1982/10/18 Caterpillar

All 3408 And 3412 Engines Equipped With Mechanical Governors

Reports show that some of the above engines have had trouble with surges (changes) in engine rpm. It has been found that the cause of this problem in many applications is low oil pressure in the governor hydraulic servo mechanism. The governor servo will operate at low oil pressure, but its action may not be correct. This is because the low oil pressure can not compensate for the internal friction in the servo. The procedure that follows can be used in troubleshooting the cause of the low oil pressure.

1. Check the governor oil pressure at the fuel transfer pump. See Illustration 1. The specification for the governor oil pressure is 620 to 690 kPa (90 to 100 psi) when the engine rpm is between low and high idle. But, a governor oil pressure of 585 to 760 kPa (85 to 110 psi) is acceptable.

Illustration 1. Test location for governor oil pressure and location of orifice.

2. Check the engine oil pressure. Acceptable oil pressures are 205 ± 25 kPa (30 ± 5 psi) at 800 rpm and 380 ± 35 kPa (55 ± 5 psi) at 2000 rpm.

These pressures are increased to 620 to 690 kPa (90 to 100 psi) by the governor oil booster pump. See Illustration 2.

Illustration 2. Location of by-pass valve and oil booster pump for governor.

3. If low oil pressures are found in Steps 1 or 2, the cause of these must be found and corrected. Some possible causes are:

a. A restriction in the oil flow, such as a plugged orifice in the fuel transfer pump housing. See Illustration 1. This orifice is a passage for oil into the governor oil booster pump. The diameter of this orifice must be 1.57 mm (.062"). If this diameter is too large, the other components in the servo mechanism will not get enough oil.

b. A failure of the governor oil booster pump.

c. By-pass valve does not operate. The valve may be held open.