1983/01/10 Caterpillar

All 3408 And 3412 Engines Equipped With Hydramechanical Governors

SERVICE MAGAZINE, OCTOBER 18, 1982, PAGE 1. Some of the information in the article "Changes In Engine RPM; Governor Does Not Operate Correctly" is not correct. Put a mark through that article, and make reference to this article.

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 move the rack as rapidly as is necessary. 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.

2. Check the engine oil pressure. Acceptable oil pressures are 205 ± 25 kPa (30 ± 4 psi) at 800 rpm and 380 ± 35 kPa (55 ± 5 psi) at 2000 rpm.

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 pump. The diameter of this orifice must be 1.57 mm (.062"). If this diameter is too small, the servo mechanism will not get enough oil.

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

b. A failure of the governor oil pump.

c. Bypass valve does not operate. The valve may be held open.

Illustration 2. Location of bypass valve and governor oil pump.