Figure 20 - Servicing Auxiliary Gauge

Figure 19 - Water Temperature Gauge Circuit Diagram

TM-5-3820-256-24-4 Drilling System Well Rotary Truck Mounted Air Transportable 600 Feet Capacity Model LP-12 NSN 3820-01-246-4276 Manual
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S-SERIES INSTRUMENTS Figure 20 - Servicing Auxiliary Gauge OIL PRESSURE GAUGE Operation The oil pressure gauge circuit (Figure 21) consists of two basic components - the cluster-mounted gauge and the sending unit. The sending unit senses the pressure of oil in the oil gallery during engine operation and registers the pressure on the gauge. Operation of the oil pressure gauge system is similar to the fuel level gauge except that the sending unit re-sistance is controlled by a diaphragm instead of a float. Testing 1. Disconnect wire at oil pressure sender unit (located under cab floor on driver’s side). 2. Connect one lead of gauge tester to end of sender wire. Connect second lead to ground. (Gauge tester now substitutes for sender unit. 3. Turn key switch "ON" (Be sure there is power to cluster. 4. Set tester for 88 ohms. (Oil pressure gauge should read at full scale. 5. Set tester for 47 ohms. Oil pressure gauge should read at half scale. (Pointer within 2 pointer widths of hash mark. 6. Set tester at 1 ohm. Oil pressure gauge should read at low scale. (Pointer within 2 pointer widths of hash mark. If oil pressure gauge responds correctly to various tester settings, gauge and wiring between gauge and sender unit are OK. Trouble is in the sender unit or the sender unit is not grounded. NOTE - Sealant or dirt on threads of sender unit or engine oil gallery may prevent a good electrical ground. Check for this condition before replacing sender unit. Figure 21 - Oil Pressure Gauge Circuit Diagram If gauge does not respond to tester: a. Check continuity of gauge wiring circuit. Make sure connector terminals are clean and tight. b. Check gauge cluster ground circuit. Make sure circuit is grounded. c. If wiring is OK, replace gauge. OIL PRESSURE WARNING LAMP A second or back-up oil pressure warning system is also used. This system uses a second oil pressure warning switch mounted on the engine. When engine oil pressure is in its normal operating range (high) the pressure switch is held in its off position and no current is sent to warning light in the instrument cluster. When engine oil pressure is below its normal operating range (low) the pressure switch will close to its "on" position and deliver current to warning light in instrument cluster and cause warning lamp to light. CTS-4140Y - Page 12

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