Need some help interpreting what my voltmeter is trying to tell me. I have owned the truck for 3 years now (since new). All this time, after starting the truck, the voltmeter would always jump straight up to 2 marks past 14 as soon as the glow plugs extinguished.

For the past few days now, it has been acting as if the glow plugs were not extinguishing and will hover around 1 mark below 14. Ever so slowly as I drive the truck, it will make its way up to where it usually runs (2 marks above 14) after about 45 minutes of driving. All the time that it is running below the normal level, the panel lights seem dimmer than usual as well.

Is this a prelude of things to come? The truck has a set of Optima batteries that are only about 8 months old.

Thanks for any input.

Mine does that but not for so long. There is an intake air heater that runs for a while after the glow plugs go out. Maybe that is staying on too long. Is this when you are towing or all the time? It take my truck about an hour to reach the above 14v. mark when I have my trailer attached.

No, I haven't had the trailer on since this started happening.

Bump

Ehh... test the alt, test the glow plugs, test the batteries...

looking at another website about glow plugs... interesting stuff about testing them:


A typical glow (or heater) plug will have a high initial current draw that will gradually drop, stabilising at a constant amperage. The current draw will be dependent on the wattage rating of the glow plug. This data is available in the appropriate diesel data books.

Once the wattage has been ascertained, multiply it by the number of cylinders and then divide by the voltage to calculate the expected stabilised current. Example:-

Each glow plug = 150 watts so 4 glow plugs = 600 watts
current = watts divided by volts: 600 watts divided by 12 volts = 50 amps

The length of time that the glow plugs are operational can be measured from the initial drop in current to the switch off point, in this case it is around 17 seconds.


What I'm thinking is going on is either the glow plugs are staying on too long, The alternator doesn't have enough oomph to charge the batteries now, or the batteries have gone bad already.

Either way, most shops like napa and stuff have alternator/battery testing equipment and should be able to help for free. If it's glow plugs, I think the computer will tell you the glow plugs are bad..

Over many years, I have discovered that the GM alternators start going out around 70K, and I usually change them out about then as a precaution. You might look at having yours tested.