Decoding Your 1994 Jeep Grand Cherokee OBD2 System: A Comprehensive Guide

As a proud owner of a 1994 Jeep Grand Cherokee, understanding its diagnostic systems is crucial for maintenance and repair. You might be wondering about OBD2 and how it applies to your classic SUV. This guide will delve into the specifics of the 1994 Jeep Grand Cherokee’s On-Board Diagnostics (OBD) system, focusing on how to retrieve and interpret diagnostic trouble codes (DTCs). Whether you’re a seasoned mechanic or a DIY enthusiast, this information will empower you to troubleshoot issues and keep your Jeep running smoothly.

Understanding OBD & OBDII for Your 1994 Jeep Grand Cherokee

The year 1994 was a transitional period for automotive diagnostics. While the OBD-II standard was being finalized and implemented across the industry, some 1994 vehicles, including certain Jeep models, may feature a pre-OBD-II system, often referred to as OBD-I or OBD 1. It’s essential to determine which system is in your 1994 Jeep Grand Cherokee to accurately diagnose any check engine light or performance issues.

Key Differences Between OBDI and OBDII for a 1994 Jeep Grand Cherokee:

Diagnostic Connector: OBD-I systems typically use proprietary connectors, while OBD-II mandates a standardized 16-pin Diagnostic Link Connector (DLC). Locate the connector under your dashboard. A 16-pin trapezoidal connector suggests OBD-II compatibility. Older, non-standard connectors indicate an OBDI system.
Code Retrieval Method: OBD-I often requires specific “key trick” methods to flash codes via the check engine light, whereas OBD-II systems are designed to communicate with external scan tools.
Trouble Code Format: OBD-I codes are typically two-digit numbers. OBD-II codes are five-digit alphanumeric codes, starting with a letter (P, C, B, or U) followed by four numbers.
Data Parameters: OBD-II provides a significantly wider range of live data parameters and diagnostic information compared to OBD-I.

Is My 1994 Jeep Grand Cherokee Obd2 Compliant?

While the OBD-II standard became mandatory for all cars sold in the US in 1996, some manufacturers started incorporating OBD-II features earlier. It’s possible that a late production 1994 Jeep Grand Cherokee might have some OBD-II components or even be fully OBD-II compliant. However, it’s more likely that a 1994 model will utilize an enhanced OBD-I system, bridging the gap to OBD-II but not fully conforming to the later standard.

Identifying Your System:

Visual Inspection: Check for the 16-pin DLC connector under the dash. Its presence is a strong indicator of OBD-II style diagnostics, but not a guarantee of full OBD-II compliance for a 1994 model year.
Owner’s Manual: Consult your 1994 Jeep Grand Cherokee owner’s manual for information on diagnostics and error code retrieval.
Test Both Retrieval Methods: Try both the OBDI “key trick” and an OBD-II scanner to see which method yields diagnostic codes.

Retrieving Diagnostic Codes on Your 1994 Grand Cherokee

Depending on whether your 1994 Jeep Grand Cherokee utilizes an OBDI-style system or a more advanced pre-OBDII setup, the code retrieval method will differ.

The “Key Trick” for OBDI Style Code Retrieval

This method, often referred to as the “key trick,” is designed for older systems and might work if your 1994 Jeep Grand Cherokee uses an OBDI-like diagnostic system.

Procedure:

Turn the ignition OFF. Ensure the engine is not running and the key is in the off position.
Within five seconds, cycle the ignition key between the ON and OFF positions three times, ending in the ON position. Specifically: ON, OFF, ON, OFF, ON. (Important: “ON” is the position just before starting the engine, and “OFF” is not “LOCK,” simply turned off).
Observe the “Check Engine” light. The check engine light will begin to flash, indicating diagnostic trouble codes.
Count the flashes. OBDI codes are typically two-digit codes.
- For example, code 23 would be indicated by: FLASH, FLASH (pause), FLASH, FLASH, FLASH (longer pause).
- A code 55 (FLASH, FLASH, FLASH, FLASH, FLASH – longer pause) indicates the “end of codes” and is normal. It signifies that the system has finished displaying all stored codes.
- Note: On older systems, the blink spacing might not be perfectly uniform. If you see repeating codes like “23 23,” it’s likely just a single “55” end-of-codes signal. Codes are generally not repeated.

