1991/11/11 Caterpillar

CMS Equipped Machines

Reference: Service Manual Module, SENR4942, "Systems Operation Testing & Adjusting" for Computerized Monitoring System.

CMS equipped machines are the first to utilize a different method of monitoring the electrical system. Use of state-of-the-art monitoring technology results in a machine with greater reliability.

CMS monitors both "R" terminal frequency and system voltage. Because of this the operator gets the earliest possible indication of an impending problem. The operator must then take appropriate action, depending upon the warning category (I or III) indicated.

A. Alternator "R" Terminal Monitoring - The alternator frequency output at the "R" terminal is monitored.

Detects: Immediate loss of alternator output, and is usually a total loss rather than partial.

* Broken/loose alternator belt.

* Internal alternator failure.

B. System Voltage Monitoring - The electrical system voltage at the alternator circuit breaker is measured continuously while the engine is running.

Present production CMS Controls provide a Category I indication if system voltage is outside of the range of 25.0 to 36.0 Volts. Beginning with 3E6136 CMS controls, a Category I indication will take place between 23.0 - 24.8 Volts or between 29.5 - 32.0 Volts. Voltages above or below these ranges will cause a Category III warning. Between these two ranges is the normal operating system voltage range.

1. High System Voltage:

a. Alternator problem causing full alternator output current with subsequent high charging voltage.

* shorted voltage regulator

* internal alternator short circuit

2. Low System Voltage:

a. Partial alternator output resulting from problem such as open rectifier diode or slipping belt (any speed).

b. Battery problem such as shorted cell (any speed).

c. Low state of battery charge following startup, if extended cranking was required or, dome light or other continuous load has caused slow drain on batteries while machine was parked (any speed).

d. Extraordinarily high electrical system loads at any speed such as:

* short circuit

* starter running engaged with engine ring gear.

* user-added components that cause the capacity of the alternator to be exceeded.

Normal electrical loads that may be too high for extended periods at LOW IDLE. Electrical loads (A/C and lighting) can be more than the alternator can supply at low engine speeds. At idle, a typical alternator can supply only about one third the current possible at high idle. Batteries must then supply the difference in required load current. During this period, system voltage will slowly drop and is an indication of an abnormal condition. Continuing to operate in this mode for an extended period may cause the batteries to lose enough charge that, if the engine is shutdown in this condition, cranking later to restart may not be possible.

The improved method of electrical system monitoring has advantages and a disadvantage.

Advantages:

Earliest possible indication of a problem in the making.

Fewer incorrect replacements of starting/charging components.

Greater machine availability.

Disadvantage:

The disadvantage applies only if the electrical system indication is due to a low system voltage.

The method of monitoring system voltage is different from EMS so operators may perceive a "malfunction" has occurred, especially during extended periods at low idle. This requires a change in thinking in that an electrical system Category I indication is not necessarily a shutdown condition. A change in operation to provide improved electrical system reliability may be all that's needed. If raising engine speed from low idle causes the indication to cease, the alternator probably cannot keep up and needs operator intervention. If the indication does not disappear, the machine should be shut down and the electrical system checked.

The electrical system Category I indication can be thought of as an ammeter showing discharge. If raising engine speed causes the indication to cease, it is like the ammeter now showing charging is taking place.

Corrective operating methods to assure electrical system reliability may be:

* Use a hand-operated governor control to raise low idle during slack periods of operation and then reduce engine speed to low idle to shift.

* Increase engine speed with foot (wheel loaders) on the governor control during extended periods of low idle.

* Reduce electrical loads when possible (lights, A/C fan speed).

Corrective service methods may include:

* Setting low idle to the high side of specification.

* Setting low idle with the normal operating electrical loads turned ON.

Corrective methods are not necessary unless a Category I indication exists continuously for a period of more than 20-30 minutes.