61 – Turbocharger boost sensor circuit high
- 62 – Turbocharger boost sensor circuit low
- 63 – Accelerator pedal position 3 circuit high
- 64 – Accelerator pedal position 3 circuit low
- 65 – Accelerator pedal position 3 circuit out-of-range
- 68 – Transmission slip detected
- 69 – Torque converter clutch (TCC) stuck “ON”
- 71 – Cruise control set/coast switch fault
- 72 – Vehicle speed sensor circuit loss
- 73 – Pressure control solenoid circuit error
- 74 – Transmission input speed sensor circuit fault
- 75 – System voltage low
- 76 – Cruise control resume/accelerate switch fault
- 78 – Turbocharger wastegate solenoid fault
- 79 – Transmission fluid over temperature
- 81 – Transmission 2-3 shift solenoid circuit fault
- 82 – Transmission 1-2 shift solenoid circuit fault
- 83 – Torque converter clutch fault
- 84 – Accelerator pedal position circuit fault
- 85 – Transmission gear error
- 86 – Transmission gear ratio error
- 87 – Transmission gear ratio error
- 88 – Top dead center offset error
- 91 – Cylinder balance fault, cylinder 8
- 92 – Cylinder balance fault, cylinder 7
- 93 – Cylinder balance fault, cylinder 6
- 94 – Cylinder balance fault, cylinder 5
- 95 – Cylinder balance fault, cylinder 4
- 96 – Cylinder balance fault, cylinder 3
- 97 – Cylinder balance fault, cylinder 2
- 98 – Cylinder balance fault, cylinder 1
- 99 – Accelerator pedal position sensor reference voltage error (5 volt ref)
Conclusion
While OBD-I is less advanced than today’s OBD-II systems, it served as a valuable diagnostic tool for the 1993-1995 6.5L GM turbo diesel engines. Understanding how to retrieve and interpret these codes by counting the “Service Engine Soon” light flashes is a fundamental skill for anyone maintaining these classic diesel trucks and SUVs. Remember that OBD-I codes are just the starting point in diagnosing powertrain issues. They provide direction for further investigation and testing to effectively resolve any problems.
Explore More 6.5 Diesel Resources:
6.5 GM diesel coolant flush and cooling system service guide
6.5 diesel oil pressure switch/sensor replacement
6.5 diesel fuel pressure test and fuel system troubleshooting guide
6.5 diesel fuel pump (lift pump) replacement procedures
6.5 diesel PMD/FSD relocation and replacement guide
6.5 diesel glow plug system troubleshooting guide
6.5 diesel glow plug replacement procedures
6.5 diesel wastegate solenoid troubleshooting and replacement procedures
