1, 2 and 3 Digit Codes: 18

Name of Code:
18-Engine Speed Sensor Circuit

When Monitored:
Ignition key is turned from "off" position to "run" position and/or ignition key is turned from "crank" position to "run" position.

Set Condition:
This code is set when the engine speed sensed by the Transmission Control Module (TCM) is different than the engine speed reported by the PCM over the CCD bus.

Theory of Operation:
The TCM uses a distributor signal or crank sensor signal to calculate engine RPM (which could be zero when the ignition key is in the "run" position and the engine is not running). When the calculated engine RPM is almost zero, it is compared to the speed received from the Powertrain Control Module (PCM) over the CCD bus to confirm that the engine is actually not running.

Monitor Condition Verification:
The following conditions must be met for a successful OBDII trip:
- The engine must run for more than two minutes
- The turbine speed must be greater than 600 RPM or the output speed must be greater than 400 RPM.
- If the transmission is below normal operating temperature, all forward gears must be applied for 3.5 seconds each and the transmission must be in either park or neutral for 3.5 seconds.
- If the transmission is hot, all forward gears must be applied for 1.5 seconds each and the transmission must be in either park or neutral for 1.5 seconds.
- No MIL lamp trouble codes have occurred or recurred.

Transmission Effects:
All MY's and calibrations - Relay open limp-in mode.

Possible Causes:
- Open or short in engine speed sensor circuit. (Crank sensor or distributor signal)
- Wrong TCM part number (89-91 MY vehicles with 92 or later TCM)
- TCM connector problem (Crank sensor or distributor signal terminals)
- Defective TCM
- Defective PCM