Using an OBDII Scanner on a 1994 Jeep Grand Cherokee

If your 1994 Jeep Grand Cherokee has a 16-pin DLC connector, you can attempt to use an OBDII scanner. Modern OBD2 scanners are readily available and offer a more user-friendly way to read codes and access live data.

Procedure:

Locate the DLC connector. It’s usually under the dashboard on the driver’s side.
Plug in the OBDII scanner. Connect the scanner to the DLC connector securely.
Turn the ignition ON. Turn the key to the “ON” position (engine off) to power up the vehicle’s electrical system and the scanner.
Follow the scanner’s instructions. Each scanner may have slightly different menus and procedures, but generally, you will need to:
- Select “Read Codes” or “Diagnostic Codes.”
- The scanner will communicate with the vehicle’s computer and display any stored trouble codes.
Record the codes. Note down the codes displayed by the scanner. They will likely be five-digit alphanumeric OBD-II codes.

Troubleshooting Scanner Issues:

Scanner Not Connecting: If the scanner fails to connect, ensure the ignition is ON, the scanner is properly plugged in, and you’ve selected the correct vehicle protocol on the scanner (if applicable). It’s also possible that your 1994 Jeep Grand Cherokee is not fully OBD-II compliant and may not communicate with all standard OBD-II scanners.
Try the OBDI Method: If the OBDII scanner doesn’t work, revert to the OBDI “key trick” method to see if that retrieves codes.

1994 Jeep Grand Cherokee OBDI Trouble Codes (Possible)

If your 1994 Jeep Grand Cherokee is using an OBDI-style system, these are some of the potential trouble codes you might encounter. Disclaimer: These codes are based on general Chrysler OBDI codes and may not be entirely specific to the 1994 Jeep Grand Cherokee. Always consult a repair manual specific to your vehicle for the most accurate information.

OBDI Codes and Meanings:

