Duty Cycle Adjustment: Overview
Number: 11-8-82Date: 6-1-82
Applicability: 1981, '82 and '83 Vehicles With ECC System.
Subject: Adjustment Of Duty Ratio
The purpose of this bulletin is to explain idle duty ratio and its relationship to vehicle operation.
Duty is defined as the period of time the duty solenoids are open during any one duty cycle measured in degrees of dwell.
As an example, the figure will explain the definition:
Since the duty is being measured using a full scale 90° dwell meter, the Dwell reading is proportional to the actual percentage of time the solenoids are open.
When the ECC system is in feedback (the 02 sensor is sending a signal to the ECM [81 ECU]),the solenoids are adjusting the air/fuel ratio to a stoichiometric mixture (A/F ratio of 14.7:1). At this time, the needle of the dwell meter will fluctuate approximately 9° This is the normal feedback swing: The center point of the needle swing will be referred to as the baseline.
As long as the needle has the capacity to swing approximately 4.5° from the baseline, the mixture will be kept in the stoichiometric range.
If the idle duty cycle baseline is within the range as specified in the charts 1 & 2 for a given altitude, attempting to adjust the idle fuel mixture will not affect the performance of the vehicle as the ECC system will keep the mixture in a stoichiometric range.
Example A: If dealer X at 4,000 ft. above sea level inspects the duty ratio of a Subaru at predelivery and finds the duty cycle baseline to be 57.5°, it can be seen in Chart 1 that the acceptable range of duty ratio at 4,000 ft. is 26.8° to 62.8°. Therefore, it is not necessary or advantageous to change the duty ratio.
Example B: If dealer Y at 500 ft. above sea level inspects the duty ratio of a Subaru vehicle at 15,000 mi. and finds the baseline to be 20°, it can be seen from Chart 1 that this falls in the acceptable 500 ft. duty ratio range of 14.6° to 50.6°. Therefore, it is not necessary or advantageous to change the duty ratio.
Example C: If dealer Z at 2,000 ft. has to remove an ECC carburetor to replace an intake manifold, when reassembled the duty ratio would be set at 37.8° +/- 4.5° Only if a carburetor is removed from a 1981 or 1982 vehicle for repair purposes will it be necessary to adjust the duty ratio to the altitude corrected ideal figure in Column 2. The adjustment time is included in the flat rate operation time for R&R of the carburetor.
Chart 1 lists the proper ideal duty ratios (corrected for altitude) along with acceptable duty ratio ranges (also corrected for altitude).
Ideal duty ratio is defined as "the average duty ratio during feedback set only after a carburetor has been completely removed and reinstalled for repair purposes. This is the listed average duty ratio (Chart 1) +/- 4.5°.
Acceptable duty ratio ranges are the ideal duty ratios +/- 18°. This is the service limit.
This Service Bulletin has been released for informational use only. Adjustment of Duty Ratio is NOT a warrantable repair.