Blog: OBD1 diagnostics for GM trucks

Practical OBD1/ALDL guides for S10, Blazer, C/K and Silverado: where the connector is, how to read codes, and real diagnostic cases.

• Where is the ALDL connector on your GM truck (1988–1995)?
  Location of the 12-pin diagnostic (ALDL) connector on S10, Blazer, C/K and Silverado trucks, and what the important terminals do.
• How to read GM OBD1 trouble codes with a jumper wire
  How to pull Check Engine codes from a 1988–1995 GM truck by jumpering A and B on the ALDL, plus the most common codes (12, 13, 24, 32, 42, 44, 45).
• Real case: the A/C idle surge was a false speed-sensor pulse
  A 1993 GMC Sonoma 2.8L whose idle surged when the A/C came on. Live data revealed a false speed-sensor pulse; here is how it was found and fixed with a resistor.