1989 plymouth horizon check engine light on

Code 34 is set when the computer does not detect a voltage to and through the speed control servo. The cruise control computer is built into the PCM, which grounds two windings in the servo mechanism to regulate the vacuum applied to the servo diaphragm.
Note: All Chrysler corporation fuel-injected engines have the computer circuitry and software for the cruise control system installed, even if there is no cruise control installed on the vehicle. If there is no cruise control installed on the care or truck, the computer will interpret this as an open winding and set code 34.
First: turn the key ON, turn the cruise switch ON, and look for +12 Volts on the dark blue/red wire that connects to the servo mechanism. This circuit is fed:

• from the ignition switch through to 10 (dark blue wire)
• from fuse 10 to the steering column (white/red wire)
• through the clock spring to the cruise control ON/OFF switch on the steering wheel (white/red wire)
• back through the clockspring to joint connect "C" (yellow/red wire)
• to the brake lamp switch under the dashboard (yellow/red wire)
• through the firewall from the brake switch (dark blue/red wire)
• to the servo (dark blue/red wire) A break anywhere in this path (broken wire, bad brake switch, loose connector, etc.) will interrupt the 12 volt supply to the servo and set a fault code. Work backwards until you locate the break.
  

Turn the key OFF. Unplug the the connector from the servo and measure the servo coil resistances.

• measure the resistance between the servo pin that connects to the tan/red wire and the servo pin that connects to the dark blue/red wire.
• measure the resistance between the servo pin that connects to the light green/red wire and the servo pin that connects to the dark blue/red wire. The resistance in both cases should be above 0 ohms and less than infinity.
  

Disconnect the battery(s). Unplug the 60 pin connector from the PCM and check for continuity between the pcm connector and the servo connector for the tan/red wire (cavity 33) and the light green/red wire (cavity 53).
If no wiring problems are found, check the PCM connectors for corrosion, and reconnect. Reconnect the battery and test the circuit again. If the problem still exists, the PCM is probably bad. NOTE: Personally, if the truck was out of warranty I would try to find for a used servo before buying a new PCM.