78-1

VTM-4 DTC Troubleshooting: 78-1

DTC 78-1: VTM-4 Control Unit

NOTE: Before you troubleshoot, review the general troubleshooting information. Initial Inspection and Diagnostic Overview

1. Turn the ignition switch to ON (II).

2. Make sure the HDS communicates with the vehicle and the VTM-4 control unit. If it doesn't, troubleshoot the DLC circuit. Testing and Inspection

3. Clear the DTC with the HDS.

4. Test-drive the vehicle, and check for DTCs with the HDS.

Is DTC 78-1 indicated?
YES - Go to step 5.

NO - Intermittent failure, the system is OK at this time.


5. Check the battery.

Is the specified battery installed, and is it fully charged?
YES - Go to step 6.

NO - Replace or charge the battery.


6. Watch the charging system indicator.

Does the charging system indicator come on with the ignition switch ON (II), and go off with the engine started?
YES - Go to step 7.

NO - Do the charging system troubleshooting. Charging System Indicator Circuit Troubleshooting


7. Check for installation of any aftermarket CB or Ham radios which may cause an RF signal interference.

Is there an aftermarket radio installed?
YES - Disconnect the aftermarket radio, and retest.

NO - Check for poor connections or loose terminals at the VTM-4 control unit connectors. If the connections are OK, substitute a known-good VTM-4 control unit Service and Repair and recheck. If the symptom/indication goes away with the known-good VTM-4 control unit, replace the original VTM-4 control unit. Service and Repair