TRUCK SERVICE MANUAL
FUEL SYSTEM
ST-790 Trouble Shooting
2. Erroneous flowmeters.
ST790 Trouble Shooting/Maintenance
3. Erroneous dynamometer load indicators.
This section of the manual should be fully understood by
c. Neglect of ST790 Maintenance.
the Injector Test Stand operator, and through this
1. Use of hose lengths, diameters, and resiliencies
knowledge he should be able to correct the test stand
other than those which are specified.
problems and subsequently produce a properly
Caution: Do not replace any hoses or lines on the
calibrated injector.
ST790 Test Stand with other than the correct ST hose as
The ST790 Injector Test Stand is the recommended
specified in the tool catalog. Hoses on ST790 should be
calibration equipment for Cummins injectors and was
replaced periodically since rubber in the hose hardens
developed to provide accurate balancing and testing of
with age causing resonance variables.
Cummins injectors in order to produce the desired
2. Fuel routing which is not to specifications.
engine performance.
3. Filter assemblies which have a different
Injector calibration on the ST790 Injector Test Stand
dampening effect on the system.
combined with fuel pump calibration on
4. Critical components of the test stand such as
Cummins Fuel Pump Test Stands with flow
check valve, gauge and hydraulic injector
meters has produced widespread acceptance of
clamping poorly maintained.
fuel system accuracy. There have been reports
of erratic results, but investigations have
Trouble Shooting The ST790
revealed that the cause for such
Injector calibration problems are discovered by
problems fall into three basic categories.
inconsistent results in service injectors or inability to flow
1. Mechanic and/or Tester Error: (Not confined to
injectors which have been calibrated at the factory or at
injector calibration alone.
Includes pump
other locations to the proper specifications. When an
calibration, engine and chassis dynamometer
injector calibration problem is indicated, the following
checks, etc.)
troubleshooting guide is to be used in the diagnosis and
2. Instrumentation Errors: (Range all way from
correction of the problem.
All injector calibration
gauge used on cuptoplunger seat check to load
complaints have been found to fall into the six basic
indicator on chassis dynamometer.
categories as follows.
3. ST790 Maintenance Status These three problem
1. Stand repeatability.
Stand will not repeat
areas are itemized as follows:
consistently on any given injector.
a. Mechanic and/or Tester Error
2. Poor results on all injector models. Injectors do
1. Calibrating a specific injector model to the wrong
not perform well in engine and do not correlate
flow value.
with injectors calibrated on other stands.
2. Calibrating a specific fuel pump model to the
3. NHRS, NRT injectors heavily overfueled. PT
wrong value.
(type B) and PT (type D) injectors slightly
3. Lack of familiarity with this bulletin and/or Bulletin
underfueled. Reasonable results on all other
No. 3379068, 983505, 3379084 and No.
injectors.
983533.
4. All PT (type B) (type C) and PT (type D) injectors
4. Use of wrong injector assembly in a specific
overfueled. Reasonable results on all other
engine model.
injectors.
5. Use of camshaft and pistons other than those
5. NHRS, NRT and all PT (type B) (type C) and PT
shown in CPL.
(type D) injectors overfueled.
Results are
6. Restricted intake air to engine.
reasonable with all other injectors.
7. Excessive exhaust back pressure.
6. All PT (type B) (type C) and PT (type D) injectors
8. Restricted fuel supply to engine.
underfueled.
9. Aerated fuel supply to engine.
10. Excessively high oil level in engine crankcase.
b. Instrumentation Errors (Other than ST790) 1.
Erroneous fuel manifold pressure gauges on
engine and/or chassis dynamometer.
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