1990/07/01 Caterpillar

All Products With Direct Electric Starting

Reference: Service Magazine; May 28, 1990; Page 4; "Jump Starting Procedures." Service Magazine; May 4, 1987; Page 10; "Alternator/Generator Output Test on the Engine."

Electrical components may suffer damage from high electrical system voltage. Damage identified by parts returned to the factory includes arc.burn spots in small switches and connectors. Electronic controls show damage to printed circuit boards. Electronic components show damage from arcing and burning. These damages are possible only from extremely high voltages.

The cause of component damage is generally one of two possibilities:

1. Incorrect jump starting procedure, where a stalled engine is jumped by a source with voltage higher than the system voltage of the stalled engine. See the first Reference Article.

Show/hide table

Jump sources higher than system voltage of the stalled engine are sometimes thought to give better cranking speeds and subsequent startability. Higher-than-rated jump sources cause damage to starters, batteries, and other electrical components. Batteries can also explode causing personal injury. NEVER jump with a source higher than the system of the stalled engine. Use a 12V source on a 12V system and 24V on 24V system, etc.

2. When the alternator voltage regulator is shorted, causing full (rated) charging current, AND an open circuit or loose connection exists in the charging circuit.

The regulator adjusts alternator output by varying field current to maintain correct system voltage. Regulators have, mainly, two failure modes: open or shorted. When open, there is no field current and no alternator output. If shorted, field current is maximum causing maximum rated, uncontrolled, output current, provided the engine is running at or above about 75% of rated engine speed.

Batteries serve two main functions in ANY electrical system:

1. Provide energy for cranking the engine.

2. Act as an accumulator to provide a "smoothing" effect for electrical system voltage variations.

Alternators serve two main functions in ANY electrical system:

1. To recharge batteries after an engine startup.

2. To supply ALL electrical system requirements after the engine is running.

If all connections are tight, full alternator output will, first, cause battery damage. Batteries become the "load" for a runaway alternator. They absorb alternator current that is above what is required for normal machine operation. This high current causes battery damage from excessive: electrolyte temperatures, plate warping, and water loss. As the electrolyte level (water) drops, the chance of battery explosion increases and system voltage will also increase out of control. The result is damage to other electrical system components like relays, lighting and electronic controls.

Full alternator output with loose or poor connections in the charging circuit (even with batteries in good condition) can cause immediate high voltage damage to electrical system components. Loose or poor connections, such as at the battery terminals, can cause short periods of an "open circuit," which has the same result as batteries with low electrolyte. Opening the disconnect during an overcharging condition also has the same effect as a loose connection.

The following are clues to high charging system voltage due to voltage regulator failure:

1. More than one lighting component blackened or dead. One or more electronic controls dead.

2. Electrolyte level low in ALL cells of ALL batteries.

3. If engine is operational, charging voltage measures over 29.0 volts (or over 15.0 volts for 12V systems).

The following are clues to incorrect jump starting:

1. More than one lighting component blackened or dead. One or more electronic controls functioning incorrectly.

2. One or more batteries exploded. Electrolyte level in undamaged cells appears normal.

3. If engine is operational, charging voltage appears normal. If alternator was damaged by reverse hookup of jump source, the most likely failure would be low or no alternator output.