How to Manually Error Test My 1990 Ford Truck


A 1990 Ford truck predates standardized On-Board Diagnostics by six years. The truck employs Ford's Electronic Engine Control (fourth version) for engine related error codes. To access the system, no scanning device is required. The Ford Truck can be put into self-testing mode, and the EEC-IV system will flash the error codes through the check engine light. Only Fords using the EEC-IV system are able to do this. Ford trucks manufactured after 1996 require OBD-II compliant hardware.

Things You'll Need

  • Jumper wires
  • Start the truck's engine and let it idle until it reaches its usual operation temperature. If you do not wish to leave the car stationary and running, you can drive the car until it fully warms up. The truck needs to be completely warm, so leave the air conditioner off.

  • Turn the truck's engine off and release the hood. Prop the hood open over the Ford's engine.

  • Locate the truck's Self-Test Out hub. Next to it, you will find the Self-Test Input receptor. The STO has a six-sided shape and features six slots. The STI is single-slotted, and it is at the end of twisted wire.

  • Insert one end of a jumper wire into the STI. Insert the other end into the STO's grounding slot. To find it, look at the row containing four slots. It will be the second from the left end.

  • Insert the key into the truck's ignition. Turn to "On" so the electrical system becomes active. However, you must leave the engine off.

  • Count how many times the Ford's check engine light flashes. EEC-IV codes consists of two numbers. The first will be seen as long flashes. The second number will be seen through shorter flashes. For example, EEC-IV code 12 would be seen as one long flash followed by two shorter ones. A brief pause will separate full code numbers. The codes will also be two numbers. There are no single digit EEC-IV codes.

  • Write each code number down. You will need to look up each code's definition. The Ford truck's manual will not offer these definitions, and you will either have to find the definitions online or obtain a Haynes or Chilton manual for your model of 1990 Ford truck. While the internet may provide the no-cost option, the Haynes or Chilton manual will be worth the investment. The book will not only explain the meaning of EEC-IV codes, it will offer detailed repair instructions for everything from the chassis electrical system to the engine and the transmission.


  • "Haynes Repair Manual: Ford Ranger and Bronco II 1983 thru 1992"; John Haynes; 1989
  • Photo Credit detail: glowing dashboard image by davidcrehner from
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