Access The Trouble Codes on a 91-93 GMC Syclone Typhoon
For anyone who has ever owned a GMC Syclone or Typhoon know all to well the check engine light and it's glowing presence means that there is something not quite right with your beloved truck.
These trucks use an OBDI system that utilizes an ALDL (Assembly Line Diagnostic Link) terminal to be able to acquire data from the 7749 ECM. Luckily, to be able to check and read codes, you dont need any equipment short of a paper clip or simple ALDL Jumper Key that I've seen for free at your local auto parts store.
Below covers the steps in how to pull up and read your CHECK ENGINE codes as well as a list of what each code means.
Accessing the Trouble Codes
Locate the Assembly Line Data Link (ALDL) port. It is located on the bottom side of the dash on the passenger side.
With the ignition OFF, jumper the A&B terminals on the ALDL port. (You can use a paperclip, wire, or ALDL Jumper Key).
Reference of Terminal Ports:
- A - Ground
- B - Test Terminal
- C - Air Injection
- E - Serial Data
- F - TCC
- G - Fuel Pump
- H - Brake Sense Speed Input
- M - Serial Data
You can keep the jumper key from AutoZone on your keyring. It's small size makes it easy to carry so it's always with you.
With the ALDL A&B terminals jumped, turn ignition switch to ON position - DO NOT START THE MOTOR!
Now, focus onto your gauge cluster and count the flashes on the "CHECK ENGINE" or "SERVICE ENGINE" light. A Code 12 will flash on the Check Engine light 3 times first and then give you any additional codes if present. Code 12 will look like this:
FLASH, pause, FLASH, FLASH = code 12
If your ECM has stored any codes, the sequence will begin with Code 12 (3 times) followed by a long pause and then begin to flash any codes stored. Once it has displayed the last trouble code stored, the system starts over with Code 12 and repeats itself.
EXAMPLE: FLASH, pause, FLASH, FLASH, pause, FLASH, pause, FLASH, FLASH, pause FLASH, pause, FLASH, FLASH -long pause- FLASH, FLASH FLASH, FLASH, pause, FLASH, FLASH, FLASH -long pause- FLASH, pause, FLASH, FLASH, pause FLASH, pause, FLASH, FLASH, pause FLASH, pause, FLASH, FLASH
This example is the showing Code 43
This sequence will repeat continuously until you shut off the ignition.
Having a pen and paper handy to follow and count the flash sequence can help you go back and determine what codes have been stored.
- Engine Diagnostics - Terminals (A) and (B).
- To Engage the Transmission Converter Clutch (TCC) - Terminals (A) and (F).
- To Engage the Fuel Pump - Connect a fused (+) lead to Terminal (G).
- Brake Diagnostics - Terminals (A) and (H).
- (M) is a Serial Data Connection for 4 Cyl? engines and needs a special tool - DO NOT USE!
Clearing Trouble Codes
Diagnostic trouble codes may be cleared by disconnecting the battery ground cable for at least 20 seconds. Reconnect negative battery cable and recheck codes to confirm the repair. OBDI can also be reset with a scanner, or by removing both ECM B and 1 fuses for 20 seconds as well.
OBDI Codes - 4.3L VIN Z and W
Code 12 – Diagnostic mode
Code 13 - Oxygen sensor circuit - Oxygen sensor voltage stays between 0.35 and 0.55 volts for 30 seconds when the TPS signal was above 0.55 volts. The engine had been running for at least 40 seconds with an engine temperature above 110? F.
Code 14 - Coolant temperature sensor or circuit/ high temperature indicated - Coolant temperature sensor signal indicates a temperature of over 284? F for 4 seconds after the engine had been running for at least 10 seconds.
Code 15 - Coolant temperature sensor or circuit/ low temperature indicated - Coolant temperature sensor signal indicates a temperature colder than -36? F for at least 4 seconds after the engine had been running for 2 seconds.
Code 16 – System voltage high - Charging system voltage was either below 9 volts or above 17 volts for 10 seconds.
Code 17 – Crank Signal Circuit - PCM did not detect any spark reference pulses within 240 fuel control reference pulses (80 crank revolutions) while engine was running below 1200 RPM.
Code 19 – Crankshaft Position Sensor or circuit
Code 21 – Throttle Position Sensor TPS or circuit voltage high - TPS voltage was above 0.8 volts for 5 seconds when engine was running and air flow was less than 15 gm /sec, or the TPS voltage was over 4.8 volts at any time.
Code 22 – Throttle Position Sensor TPS or circuit voltage low - TPS voltage was under 0.2 volts for 4 seconds when the ignition was on.
Code 23 – Intake Air Temperature (IAT) sensor circuit – low temperature indicated - Inlet air temperature sensor signal showed an air temperature of -29? F for 4 seconds.
Code 24 - Vehicle Speed sensor circuit - Vehicle speed sensor signal showed less than 3 MPH for 2 seconds when engine speed was greater than 3000 RPM and transmission has not been in park or neutral for more than 4 seconds. Or vehicle speed goes from above 18 MPH to 0 in less than 2 seconds without applying brake.
Code 25 – Intake Air Temperature (IAT) sensor or circuit – high temperature indicated - Inlet air temperature sensor signal showed an air temperature of over 218? F for 5 seconds when vehicle speed was greater than 35 MPH.
Code 26 - Quad-Driver sense voltage shows a low voltage when battery voltage should be present, or if the sense voltage shows battery voltage when a low voltage should be present for at least 5 seconds when the engine was running.
