Usage:
D11N Tractors;
777B, 785 Trucks;
3500 Family Of Engines
Reference: Caterpillar 3161 Service Manual, SENR3028. Add a Note to this publication on Page 43 referring to this article. The Service Manual states that the cocking pressure (or arming pressure) for the fuel ratio control has to be set on a test bench. This is no longer necessary. The following procedure makes it possible to adjust the arming pressure on the engine in any application.
If the fuel ratio control diaphragm or the control itself is replaced, the control arming pressure, as well as the fuel limit level for the fuel ratio control should be adjusted. The 3161 Service Manual already covers the fuel limit adjustment on Page 43. However, when the following procedure is used to adjust the arming pressure, the fuel limit level is also adjusted at the same time.
1. From the Fuel Setting And Related Information Microfiche or the engine information plate, write the figure for the dynamic fuel ratio control setting. Also, the arming pressures for the fuel ratio controls are:
... 777B, 785 Trucks: 26.9 to 41.0 kPa (3.9 to 5.9 psi)
... 3500 Family Marine: 5 to 12 kPa (.7 to 1.7 psi)
... 3500 Family Industrial, Generator Sets: 10.3 to 24.1 kPa (1.5 to 3.5 psi)
These arming pressures will be in a future issue of the microfiche.
2. Install and zero in the dial indicator on the side of the engine front drive housing for checking fuel setting. See Illustration 1. See FUEL SETTING in the Testing and Adjusting Section of the Engine Service Manual for the procedure to use.
3. Disconnect boost line from the fuel ratio control. Plug the boost line to keep dirt and foreign material out of the engine.
4. Connect an outside air supply with a pressure regulator and gauge assembly to the fuel ratio control. See Illustration 1.
Illustration 1. Setup for using outside source of air pressure.
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Work carefully around an engine that is running. Engine parts that are hot, or parts that are moving, can cause personal injury.
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5. Start the engine and apply an air pressure of 70 kPa (10 psi) to arm (activate) the fuel ratio control. Now, lower the air pressure to the specified arming pressure.
6. Remove the plug from the right front corner of the top cover. See Illustration 2. Put a 3/16" hexagonal wrench through the hole in the top cover until it engages the fuel limit adjustment screw. Turn the adjustment screw clockwise until engine speed starts to drop. The engine should stop. Count and record the number of turns required for adjustment so you can use this same setting later in this procedure.
Illustration 2. Location of screw for fuel limit adjustment.
7. Shut off engine if it did not stop in previous Step. Remove the aluminum plug from top of fuel ratio control. Use a pair of locking pliers to grip and turn out this plug.
8. Use a 1/8" hexagonal wrench to turn the arming adjustment screw two turns clockwise. See Illustration 3.
Illustration 3. Location of arming adjustment screw.
9. Start engine and apply the specified arming pressure to the fuel ratio control. See Step 1.
10. Turn arming adjustment screw counterclockwise slowly until engine speed begins to drop. This is the CORRECT SETTING for the arming adjustment screw. Check this setting as follows:
(a) Stop engine and wait one minute for oil pressure to drop.
(b) Reduce the air pressure to the fuel ratio control to zero and start the engine.
(c) Raise the fuel ratio control air pressure until the control causes the engine speed to drop. Note this pressure.
(d) If the arming pressure found in Step (c) is not correct, turn the arming adjustment screw clockwise to increase arming pressure. Go back to Step (a) to re-check arming pressure.
11. Shut off the engine and reduce the air pressure, that is connected to the fuel ratio control, to zero.
12. Turn fuel limit adjustment screw counterclockwise the same number of turns, as recorded in Step 6, to put the adjustment back to its original setting.
13. Start engine and again apply the specified arming pressure to the fuel ratio control. Then reduce the air pressure to zero.
14. Rapidly move the speed control shaft in the speed increase direction. The dial indicator will go up to the fuel limit position, pause momentarily, then drop back. Do this several times and read the dial indicator during each pause. Or, load the engine until the dial indicator stops. Where the dial indicator pauses or stops, is the limit for fuel control linkage movement set by the fuel ratio control.
NOTE: If the dial indicator does not pause or stop, the arming adjustment is not correct. See Step 10. Apply air pressure above the arming pressure specified for the fuel ratio control. Reduce air pressure to zero and repeat Step 14.
15. Adjust the fuel limit by turning the adjustment screw. Turn the adjustment screw counterclockwise to reduce the fuel limit reading on the dial indicator when it pauses or stops.
16. Again, check the fuel limit setting by rapidly moving the speed control shaft in the speed increase direction and read dial indicator when it pauses. The fuel limit adjustment is correct when the dial indicator pauses at the specified Dynamic Fuel Ratio Control Setting.
17. Shut off the engine and reinstall the aluminum plug in top of fuel ratio control. Tap the plug in the hole with a small hammer. Reinstall the plug over the fuel limit adjustment screw.
18. Remove the external air supply line and reconnect the boost line.
19. Remove the dial indicator and reinstall the plug at this location.