Usage:
The acceptable tolerance for dynamic (operating) fuel injection timing for engines with or without timing advance mechanism can be determined from the data available in the "Fuel Setting And Related Information" microfiche. The specified fuel system data must be plotted (recorded) on a Dynamic Timing Chart, Form SEHS8140. The actual dynamic fuel injection timing of an engine, when measured with the 6V3100 Timing Indicator and plotted on the timing chart, must fall within the borders of this tolerance. If not, an adjustment or repair may be necessary.
Two examples are shown of plotting Fuel System Data on Dynamic Timing Charts. In each example the dynamic timing tolerance is constructed from the minimum and maximum values of the timing degrees and engine speed provided in the Fuel System Data section of Fuel Setting and Related Information microfiche.
Example 1 uses Fuel System Data from 0T4858. This example has no timing advance. The Fuel System Data for 0T4858 is shown in Illustration 1 and the dynamic timing tolerance is shown in Illustration 2. 0T4858 specifies a minimum and maximum timing of 20.5 to 22.9 degrees at a minimum and maximum engine speed of 720 to 780 rpm for the low idle condition (see Illustration 1). This 2.4 ° tolerance is a combination of engine tolerance and the measurement tool tolerance. These two tolerances and the advance tolerance, if used, are included in the minimum and maximum calculations. Each timing is plotted on the dynamic timing chart at each speed making four data points that are then connected to form a window (see Illustration 2). For an engine with no timing advance mechanism, a second window is constructed in the same manner for the remaining condition (stop rpm) and the extreme limits of the two windows are connected to form the dynamic timing tolerance (see Illustration 2).
The slope of the lines, connecting the low idle window to the stop window, is the result of injection port effect, which is included in the Fuel System Data.
Illustration 1. Fuel System Data from "Fuel Setting And Related Information " microfiche 0T4858.
Illustration 2. Dynamic timing tolerance for 0T4858.
Example 2 uses Fuel System Data from 0T4442. See Illustration 3. This is an example of an engine equipped with a timing advance mechanism. This dynamic timing tolerance will have three windows instead of two as in Example 1. Four data points are located and connected, as in Example 1, for low idle rpm, start rpm and stop rpm. This forms three windows. The extreme limits of all the windows are then connected to form the dynamic timing tolerance as shown in Illustration 4. The slope of the lines, from the start window to the low idle window, is the result of port effect. The slope of the lines, connecting the low idle window and the stop window, is the result of port effect and the timing advance mechanism spring rate. The scope of the lines can not be changed unless a different spring is installed in the timing advance mechanism. Only the points where the slope starts and stops can be adjusted.
Illustration 3. Fuel System Data from "Fuel Setting and Related Information microfiche 0T4442.
If the measured fuel injection timing passes through each window of the dynamic timing tolerance, and does not fall outside the lines connecting the windows, no adjustment or repair is required. If measured timing is outside the Low Idle or Start RPM dynamic timing tolerance window an appropriate adjustment to static timing will be necessary to bring the measured timing back within tolerance. If the engine has acceptable Start RPM advance, but incorrect Stop RPM advance (outside Stop RPM window), an adjustment or repair of the timing advance mechanism is required. The change in slope of the measured dynamic timing indicates the start or stop of timing advance and should occur within the appropriate window. If not, an adjustment or repair of the advance mechanism is required. Consult the appropriate service manual for the repair or adjustment procedure necessary to achieve the correct dynamic timing.
Illustration 4. Dynamic timing tolerance for 0T4442.