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Symptom - Gauge passes initialization but is inaccurate in actual use; false warning
message or alarm present.
Probable Cause - Incorrect data being sent by the Engine ECU.
Gauge Types - SMC, Tachometer, Engine Oil Pressure, Engine Coolant Temperature,
Engine Oil Temperature, % Engine Load, Manifold Boost Pressure.
Procedure:
1. Connect the HDT to the J1708 diagnostic connector and compare the Engine ECU
data displayed on the tool to the reading on the suspect gauge.
a.
If the readings are about the same and you still suspect the reading is in-
accurate, the problem is in the Engine ECU. Follow the engine manufac-
turer's procedures to diagnose and repair the Engine ECU.
b. If the readings are different, connect the HDT to the private data bus and
compare its reading to the suspect gauge's reading. If the readings are
about the same, replace the Interface Module; otherwise, replace the
gauge.
Symptom - Gauge passes initialization but is inaccurate in actual use; pointer stays at
7:00; false warning message.
Probable Causes - Defective vacuum source Interface Module.
Gauge Types - Air Filter Restriction, Fuel Filter Restriction
Procedure: (see Figure 5-5 on page 5-12)
1. If the gauge is an Air Filter Restriction gauge, disconnect the hose from port 5A
on the Interface Module. If the gauge is a Fuel Filter Restriction gauge, discon-
nect the hose from port 5B.
2. While observing the vacuum gauge, connect a known good vacuum source to the
input of the vacuum block and apply the vacuum given in the following table:
To check this gauge- Apply this test vacuum- At this port-
Air Filter Restric-
Three inches of mercury.
tion
Fuel Filter Restric-
Ten inches of mercury.
tion
CAUTION: Most vacuum test sources read in inches of mercury. If using this type of
vacuum source when applying vacuum to port 5A, do not exceed three inches of mer-
cury. If you do, you can destroy the VTM.
Correct Reading-
5A
About 40 inches of water.
5B
About 10 inches of mer-
cury.
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