Intermittent Low-Voltage Light Sparks Curiosity
Question: The low-voltage light on my 1996 Pontiac Bonneville comes on intermittently. My gas station mechanic said he checked the charging system and determined that the alternator is putting out more than enough voltage to keep the battery charged. But he can’t figure out why the light is coming on. Is the light wrong, or do I have an electrical problem? --P.M.
Answer: If the alternator is supplying 14.5 volts to the battery, then the system should be providing the power the car needs.
You can generally tell if the system is not putting out enough voltage because the lights will be dim and the turn signals will typically blink more slowly. Conversely, if the battery is being overcharged, you will often see signs of acid splatter around the filler holes on the battery surface.
Let’s assume the mechanic is correct that the alternator and charging system are operating properly and that the problem is that the voltage warning light is giving a false reading.
There is a series of tests devised by Alldata, a publisher of automotive diagnostic and repair information, that your mechanic can perform to isolate the problem. The tests would tell whether the alternator has an internal electrical problem, the car’s wiring harness has a broken wire or the car’s central computer is malfunctioning.
The central computer, known as a powertrain control module, or PCM, costs about $220 plus labor to replace. A broken wire can be fixed for pretty much the labor cost, though finding the wire can take a lot of time. And a new alternator costs not quite $200 plus labor.
But if the problem is intermittent and the car is running fine, you might just want to ignore the light. Chances are you have a loose connection or broken wire in the line that transmits the low-voltage signal to the control module and instrument cluster.
Q: I have a question about mixing tires. I have a set of Pirelli P225/60R16 tires on my 1993 Lexus. Because of previous road damage on one tire and a defect on another, the tires have different mileages. Now one needs to be replaced because of wear. My spare is a Goodyear Eagle GA Touring P225/60R16. It has good tread but a plug repair in the center. Rather than buy one new Pirelli, can I use the Goodyear with the three good Pirellis? --R.M.B.
A: Most tire professionals would recommend against mixing the tires. At least your tires are all the same size, 16 inches. But they certainly have different tread patterns, so you’re going to get different traction out of those tires on different wheels.
In certain kinds of turns or emergency maneuvers, that mismatching could contribute to a loss of control. It could also pose a problem with braking, even with anti-lock brakes.
It’s not the worst thing in the world to mix different brands of tires, but at least you should keep the same type and brand on the front and likewise on the rear.
So you might consider buying two new Goodyear tires for the rear and run the two Pirellis on the front. Then you’d have a spare Pirelli for the front and a spare Goodyear for the rear.
As for the plug, if the tire was properly repaired, you should be OK. That’s assuming the problem was simply a puncture that did not damage the structural integrity of the belts or radial cords.
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Ralph Vartabedian cannot answer mail personally but responds in this column to automotive questions of general interest. Please do not telephone. Write to Your Wheels, Business Section, Los Angeles Times, 202 W. 1st St., Los Angeles, CA 90012. E-mail: ralph.vartabedian@latimes.com.
