Me and the wife were coming back from Alki Beach in West Seattle today. Had the urge for Spuds Fish and Chips. Anyway, we were in slow traffic, kind of like bumper to bumper- stop and go, I had the radar detector, A/C and stereo going and the radar detector started to flash "LOW VOLTAGE" and sure enough the dash gauge was down in an area it normally isn't. When I sped up the gauge would come up to normal and the radar detector went back to normal. Any ideas as to what could be happening? I'm gonna be taking it down for a load test to see what's up. Battery? Alternator? I don't want any electrical problems in the heat of the Summer. Thanks, Wayne

 

Alternators cannot generate their rated current at idle speeds so their output is current limited and any load current higher than the alternator output will be provided by the battery which will show as a system voltage lower than 13 volts. The radar detector draws very little current (about 0.2 amps), but the a/c makes the radiator fans come on (14 amps apiece) and the blower motor probably on high (haven't measured this one, but I'll guess 8 amps). The compressor clutch draws lots of current. Unless you are stuck for more than 1/2 hour, I wouldn't worry about it since the alternator will pick up the load when you get underway.

 

Here's what GM had to say in 1997:

Charging - Low Voltage Reading Or Dim Lights At Idle

File In Section: 6 - Engine

Bulletin 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.



I'm not saying you don't have a problem and if it is, I'd suspect the battery's reserve capacity first. Assuming you have the CS130 105 amp alternator, the last time a member slapped a clamp-on meter on one, they found a measly 30 to 35 amps at idle. I'd forget about load testing the alternator since that will be done cold. GM'S chart leaves out the fans - probably 10 to 15 amps on top of the blower motor which if worn, may be drawing 25 and not the 20 in the chart.

Your Vette has 15 year old wiring so you can be sure its resistance is considerably higher than when it was new.

You could upgrade to a higher rated output though be sure the low rpm operation is rated higher too - many seem to produce the same meager figures until they reach full speed. www.alternatorparts.com advertises a few, though I recall a member buying one of their "Iceberg" units (it may have been purchased from another source) and it fell apart or failed within a few weeks.

 

It could be normal or it could be a faulty alternator.
Mine started acting up like yours and continued to get worse with the system chaage light coming on even at running speeds. A quick drive to Autozone and testing on the car with a Baer diagnostic unit showed a faulty diode pattern.
The replacement diode cost me $6 and I learned the how to's of rebuilding an alternator

 

Thanks for all the good input. I'm gonna have to get things checked out just to make sure it's all good. I will also have to spend more time in the C4 Tech/Performance area than in the General Discussion area. Again, thanks for the wealth of Info.