Checking Connections And Leads; Electric Governor Trouble Shooting; Engine Trouble Shooting Procedures; General - Hobart 6921 Series Operation And Maintenance Manual

OM-2040 / Operation and Maintenance Manual
90D20 / Generator Sets
• Underfrequency relay trips at 375 to 385 Hz, resets at 385 Hz.
• Overload relay trips at 125% rated load in 5 minutes.
See 2-3 Fig. 13 for typical test values at test receptacle and test box.
7. Check Connections and Leads
ALWAYS make a check of connections and leads to a component suspected of being faulty. With the
exception of a few instances, we will assume that connections and wiring have always been checked
first and that power has not been lost as a result of defective wiring or connections.

8. Electric Governor Trouble Shooting

The following facts concerning the operation of the electric governor may be helpful in understanding the
system and in determining which unit in the system is faulty in case of troubles.
a. The system requires two sources of power to operate normally.
(1) 12-V DC input power (from engine electrical system)
(2) AC input power (from magnetic pickup)
b. Assuming other conditions are normal, the actuator will go to, or remain in, IDLE position
under the following conditions.
(1) No 12-V DC power to control box.
(2) No voltage from control box to actuator.
(3) No signal from magnetic pickup.
c. The actuator will "surge" under the following conditions:
(1) "Stability" or "gain" adjustment set too "high".
(2) Throttle linkage binding.

9. Engine Trouble Shooting Procedures

a. General
The following engine troubleshooting procedures were taken from Detroit Diesel's "Field Service
Data Book". Certain abnormal conditions which may interfere with satisfactory engine operation,
together with methods of determining the cause of such conditions, are covered in the Trouble
Shooting Chart.
The ability of the engine to start and operate properly depends primarily on two things:
(1) The presence of an adequate supply of air, compressed to a sufficiently high compression
pressure.
(2) The injection of the proper amount of fuel at the right time.
Lack of power, even running, excessive vibration, stalling, and hard starting may be caused by
low compression, faulty injection in one or more cylinders, faulty timing or valves, or lack of
sufficient air.
Since proper compression, fuel injection, and the proper amount of air are important to good
engine performance, procedures for checking them are given here.
b. Locating a Misfiring Cylinder
(1) Start engine and allow it to run until it reaches normal operating temperatures.
(2) Remove valve cover.
(3) Run engine at IDLE speed and check valve clearance.
(4) Hold the No. 1 injector follower down with a screwdriver, or with a special injector cut-out tool,
thus preventing operation of the injector.
February 28/94 Revised Chapter 3-1
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