Questions and Answers
Fluctuating Voltage Readings
Q. Vehicle was purchased new 3/96. Beginning at the end of last summer, the dash volt-meter began to show wavering readings. Prior to this time the meter held a steadfast reading under all driving conditions. The needle held at 14.5 during normal driving and held at 14.0 while towing (24' travel trailer) no matter what accessories were in use, on the road or at idle. (all readings are estimates based on volt-meter scale).
Last Spring the A/C compressor and serpentine belt were replaced. A/C went bad and belt was done as normal maintenance item. Starting last summer, the volt-meter needle began to waiver once the vehicle comes to a braked stop. Depending on accessories in use the needle fluctuates (ie: in time with a turn signal) to readings as low as 13.0, until the vehicle starts to travel again.
I brought it in to the repair shop. First they checked the serpentine belt and it's tension (found both to be okay) and then they replaced the battery (it was the original and was "bloated"). Problem remained. Then they put on a new alternator (the output on the old one was fine, but they felt the regulator might be the problem). This also made no difference, so they put the original back on.
The wavering is worse as the truck and the outside temperature warms. In cold weather (cold starts) the meter, now, initially reads 15.5, and will steadily decrease to 14.5 while driving and the engine warms. Is this difference attributable to an aging vehicle, or should I be looking for another cause?
1996 GMC Yukon
5.7 liter
Automatic transmission
48,000 miles
ABS brakes
P/S - A/C
Cruise controlThank you...
A. Usually when this happens, it is due to a weak battery or a battery too small for the vehicle. Since the battery was replaced, I'm going to assume the new battery is good and of equal, if not higher, rating.
GM issued a bulletin, #43-64-07A, issued in January, 1997. From the bulletin:
Subject:
Low Voltage Reading or Dim Lights at IdleModels:
1990-97 Passenger Cars and TrucksAny 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 set point 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. 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.
So, according to General Motors, this is normal. But I think it can be corrected by installing a battery of a higher rating than that called for by GM.
Additional Information provided courtesy of ALLDATA
