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.
Monday, March 15th, 2010 AT 9:01 PM