1996 Chevy Silverado V8 Two Wheel Drive Automatic 240000 miles
When at an idle or driving, the engine wants to cut when I turn on the lights or brakes. Once when I even opened the drivers door at an idle. Battery at 12.86 volts DC and Alternator at 14.75 volts DC at an idle, at fire up, 15.25 volts DC. At one point the engine cut out and I had no power at all. No lights or dome light, nothing. Battery was good. After a few attemps, the power came up and the engine fired. Fuses all good. Ign. Switch replace in 2004. Today, engine cut out and took a few cranks to fire it up. Any thoughts?
Can you clarify the voltages? When is the battery at 12.86 volts? Engine off? Engine running? If the generator and battery are not at the same voltage, suspect a problem with the heavy wire going back to the battery.
14.75 volts with the engine running is perfect. 15.25 volts is a little high and suggests, along with some of your other observations, loose or corroded battery cables.
A running problem and a confused voltage regulator can also be caused by voltage spikes from the generator. The generator will keep the battery charged, but the spikes will confuse some of the computers on the truck.
March, 15, 2010 AT 3:35 PM
Battery is side post and the connections are clean and tight. Cable from battery to alternator is in good shape, no breaks and is also tight. Battery is at 12.86 volts with out the engine running. The 15.25 volts was when then engine first started up in the morning and after if warmed up, the volts dropped to 14.75. At the alternator and battery. Belt is good and not slipping. Did see a change in the dash gauge at one point, was over center point of 14 amps then went under 14 amps then back to 14 amps. Seems to run pretty much below 14 amps, say 13 or so.
March, 15, 2010 AT 4:43 PM
Don't rely too much on the dash gauge for diagnosis except that you are familiar with what is normal. The voltage readings at the battery and generator seem fine. Those are what really count.
At 240,000 miles, I would guess the generator has been replaced a couple of times already. If not, suspect worn brushes causing an intermittent no-charge condition. You will have to make some measurements when the problem is acting up.
12.8 volts at the battery suggests it's in dandy condition although an intermittently shorted cell, (not common), could cause the symptoms you listed. If you see your dash gauge drop to 13 volts or less, jump out, (stop the truck first : )), and measure the battery voltage. If it is indeed below 13.75 volts, measure at the back of the generator too. If both are low, suspect a defective generator, but first measure the voltage on the small plug-in red or brown wire, (I can't remember the color). It must have full battery voltage on it while the engine is running. It should have around 2 volts with the ignition switch on and the engine not running. That is the circuit that starts up the generator and turns the battery light on the dash on and off. A resistor is used if you don't have a battery light.
If you can catch it not cranking again, measure the voltages at the battery while a helper holds the ingition switch in the " crank" position.
March, 15, 2010 AT 6:45 PM
Thanks, I will give this a try. I have never really had any troubles with this truck other than fuel pump issues. I do pull a camper, 24 footer with this truck. I do think myself, that this could be an alternator issue do to the fact that the truck also charges the trailer battery when hooked to the trailer. Do you recommend a heavy duty alternator and will the computer handle more amp output from the alternator?
March, 15, 2010 AT 9:01 PM
Don't jump the gun until you're sure the alternator is the problem.
As for going to a heavy duty unit, you won't gain anything. The smallest unit you're going to find will still deliver well over 70 amps. A totally dead battery will only draw around 20 amps for a minute or two when it first starts to recharge. It only takes little more than that to maintain three fully charged batteries and run the rest of the electronics on the truck. The reason for the much higher capacity of your alternator is to prevent a drop in voltage when it is momentarily stressed to provide for power windows, power seats, wipers, radio, head lights, and heater fan on high all at the same time. A drop in system voltage can cause the many computer modules on the truck to become confused and do strange things. It is very unlikely you will ever need more current than the alternator can deliver.
Going to a larger alternator will only provide a larger maximum capacity. It will not charge batteries any faster or deliver more current then the truck demands. One potential unintended consequence has to do with the wire between the alternator's output terminal and the battery. In the late '70s - early '80s, for example, Chrysler products came with one of three different size alternators depending on the accessories on the car. A fuse link wire was part of the wire between the battery and alternator output terminal. All of the alternators mounted and plugged in the same so sometimes people installed one with the larger capacity. That would not cause a problem in itself, but if someone performed a ful-field load test to determine its maximum output, more current might be drawn through the fuse link than it could handle. To complete the modification properly, it was necessary to use a larger diameter wire with its larger fuse link.
If you haven't had a problem with the trailer attached before, there is no reason to need a higher capacity alternator now. It won't last any longer or run the electronics any better unless the trailer causes the current one to be near its maximum output all the time.
As for the Engine Computer, he has no idea what the charging sytem is doing or how much current it's delivering. The voltage regulator inside the alternator watches system voltage. An increase in current demand lowers sytem voltage. That tells the regulator to run the alternator harder to increase current flow. That results in voltage going back up.
Here's another way to visualize what you can't see. Think of a municipal water tower. As long as it's full, a pressure gauge at ground level says " 100 psi". When a little water, (current) is drawn off, the water level, and its pressure, (voltage) goes down. The water pump has no idea what the water level is in the tank, but it sees the drop in pressure so it delivers more water volume to build the pressure back up. There is a limit to how much water can be drawn at any given time based on the number of homes connected to the system, so all that's needed is a pump that's big enough to meet the demand if every home turned the toilet, sink, shower, and garden hose on at the same time. Installing a pump with a higher capacity would fill the tank faster, ... If the pipe diameter was also increased.
Hope that all made sense. Electrical was the hardest subject I taught because it can't be seen, touched, or manipulated. I had real good results by comparing anything electrical to water flow in a pipe or river.
March, 16, 2010 AT 7:58 AM
Thanks, with the help. I am a HVAC Service Tech, so I do the same with my customers, most customers can not understand the technical but using different ways to explain the seq. Of events, helps.
March, 22, 2010 AT 11:09 AM
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