Tag Archives: Electrical

1990 GMC trouble codes and how to get them

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Diagnostic Trouble Codes – 1990 GMC

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13 Open Oxygen Sensor Circuit
14 Coolant Sensor Circuit Shorted
15 Coolant Sensor Circuit Open
21 Throttle Position Sensor (Signal Voltage High)
22 Open Throttle Position Sensor Circuit
24 Vehicle Speed Sensor (VSS) Circuit
32 Exhaust Gas Recirculation System (Over 8500 GVW)
32 Exhaust Gas Recirculation System (Under 8500 GVW)
33 Manifold Absolute Pressure Sensor (MAP) Voltage High)
34 Manifold Absolute Pressure Sensor (MAP) Voltage Low)
35 Idle Air Control (IAC) System
42 Electronic Spark Timing (EST) Circuit
43 Electronic Spark Control (ESC) Circuit
44 Oxygen Sensor Circuit Lean Exhaust Indicated
45 Oxygen Sensor Circuit Rich Exhaust Indicated
51 Faulty Mem-Cal or PROM Problem
52 Fuel CALPAK Missing
53 Over Voltage Error
54 Fuel Pump Circuit (Low Voltage)
55 Faulty ECM
To extract a trouble from the computer for diagnostic purposes, the Assembly Line Communication Link (ALDL) connector is used.

Ground terminals A and B with the ignition “ON” and engine not running. The system will enter the diagnostic mode. In this mode, the computer will display a 12, three consecutive times, by flashing the “CHECK ENGINE”, “SERVICE ENGINE SOON” or “MALFUNCTION INDICATOR LAMP” or light. A 12 consists of one flash, a short pause, then two flashes in rapid succession.

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After 12 is displayed, any stored trouble s will be displayed by flashing the “CHECK ENGINE”, “SERVICE ENGINE SOON” or “MALFUNCTION INDICATOR LAMP” light. All s are displayed 3 times each then a 12 will repeat. In the diagnostic mode, the computer will also energize all computer controlled relays and solenoids.

Diagnostic Trouble Codes - 1990 GMC
ALDL Connector (Located Underdash)
The ALDL diagnostic connector is located in the passenger compartment.

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You put a paperclip into pins A and B so they are connected together, (There is a picture in one of the links), then turn the ignition to run but don’t crank it. The “check engine” light will start flashing. I will flash once, pause, twice, long pause, once, pause, twice, long pause, once, pause, twice….that is showing code 12. It repeats each code 3 times so you are sure what it is telling you. It will continue on to the next code, and the next, until it is done. There will be a longer pause (about 5 seconds) and start over again.

If it flashes code 12, 6 times that means that there are no codes stored.

There are only a few sensors on a OBD 1 system like you have and the computer is very basic, so it is a good one to learn computer controlled systems.

Do not short pins on newer vehicles which are OBD2 (1995ish and newer) because you will fry things inside. For those you need a code reader or scanner which is a hand held specialized computer.

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GM 2 Digit codes for 82 and up (non OBD-II)

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code code definition
12 diagnostic mode
13 02 sensor or circuit
14 coolant sensor or circuit/ high temp indicated
15 coolant sensor or circuit/ low temp indicated
16 system voltage out of range
19 crankshaft position sensor or circuit
21 throttle position sensor or circuit- voltage high
22 throttle position sensor or circuit- voltage low
23 mixture control (m/c) solenoid or circuit (carbureted models)
23 manifold air temperature (mat) sensor or circuit (1990 and earlier models) 23- intake air temperature (iat) sensor circuit (fuel-injected models)
24 vehicle speed sensor (vss) or circuit
25 manifold air temperature (mat) sensor or circuit – high temperature indicated (1990 and earlier models)
25 intake air temperature (iat) sensor or circuit – high temperature indicated (1991 and later models)
26 quad driver module circuit
27 quad driver module circuit
28 quad driver module circuit
29 quad driver module circuit
31 park/neutral position (pnp) switch circuit
32 baro sensor or circuit (carbureted models)
32 egr circuit (fuel-injected models)
33 manifold absolute pressure (map sensor signal voltage high
33 mass air flow (maf) sensor or circuit – excessive airflow indicated
34 manifold absolute pressure (map) sensor signal voltage low
34 mass air flow (maf) sensor signal – low airflow indicated
35 idle speed control (isc) switch or circuit (shorted) (carbureted models)
35 idle air control (iac) valve/ circuit
38 brake switch circuit
39 torque converter clutch (tcc) circuit
41 no distributor signals to ecm, or faulty ignition module ( carbed models)
41 cylinders select error- mem-cal or ecm problem ( fuel-injected models
41 cam sensor circuit (3.8 engine)
42 bypass or electronic spark timing (est) circuit
43 low voltage at ecm terminal l (carbureted models)
43 knock sensor circuit
44 oxygen sensor or circuit – lean exhaust detected
45 oxygen sensor or circuit – rich exhaust detected
46 power steering pressure switch circuit
48 misfire diagnosis
51 prom, mem-cal or ecm problem
52 calpak or ecm problem
53 egr fault (carbureted models only)
53 system over-voltage (ecm over 17.7 volts)
54 mixture control (m/c) solenoid or circuit (carbureted models)
54 fuel pump circuit (1986 and later models)
55 oxygen sensor circuit or ecm
55 fuel lean monitor (2.2l engine)
61 oxygen sensor signal faulty (possible contaminated sensor)
62 transaxle gear switch signal circuits
63 manifold absolute pressure (map) sensor voltage high (low vacuum detected)
64 manifold absolute pressure (map) sensor voltage low (high vacuum detected)
66 pressure sensor or circuit air conditioning