Answer

Sometimes when an alternator is working on and off interminently it takes a load off the engine when it is not working and that could be why it revs up. That will also cause the lights to dim.

 

There has been a service bulletin that seems to cover that type of concern. Here is what it states.

 

Bulletin No.: 02-06-03-008C

Date: May 30, 2007

INFORMATION

Subject:
Low Voltage Display on IP Gauge, Lights Dim at Stop Lights, Battery Discharged, No Start, Slow Cranking, Dim Lights at Idle, Low Generator Output

 

Models:
1990-2008 Passenger Cars and Light Duty Trucks (including Saturn)
2003-2008 HUMMER H2, H3
2005-2008 Saab 9-7X

 

Supercede:

This bulletin is being revised to add the 2008 model year. Please discard Corporate Bulletin Number 02-06-03-008B (Section 06 - Engine).

Any vehicle may have a low voltage display (if equipped with gauges), lights that dim at stop lights, slow cranking, no start, low generator output at idle or dim lights at idle when electrical loads are heavy at idle or under slow driving or infrequent usage conditions. These characteristics may be more noticeable with customer added electrical accessories, or with a discharged battery. These are normal operating characteristics of a vehicle electrical system and no repairs should be attempted unless a proven fault has been diagnosed.

During normal driving conditions, when engine speed is above 1000 RPM, the generator is designed to do two things:

Supply the current necessary to operate the vehicle's originally equipped electrical devices (loads).

Recharge/maintain the battery's state of charge.

The following factors may affect generator and battery performance:

Non-usage of the vehicle for extended periods of time. The vehicle's computers, clocks and the like will cause the battery state of charge to drop (For example; 30 days in a parking lot and the vehicle may not start because of a dead battery or a vehicle which is driven only a short distance once a week may end up with a discharged battery to the point where the vehicle may not start). This would be considered abnormal usage of the vehicle and the normally expected result for the vehicle battery, generator and electrical systems.

 

 

At idle, vehicle electrical loads may exceed the low speed current (amperage) output of the generator and when this happens the shortfall comes from the battery. This will result in a drop in the electrical system voltage as the battery delivers the additional electrical current to meet the demand. This is equivalent to the brown outs experienced by homes and businesses when the electrical demand is more than the supply. See Figure 1.

Extended periods of engine idling, with high electrical loads, may result in a discharged battery. Attempting to recharge a battery by letting the engine run at idle may not be beneficial unless all electrical loads are turned "OFF".

Increased internal generator temperatures from extended idling can also contribute to lower electrical system voltage. As the generator's internal temperature rises, the generator's output capability is reduced due to increased electrical resistance.

 

Expert:	Super Mechanic
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Accepts:
Answered:	1/1/2009

Master technician

29 yrs. exp., ASE master, cert. in G.M. electrical, a/c, susspension, brakes, electronic steering