11: No ignition reference signal detected during cranking OR timing belt skipped one or more teeth; OR loss of either camshaft or crankshaft position sensor.
12: Battery or computer recently disconnected. (This code is often normal if the battery has been disconnected recently).
13: MAP sensor or vacuum line may not be working.
14: MAP sensor voltage below .16V or over 4.96V.
15: No speed/distance sensor signal.
16: Loss of battery voltage detected with engine running.
17: Engine stays cool too long (bad thermostat or coolant sensor?).
21: Oxygen sensor signal doesn’t change (stays at 4.3-4.5V). Probably bad oxygen sensor.
22: Coolant sensor signal out of range – May have been disconnected to set timing.
23: Incoming air temperature sensor may be bad.
24: Throttle position sensor over 4.96V.
25: Automatic Idle Speed (AIS) motor driver circuit shorted or target idle not reached.
26: Peak injector circuit voltage has not been reached (need to check computer signals, voltage reg, injectors).
27: Injector circuit isn’t switching when it’s told to (TBI) OR (MPI) injector circuit #1 not switching right OR (turbo) injector circuit #2 not switching right OR (all 1990-) injector output driver not responding – check computer, connections.
31: Bad evaporator purge solenoid circuit or driver.
32: EGR gases not working (check vacuum, valve) OR computer didn’t see change in air/fuel ratio when EGR activated – check valve, vacuum lines, and EGR electrical.
33: Air conditioning clutch relay circuit open or shorted (may be in the wide-open-throttle cutoff circuit or normal on early 80’s models if you don’t have air conditioning).
34: Speed control shorted or open.
35: Cooling fan relay circuit open or shorted OR idle switch motor fault – check connections.
36: (turbo only) Wastegate control circuit open or shorted.
37: Shift indicator light failure, 5-speed OR part throttle lock/unlock solenoid driver circuit OR solenoid coil circuit OR Trans temperature sensor voltage low (1995 and on).
41: Alternator field control circuit open or shorted.
42: Automatic shutdown relay circuit open or shorted OR Fuel pump relay control circuit OR Fuel level unit – no change over miles OR Z1 voltage missing when autoshutdown circuit energized.
43: Peak primary coil current not achieved with max dwell time OR Cylinder misfire OR Problem in power module to logic module interface.
44: No FJ2 voltage present at logic board OR Logic module self-diagnostics indicate problem OR Battery temperature out of range.
45: Turbo boost limit exceeded (engine was shut down by logic module).
46: Battery voltage too high during charging or charging system voltage too low.
47: Battery voltage too low and alternator output too low.
51: Oxygen sensor stuck at lean position (this may be tripped by a bad MAP sensor system causing a rich condition). OR Internal logic module fault (’84 turbo only).
52: Oxygen sensor stuck at rich position OR Internal logic module fault (’84 turbo only).
53: Logic module internal problem.
54: No sync pickup signal during engine rotation (turbo only) OR Internal logic module fault (’84 turbo only) – or camshaft sensor/distributor timing.
55: End of codes (Normal – not an error).
61: “Baro” sensor open or shorted.
62: EMR mileage cannot be stored in EEPROM OR PCM failure SRI mile not stored.
63: Controller cannot write to EEPROM.
64: Catalytic converter efficiency failure.
65: Power steering switch failure.
66: No CCD Message From TCM and/or No CCD Message From BCM.

1994 Jeep Grand Cherokee OBDII Trouble Codes (Possible)

If your 1994 Jeep Grand Cherokee is indeed equipped with a more advanced system that resembles OBD-II, you might encounter standard OBD-II codes. These are five-digit alphanumeric codes. Here is a partial list of potential OBD-II codes. Again, consult a specific repair manual for your 1994 Jeep Grand Cherokee for the most accurate code definitions.

OBDII Codes and Meanings (Partial List):

P0300-P0308: Misfire detected in multiple/specific cylinders.
P0106-P0108: MAP sensor voltage range/performance issues.
P0112-P0113: Intake air temperature sensor voltage low/high.
P0117-P0118: Engine coolant temperature sensor voltage low/high.
P0122-P0123: Throttle position sensor voltage low/high.
P0130-P0160: O2 Sensor Circuit Malfunction (Bank 1/Bank 2, Sensor 1/Sensor 2/Sensor 3). Various O2 sensor circuit issues including low/high voltage, slow response, no activity, heater circuit malfunction.
P0171 & P0174: System too Lean (Bank 1 & Bank 2).
P0172 & P0175: System too Rich (Bank 1 & Bank 2).
P0201-P0208: Injector #1-#8 control circuit problem.
P0320: Crankshaft position sensor reference signal issue.
P0325 & P0330: Knock sensor signal issues.
P0340: No camshaft signal.
P0401: EGR flow insufficient detected.
P0403: EGR solenoid control circuit problem.
P0420 & P0432: Catalyst system efficiency below threshold (Bank 1 & Bank 2).
P0441: Evaporative Emission Control System Incorrect Purge Flow.
P0442 & P0455 & P0456: Evaporative Emission Control System Leak Detected (Small/Large Leak).
P0500: Vehicle Speed Sensor Malfunction.
P0505: Idle Air Control System Malfunction.
P0700: Transmission Control System Malfunction (Requires further diagnosis via transmission controller).
P1192 & P1193: Inlet air temperature sensor voltage wrong.
P1194-P1197: Oxygen sensor heater/slow response issues (Bank 1 & Bank 2).
P1198 & P1199: Radiator coolant temperature sensor voltage wrong.
P1281: Engine staying cold too long (thermostat issue).
P1282: Fuel pump relay circuit problem.
P1294: Cannot reach target engine speed (vacuum leaks, idle speed motor issues).
P1299: MAP/TPS signals don’t match (vacuum leak).
P1388 & P1389: Auto shutdown relay circuit problems.
P1390 & P1391: Cam/Crank sensor signal mismatch/intermittent loss.
P1403: EGR position sensor voltage issue.
P1491: Radiator fan control circuit problem.
P1492 & P1493: Ambient/Battery temperature sensor voltage wrong.
P1494 & P1495: Leak Detection Pump (LDP) pressure switch problems.

