LAMP GP 9M1305 - Caterpillar

If the tachometer system of the 4S6553 Engine Instrument Group does not operate correctly, test the separate parts of the tachometer system. These tests can be made with a volt-ohmmeter and a continuity light. Use the procedure that follows to find if replacement of all or some of the parts is necessary.
1. Use the continuity light and test the 1P7445 Tachometer Cable. Order a new 1P7445 Tachometer Cable if necessary.2. Use an ohmmeter and the schematic drawing (Illustration 1) to test the 1P7443 Tachometer. Use the chart below to see if the tachometer operates correctly: The 1P7443 Tachometer can come from three different suppliers. Each column and each set of values in the chart above are for the three separate suppliers. The values for all three contact points can fall into the ranges shown. But, each set of values must be similar, or a replacement tachometer must be ordered. If the tachometer tests correctly, go to the next section of testing.3. Use an ohmmeter and the schematic (Illustration 1) to test the 1P7444 Tachometer Generator. Use the chart below to see if the generator operates correctly. If you do not get the values shown above, order a new 1P7444 Tachometer Generator.
To find the maximum and minimum output voltage from the 1P7444 Tachometer Generator, use an AC voltmeter and the procedure that follows: 1. Install the tachometer generator on an engine, or fuel injection test bench, and connect the tachometer to the tachometer cable.2. Disassemble the connector on the tachometer end of the cable (see Illustration 2), and make the tests at this point.3. Increase the engine speed to 2500 RPM, 1250 RPM on the fuel injection test bench. The maximum voltage between any two contact points is 24 VAC.4. Decrease engine speed to 600 RPM, 300 RPM on the test bench. The minimum voltage between any two contact points is 3.5 VAC.
Order a new 1P7444 Tachometer Generator if either the maximum or minimum voltage is wrong.
Illustration 1.
Illustration 2.