A. THE DIAGNOSTIC PROCEDURE - WHERE TO START
When diagnosing the cause for engine performance, fuel economy or exhaust system complaints, perform normal
checks (non DDEC-III items) before considering DDEC as the possible source of the problem.
When diagnosing the system, always start with the first chart (labeled "START) on page 3-345.41. This will ultimately
lead to other diagnostic charts, even in the cases where no fault codes were logged but a symptom(s) was noted. In
fact, If no faults were recorded (but a symptom remains),the "START" chart will refer you to the "Customer Complaint"
chart 1 on page 3-345.57, which can identify fault trees to use based on the customer complaint.
Although there are many charts connected with diagnostics, only one is needed to
determine that the system is operating properly. Normally, only two charts are
necessary to find a problem.
B. DDEC III DIAGNOSTIC CODES/WHAT THEY MEAN
The following pages give a brief description of each diagnostic code. Basic facts about these codes are given below:
Most problems must occur for a total of at least two (2) seconds before the "Check Engine" light comes on and a code
If a problem goes away, the "Check Engine" light will turn off. But the code will remain stored in the ECM.
Code 25 means no codes were stored at all.
VSG SENSOR INPUT VOLTAGE LOW
SAE J1587 CODE:
PID: 187 FMI: 4
Indicates that the Variable Speed Governor (VSG) input to the ECM has dropped below 5% (normally < 0.25 volts)
of the sensor supply voltage. This diagnostic condition is typically:
open sensor signal circuit
open sensor +5 volt supply circuit
sensor signal is shorted to the sensor return circuit or to ground
sensor +5 volt supply is shorted to sensor return circuit or to ground
VSG SENSOR INPUT VOLTAGE HIGH
SAE J1587 CODE:
PID: 187 FMI: 3
Indicates that the Variable Speed Governor (VSG) input to the ECM has exceeded 95% (normally > 4.75 volts) of
the sensor supply voltage. This diagnostic condition is typically:
open sensor return circuit
sensor signal circuit is shorted to the sensor +5 volt supply
Change 3 3-345.19