It’s doing this because the voltage set point of the L-N alternator is higher then the voltage the truck was designed around, it goes to 14.5 to 14.6 volts at initial startup. With the smaller pulley it gets there faster then it would with the stock size pulley. The ramp up changes though depending on battery conditions. I did not notice the issue much until I installed new batteries. New batteries have a higher CC/CCA capacity since the plates are not oxidized, so they alter the situation from my experience.
Again, I don’t have all the recorded data I did testing this alternator or all the things I tried at Prestolite’s suggesrions that might be causing this. None of their ideas changed my results and it got to the point where I thought they had no plans to do anything other then wear me out so I would go away. But here are a small pulley / stock pulley vids to show the differences.
Small pulley, you can see the GPCM cut out once volts go above the voltage, the alt backs off amp output, volts drops just below threshold, GPCM turns the glow plugs back on and the alternator reads the voltage drop due to the higher load, alternator kicks in full amps, volts exceed threashold. This was done on a spring day so it doesn’t take long for the batteries to fill up more and everything. Everything calms down once the voltage is high enough to just keep the GPCM turned off waiting for the PCM two minute time out.
This is the same batteries but the larger stock pulley and a colder temp. The alternator output amps is lower at idle so the ramp of the batteries replenishment is slower. The voltage still reaches over the threshold, but with the feed to the batteries slower and the slower rpm of the alternator the effect is dampened. Again, the GPCM still has shut down early with the higher volts, before the two minute time out.
This is from when I first started the project to test alternator output differences. After I completed all this work I decided to redo it as I needed to separate out the GP data and time it. But this recording is a warm summer morning with my old tired batteries. Again it’s the stock pulley and idle output here is 160-180a. Honestly, when we get to a alternator of this size output we really don’t need the smaller pulley. I fell into this trap too. We all started to go large with the assumption it’s after start when the FICMs fail, I no longer believe that. But that’s another story and we’re on alternators here.
I’ll throw this out too, it’s a combination of two variables, first altering the voltage sense connect direct to the alternator output, the second as factory, from the splice of all the fusible links, stock size pulley. Voltage still going over 14.4 and GPCM still reacting and shutting down as it should.
This time moving the sense connection all the way to the battery, no change as the voltage is still exceeding 14.4 and shutting down the GPCM before the two minute time out. But with the smaller pulley most times on my truck there is not the wild swings of the voltage regulator. You still can have the issue with the stock pulley, but the action is dampened. Now this is my truck, my batteries. The situation can still get reactive during some conditions. Some people have found providing a slightly higher load by keeping the HVAC fan on a notch or two helps during those first two minutes of startup, but IMO that’s very dependent on the individual situation.
Again, this is only an issue during the time the glow plugs are on. After that everything is normal.