How To Measure Duty Cycle On A Feedback Carburetor - Fluke 78 User Manual

78
Users Manual
x How to Measure Duty Cycle on a Feedback
Carburetor
Duty cycle is the percentage of time (0-99.9%) a voltage is
positive or negative. Most cars have points closed for a
duty cycle of 50 to 70%, or mixture control solenoids that
are set to 50% in closed loop.
To measure duty cycle on a feedback carburetor:
1. Insert the test leads in the jacks as shown in Figure
14.
Turn the rotary switch to x.
2.
Press R for 2 seconds to toggle to duty cycle.
3.
When the meter is turned on or comes out of standby
(Sleep) mode, the trigger slope is negative (-). The
slope is indicated by a + or - sign at the left of the bar
graph.
To toggle between a positive and negative trigger
slope, press R momentarily.
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4. Connect the black test lead to ground and the red
lead to the green mixture control test connector (GM
cars) at the base of the carburetor.
5. Start the engine and read display when the engine is
cold (open loop).
When the engine is in open loop, duty cycle is a
steady value. See the car's specifications.
6. Read the display again when the engine is warm
(closed loop).
When the engine warms up and goes into closed
loop, the reading duty cycle should fluctuate but av-
erage 50%.