Your voltage SHOULD rise with engine rpm. Today's charging system testers have you check the voltage at idle and again with the engine revved up. If it doesn't see a difference in voltage then it says there is a problem.
Even though yours is older than these, I think it still applies. I just read this from ALLDATA:
The voltage guage in the instrument panel may indicate low voltage on all 1990-2003 GM passenger cars and light duty trucks, including 2003 Hummer H2.
Any vehicle may have a low voltage display (if equipped with guages), 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, during slow speed driving, or when the veh. is used infrequently. These characteristics may be more noticeable with customer-added electrical accessories, or with a discharged battery. All of these are considered 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 the engine speed is above 1000 rpms, the generator (alternator) is designed to do two things:
1) Supply the current necessary to operate the vehicle's originally equipped electrical devices.
2) Recharge\maintain the battery's state of charge.
The following factors may affect alternator output and battery performance:
* Non-usage of the veh. for extended periods of time.(For example: 30 days in a garage or driving veh. only once a week). The vehicles computers, clocks, etc. will cause the battery state of charge to drop. These examples would be considered abnormal useage of the veh., with the normally expected result for the veh. battery, alternator, and electrical system.
* At idle, veh. electrical loads may exceed the low speed current (amperage) output of the alternator. When this happens, the shortfall is supplemented by the vehicle's battery. A drop in the electrical system voltage will result during times when the battery is supplementing the additional electrical current demand.
* Extended periods of idling, with high electrical loads, may result in a discharged battery. Attempting to recharge a battery may not be beneficial, unless all electrical loads are turned OFF.
* Increased internal alternator temperatures from extended idling can also contribute to lower electrical system voltage. As the alternator's internal temperature rises, the output capability is reduced due to increased electrical resistance.
Depending on the veh. application, alternator current (amperage) output at engine idle speeds of 600-700 rpms can be as low as 35% of the full rated output. With enough electrical loads ON, it is easy to exceed the alternator current (amperage) output when the engine is at an idle speed of 600-700 rpms. This is a normal condition. Under these extreme circumstances, the battery supplements the demand shortfall for short periods of time. When the vehicle road speed is above approx. 15 mph, the engine\alternator rpm is high enough so that the alternator current (amperage) output is sufficient to suppl the current requirements of the vehicle, as originally equipped, and recharge the battery.
Dimming lightsat idle may be considered normal for two reasons:
1) As the engine's alternator speed changes, so will the current output of the alternator. As the vehicle speed slows, engine\alternator rpm slows, and the current (amperage) output of the alternator may not be sufficient enough to supply the current necessary to support the electrical loads. Under these conditions, the charging system voltage will drop, and the lights will dim. Dimming of the lights is an indication that the battery is supplementing the low current supply. If the battery is already in a low state of charge, the driver may notice a more pronounced dimming in the lights than a veh. with a fully charged battery.
2) When high current loads (such as the blower, rear defogger, headlamps, cooling fan, etc.) are operating or cycled ON, the alternator's voltage regulator can delay the rise in output. This delay, usually at low engine speeds, can take up to ten seconds to ramp-up the alternator output. This delay is used in order to avoid loading the engine severely. In order toincrease current (amperage) output, additional engine torque is required by the alternator, which could cause engine rpm fluctuations. The powertrain control module (pcm) will ramp-up engine\alternator speed in small steps, so engine speed variations are not noticeable to the driver of the veh. Refer to TSB#02-06-03-008 for more info.
Source:General Motors Corp. (courtesy of ALLDATA LLC)
Thanks for the advice! I really appreciate it. The truck is running fine so I will just ignore the low voltage at idle.
