HOUSING 8R4664 - Caterpillar

General Information
The start switch on the backhoe loaders has two main functions. They are: ... To crank the engine.... To turn on the main power relay.
When the start switch is turned on, a "click" should be heard as the main relay is energized. This relay is powered any time the start switch is in either the "ON" or "START" position.
When the start switch is in the "ON" or "START" position, the battery voltage at TP1 (Test Point 1) is applied to TP4. If the voltage on TP4 is within 0.5 V of TP1, but the start relay will not click, check for an open coil (approximately 20Ohms) or a poor connection where the coil is grounded. When the relay is powered correctly, the output at TP5 should be within 0.5 V of TP1. See the reference article, "Improved Switch Assembly Is More Reliable."
All accessories are powered through the main relay. In order for the warning horn, gauges and other electrical accessories to be powered, the main relay must be energized or the contacts have stuck in the closed position. It is also possible that a machine could be incorrectly wired so that some electrical functions may bypass the main relay contacts and are energized by the start switch "R" to "B" contacts.
Putting the start switch in the "Start" position will cause the start relay to produce an audible click. This click cannot be heard if the starter cranks normally. While the starter is cranking, the voltage at TP1 is connected to the start relay coil as monitored by TP6 which should be within 0.5 V of TP1.
Starting
If the engine does not crank, use the sequence that follows to locate the problem.
1. Check the battery voltage at TP1. Check for a dead battery or open circuit between the battery and start switch Terminal B. The battery must have a good ground connection at the frame.2. Check the battery voltage across the start relay coil while cranking. Look for an open wire from the start switch Terminal "S" to the start relay coil or an open relay coil (should be approximately 3Ohms). Also, make sure the coil has a good ground connection.3. Check the battery voltage at TP2 while attempting to crank the engine. The voltage should be within 0.5 V of TP1. The start relay is an electrically-controlled switch. If the start relay coil is good (a "click" is heard while attempting to crank the engine) but there is no voltage at TP2, check for an open circuit between TP1 and the starter solenoid terminal marked "Start." This open circuit can include high resistance (causes more than a 0.5 V drop) or intermittent contacts in the start relay.4. If there is battery voltage at TP2, but the starter will not crank, check for: an open starter motor, butt engagement of the starter pinion with the ring gear, or defective (open) solenoid contacts.Main Relay/Warning Horn
If either the warning horn or dash circuits are energized when the start switch is in