Using an OBD2 Scanner for Enhanced Diagnostics

Regardless of whether your 1994 Jeep Grand Cherokee is strictly OBDI or has some OBD-II features, using a modern OBD2 scanner is highly recommended for more in-depth diagnostics.

Benefits of Using an OBD2 Scanner:

Read and Clear Codes: Scanners provide a clear readout of DTCs and allow you to clear codes after repairs are made.
Live Data Streaming: Many scanners can display real-time data from various sensors, helping you pinpoint intermittent problems or monitor engine performance. This data can include engine temperature, RPM, sensor voltages, fuel trims, and more.
Enhanced Code Definitions: OBD2 scanners often provide more detailed descriptions of the trouble codes compared to generic OBDI code lists.
Advanced Functions: Some scanners offer advanced functions like freeze frame data (snapshot of sensor readings when a code was triggered), O2 sensor testing, and EVAP system testing.

Choosing an OBD2 Scanner:

For a 1994 Jeep Grand Cherokee, a basic OBD2 scanner that can read and clear codes and display live data is sufficient for most DIY needs. Consider these features:

Compatibility: Ensure the scanner supports OBD-II protocols (and ideally, has some backward compatibility or Chrysler-specific modes if your Jeep is borderline OBD-II).
Ease of Use: Look for a scanner with a clear display and intuitive menus.
Features: Decide if you need advanced features like live data, graphing, or bidirectional controls.
Price: OBD2 scanners range from basic, affordable models to professional-grade tools. Choose one that fits your budget and diagnostic needs.

Disclaimer and Important Notes

This code list is not exhaustive and may not be perfectly accurate for every 1994 Jeep Grand Cherokee. Variations may exist based on production date, engine type, and specific vehicle configuration. Always consult a factory service manual or a reputable online database specific to your 1994 Jeep Grand Cherokee for the most precise code definitions and diagnostic procedures.
Diagnostic trouble codes are an aid to troubleshooting, not a definitive repair guide. A DTC indicates a potential problem area, but further diagnosis is always necessary to pinpoint the root cause. Sensors can fail, wiring can be damaged, and sometimes a code can be triggered by a problem in a different system.
“Limp-in” Mode: In severe cases, the engine computer may activate “limp-in” mode to protect the engine. This mode often limits engine power and transmission function (e.g., loss of 3rd and 4th gear in automatic transmissions) to allow you to drive the vehicle to a repair shop.
Clear Codes Responsibly: After you have repaired the issue causing a trouble code, it’s generally safe to clear the code using a scanner or by disconnecting the battery (though battery disconnect is less reliable for clearing all codes and may reset other systems). However, never clear codes without addressing the underlying problem. The check engine light will simply return if the issue persists, and you will have lost valuable diagnostic information.

Conclusion

Understanding the OBD system in your 1994 Jeep Grand Cherokee is a vital step in maintaining its reliability and performance. By learning how to retrieve diagnostic codes, whether through the key trick or an OBD2 scanner, and by understanding the meaning of these codes, you can effectively troubleshoot issues and make informed repair decisions. Remember to always consult reliable repair resources and prioritize thorough diagnosis over simply replacing parts based on codes alone. Keeping your classic Jeep running smoothly is easier with the right diagnostic knowledge in your toolkit!