6.3 Reading Engine ECU Gauge Data
This section describes how to use the HDT to see gauge data from the Engine ECU
on the J1708 data bus. This is useful while troubleshooting to help determine if the
Engine ECU is broadcasting gauge data to the Interface Module and SMC. If no
gauge data is being broadcast, the Engine ECU, the J1708 data bus wiring, or another
device connected to the J1708 data bus could be defective. The HDT can also be used
to compare gauge data from the Engine ECU to individual gauge readings in the in-
strument system to help determine if the gauge or the Interface Module is defective.
6.3.1 Procedure
1. With the ignition off, connect the HDT to the diagnostic connector on the vehicle
using the appropriate adapter. See Figure 6-2. The diagnostic connector is usu-
ally located under the dash on the driver’s side.
2. Turn the ignition on. The HDT will look for the Interface Module and try to es-
tablish communications with it while displaying:
Typically, this takes less than 30 seconds but can take up to 2 minutes.
Model 387 Instrument System Service Manual
072-40245 B 6-3 Using the Handheld Diagnostic Tool
Figure 6-2 Connecting the HDT to the Diagnostic Connector
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