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2005 Pontiac Bonneville - voltage meter reads high

My voltage meter on the dash is reading 15-15.5 volts when running (pretty far to the right of the 14V the gauge has as the middle). Just replaced the alternator and the problem remains. I’m afraid it’ll blow out my lights or more important electrical equipment. What else could it be?

The car’s ECU controls the alternator’s output. There isn’t a voltage regulator anymore. The PCM controls it based on a number of things.

First off, is the car’s battery old? Have it checked, the alternator may be running overtime to keep a failing battery charged. Second, is the check engine light on? What are the codes?

Check it with a digital volt meter while running for an accurate reading.

The battery is new (1 yr old) and no check engine light. Also, an electric multimeter confirms the voltage.

Seems like enough info. I’m not as experienced as some of these guys with CPU regulation, but my next plan of action would be to confirmation the condition of the battery. Can be done at parts store. Then I would check to see if alternator and CPU are communicating accurately. Speculation: if there is unwanted resistance in either battery cable could that fool the computer to demand extra charge? If cables are good, theres going to be communication between CPU and charging system. Find where these are and what they are supposed to do. Make sure there’s no damage to this wiring. If the alternator, battery and all wiring is confirmed good, then the problem lies in the CPU, which is not likely. Somebody can correct me if I’m wrong, but that would be my flight plan.

Update: i like the idea of unwanted resistance in the loop that requires more oomph from the alternator. I just took apart all the wires and connections I could find in the electrical system, and I discovered that no matter how tight I tighten the positive terminal cable to my battery, it seems stripped and won’t fully tighten. Sounds about right to me. Now theres a new problem! Any ideas on how to fix this without buying a new battery?

You could cut a piece of thin copper to wrap around the battery terminal. Or you could file off some of the “ears” on the clamp, so the clamp can be tightened further.

Classic GM side terminal batteries! Check to see if it is the bolt that is stripped. They are readily available. Next see if you have a metric bolt a little bit bigger. A 10mm bolt is 0.5 mm bigger than the 3/8 inch battery bolt. A standard thread 10 mm bolt about the same length as the battery bolt will thread in and allow you to tighten the terminal.

Have you known the vehicle long enough to know whether this has always been the charging voltage or is it a new development? If new can you associate it with any changes or events? Whatever the reason, if this truly is the charging voltage as measured at the battery terminals and it continues for long periods of time it may be exceeding the healthy limit for the battery, can cause a lot of gassing and electrolyte loss, damage the plates, etc. Brief periods around 15vt. maybe okay, you can check both the battery and charging system specs for your specific ambient temperature, but if high voltage continues after the battery is charged, if the battery is heating more than normal or you are losing electrolyte excessively you have a problem.

One thing to try is slow charging the battery completely with a smart charger, one that knows when to taper down or turn off, followed by applying a moderate load for a few minutes (running the lights or heater motor) to remove the “surface charge” followed by measuring the terminal voltage with an accurate meter. If in the high 12’s to mid 13vt range the battery probably has a normal full charge. Then see what the (accurately measured) terminal voltage is while the car is running after starting the car and running it for perhaps 20-30 minutes - one normally would expect it to settle in the high 13 to low or mid 14vt range, (consult the specs). If you can get your hands on an appropriate amp meter (maybe the clamp on variety) you can check for excessive charging current. You also can check whether the suspect terminal is getting hot, confirming a poor connection. While not the norm it is possible for a battery to go bad within a year, some due to infant failures and others to excessive charging or discharging. Or it could be bad connections or something with the charge control system, as mentioned.