DTC 18

Fig. 16 Crank signal wiring circuit. 1984:

Fig. 17 Crank signal wiring circuit. 1985:

During engine cranking, a 12 volt signal is present in the cranking circuit between the ignition switch and ECM.
Code 18 indicates that the ECM is not receiving the 12 volt signal while the engine is being cranked. Possible causes of this condition are:
a. Defective ignition switch.
b. Defective wiring harness or electrical connectors.
c. ECM not processing signal.
d. Blown 3 amp ``Crank'' fuse.

To diagnose a Code 18, proceed as follows:
1. Check condition of 3 amp ``Crank'' fuse. If fuse is blown, check circuit 6 (1985, 806) for a short circuit to ground between the fuse and ECM. If circuit is satisfactory, replace fuse. If fuse blows again, check for faulty ECM or ECM connector and repair or replace as necessary.
2. Crank engine while checking for voltage at crank fuse using voltmeter J-29125 or equivalent connected between fuse and ground. If reading of zero volts is obtained, check for open in circuit 6 (1985, 806) and repair as necessary.
3. If reading of 12 volts is obtained at fuse, connect voltmeter lead to pin No. 1 at blue electrical connector. If voltmeter indicates 12 volts while cranking, check terminal for open condition. If terminal is in satisfactory condition, check for faulty ECM or ECM connector and repair or replace as necessary.
4. If voltage reading is zero, check for open in circuit 6 (1985, 806) and repair as necessary.