Code 27 - 2nd gear switch was closed or grounded for 10 seconds when vehicle was in 4th gear, or 2nd or 4th gear switch was open when engine was first started.
Code 28 - 3rd gear switch was closed or grounded for 10 seconds when vehicle was in 4th gear, or 3rd or 4th gear switch was open when engine was first started.
Code 29 - 4th gear switch was closed or grounded for 10 seconds when vehicle was in 4th gear, or 4th gear switch was open when engine was first started.
Code 31 - Park/Neutral Position (PNP) switch circuit - PRNDL indicates a gear other than park or neutral for 13 seconds for 3 consecutive startups when ignition was on, engine was off and vehicle speed was 0. Or the PRNDL status indicators (A, B, C, P) showed an incorrect combination of Lo and Hi for more than 10 seconds.
Code 32 - Exhaust Gas Recirculation (EGR) Circuit - EGR open command did not change the fuel mixture when the command was given during off idle operation. EGR opening should cause the mixture to be slightly richer because of the low oxygen content of exhaust gas.
Code 33 - MAP sensor signal voltage was too high (low vacuum) for 5 seconds when throttle opening was under 2%.
Code 34 - MAP sensor signal voltage low - MAP sensor signal voltage was too low (high vacuum) when engine speed was over 1200 RPM, or the throttle angle was above 21% when engine speed was over 1200 RPM.
Code 35 - Idle Air Control (IAC) System
Code 38 – Brake Switch
Code 41 – Camshaft Sensor Circuit
Code 42 – Bypass or Electronic Spark Timing circuit - The EST signal did not change when the ECM applied bypass voltage to the ignition module.
Code 43 – Knock Sensor Ciruit - Knock sensor signal was either above 3.5 volts or below 1.5 volts for at least 20 seconds.
Code 44 - Oxygen sensor or circuit – lean exhaust detected. - Oxygen sensor voltage was under 0.25 volts for up to 4 1/2 minutes of closed loop operation.
Code 45 - Oxygen sensor or circuit - rich exhaust detected. - Oxygen sensor voltage was over 0.750 volts for 2 minutes in closed loop operation with a throttle angle between 0.6 and 1.9 volts.
Code 46 – Power Steering Pressure Switch circuit
Code 47 - Momentary loss of power to Body Computer Module (BCM).
Code 48 – Misfire Diagnosis
Code 51 - MEM-CAL, PROM, or ECM Error (Faulty or Incorrect Calibration)
Code 52 – CALPAK or ECM problem
Code 53 – System over-voltage – ECM over 17.7 volts
Code 54 – Fuel Pump Circuit
Code 55 - ECM problem
4-Wheel Anti-Lock Diagnostic Codes:
To do a diagnostic on 4WAL codes you take a paper clip and connect terminal (A) and (H) together. After 20 seconds if any codes are present they will be displayed by the flashing of the Anti-Lock light. A code is displayed for example 12 (1 flash, pause, 2 flashes).
To clear the code from the ECU memory, with the ignition On, place the jumper between (A) and (H) for two seconds, remove for one second, and replace again for two seconds. The Antilock/Brake light should light, then go off.
Code 21 - Right Front Wheel Circuit Faulty wheel sensor or open circuit.
Code 22 - Right Front Wheel Circuit No signal. Remove the wheel. Check for a damaged sensor or loose electrical connectors.
Code 23 - Right Front Wheel Circuit Intermittent signal, indicating a loose electrical connector.
Code 25 - Left Front Wheel Circuit No signal. Remove the wheel. Check for a damaged sensor or loose electrical connectors.
Code 26 - Left Front Wheel Circuit Intermittent signal, indicating a loose electrical connector.
Code 27 - Left Front Speed Sensor Circuit Faulty sensor or loose electrical connector.
Code 28 - Speed Sensor Signal Erratic brake action at low speeds and pedal pressures indicates a fault in the speed sensor circuit or speedometer.
Code 29 - EHCU Circuit A loss of signal from all four sensors indicates a loose EHCU connector.
Code 31 - Right Rear Speed Sensor Circuit An intermittent code indicates a loose electrical connector.
Code 32 - Right Rear Wheel Circuit No signal. Remove the wheel. Check for a damaged sensor or loose electrical connectors.
Code 33 - Right Rear Wheel Circuit Intermittent signal, indicating a loose electrical connector.
Code 35 - Left Rear Wheel Circuit Intermittent signal, indicating a loose electrical connector.
Code 36 - Left Rear Speed Sensor Circuit No signal. Remove the wheel. Check for a damaged sensor or loose electrical connectors.
Code 37 - Left Rear Speed Sensor Circuit An inconsistent anti-lock action at low speeds and pedal pressure is a sign of a fault in the speed sensor unit or connectors.
Code 38 - EHCU valve circuit A pulsing pedal indicates a defective wheel sensor or faulty EHCU valve.
Code 41 through 66 - 4WAL control unit or motor circuit Check for a loose ground connection at the motor circuit. Clear the code and drive the vehicle. If any of these codes return, the EHCU valve may be faulty.
Code 67 - Motor circuit Check the motor for loose electrical connections.
Code 68 - Motor circuit A locked motor or an open circuit. Check the motor for loose electrical connections.
Code 71 through 74 - 4WAL control unit Clear the code and drive the vehicle. If any of these codes return, the EHCU valve may be faulty.
Code 81 - Brake switch circuit Make sure the driver hasn't been riding the brake. Check the brake switch adjustment and electrical connections
Code 86 - Anti-Lock warning light Check the light and wiring for a short circuit.
Code 88 - Brake warning light Check the light and wiring for a short circuit.
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