Charging - Low Voltage Reading Or Dim Lights At Idle
File In Section: 6 - EngineBulletin No.: 43-64-07A
Date: January, 1997
INFORMATION
Subject:
Low Voltage Reading or Dim Lights at Idle
Models:
1990-97 Passenger Cars and Trucks
This bulletin is being revised to add the 1996-97 model years and to update text. Please discard Corporate Bulletin Number 43-64-07 (Section 6 - Engine).
Any vehicle may have a low voltage reading (if equipped with gauges) or lights that dim, when electrical loads are heavy at idle, or under very slow driving conditions. This condition may be worse with owner added electrical accessories, or with a discharged battery. THIS CONDITION IS A NORMAL OPERATING CHARACTERISTIC OF THE VEHICLE, AND NO REPAIRS SHOULD BE ATTEMPTED UNLESS A PROVEN FAULT HAS BEEN FOUND.
At idle, vehicle electrical loads may exceed the low speed output of the generator, but the battery can make up for this shortfall from its reserve capacity for short periods. During normal driving conditions, the generator is designed to do two things: supply the necessary vehicle loads, and recharge the battery. Long periods of battery discharge due to high accessory loads at idle will cause the electrical system voltage to drop as the battery continues to deliver the electrical power. Increased generator temperatures from extended idling can also contribute to lower electrical system voltage. As temperatures rise, the voltage setpoint is reduced to avoid battery overcharge, and the generator's output capability is reduced due to increased electrical resistance.
Depending on the vehicle application, normal generator output at idle can be as low as 35% of the full rated output (see Figure 1). With enough electrical loads, it is easy to exceed the low speed generator output at idle. This is a NORMAL condition that the battery can compensate for during short periods. Items that affect the vehicle system voltage at idle are driving conditions, the number of electrical loads being used, add-on accessories, and extended idle times. Normal driving conditions will recharge the battery and restore the charging system to its normal state.
Some typical electrical loads are:
Load Amps
Rear window defogger 25
Headlamps (low) 15
Headlamps (high) 20
High blower 20
Windshield wipers 6
Ignition 6
Brake lights 5
Dimming lights at idle may be considered normal for two reasons. First, a vehicle with a low state-of-charge battery or under a discharge condition will have a normal low voltage condition. As vehicle speed changes, so will the output of the generator. As a vehicle slows, the output of the generator may not be sufficient to supply all demanded loads, so the vehicle system voltage will drop, and the lights will dim. Secondly, as high current loads (blower, rear defogger, headlamps, cooling fan, power seats or windows) are operated, the regulator can delay the rise in output. This effect, usually at lower engine speeds, can take up to ten seconds to ramp up the generator output. This is done to avoid loading the engine severely, so that engine speed variations caused by the increase in generator output are not noticeable to the driver.
Component Functional Tests
Make sure all battery and ground connections throughout the engine compartment are secure and tightened. The battery and generator performance can be checked as follows:
Battery
1. Turn the ignition switch and all accessories "OFF."
2. The battery should have a "green eye" and the terminal voltage should be greater than 12.0 volts.
3. If not, the battery should be charged and tested following the instructions in Section 6D1 of the appropriate Service Manual before testing the generator.
Generator
The regulator lamp circuit provides the best diagnostic indicator for generator faults. Confirm that the lamp is operating properly, and isolate the unit with the Universal CS Generator Tester (J 41450-B).
1. WITH ALL ACCESSORIES OFF, run engine above 2500 RPM.
2. Diagnostic lamp should remain off. Voltage at the battery terminals should be between 14.0 and 16.0 volts when cold, and between 13.0 and 15.0 volts with engine at normal operating temperature.
3. Install a carbon pile tester (VAT-40 or equivalent equipment) across the battery and an inductive ammeter on the generator output wire(s).
4. Run the engine above 2500 RPM and load the charging system until peak output is reached.
5. If the peak measured current is within 70% of its rated output and the diagnostic lamp remains off, the generator is good.
If the generator does not perform to the above tests, refer to Section 6D3 of the appropriate Service Manual.