Voltage drop!!??

[Trenton Sauer]

My truck recently started dropping voltage really really bad when headlights are on or the heating fans. I replaced the alternator I believe the battery is ok... is it possible that the battery can start the truck just fine but cant run everything at once?

 

[Trenton Sauer]

I ****** the battery and put a brand new alternator in it. Its almost like it isnt generating enough power to run everything. i dont understand how because I changed the alternator right after the problem started and the battery is new a month or two ago. Im so lost

 

[Oldblue98]

What does charging volts say with everything on ? Not by dash gauge but with a multi meter Turn all accessories on including lights and see what you got !? Check grounds for corrosion , including body to frame, engine to frame etc etc. Check plug terminal on alt to make sure it has a good connection with no hot spots. Check for Corrosion through out the battery cables not just the part you can see at the terminals.
Check battery with a load tester.

 

[sewlow]

Good grounds & good wires are important because poor grounds & poor wires cause voltage drop. The consequence of this voltage drop is that the systems in that circuit can malfunction. How do bad grounds/wires cause voltage drop? Well, the answer is Ohms Law.
Ohm's Law states that V=IR. This means that the voltage drop (V) is proportional to the current through the circuit (I) and the resistance (R) of the resistor (or bad ground in our case). An easy example is attempting to draw 10 amps through a bad ground with a resistance of 1 Ohm. So V=10(1) thus making the voltage drop a whopping 10V. This leaves only 2 volts available (in a standard automotive 12V system) to power the circuit and means that all that extra power must be dissipated as heat at the resistor (bad ground). Generally this heat makes the condition even worse, causing more voltage drop and more heat. In the most severe cases the ground will function under light loads yet fail under high current draws causing what appears to be an intermittent failure. Many people have seen this behavior caused by a loose (high resistance) battery terminal.

Bad grounds/poor wires can cause poor performance in almost any circuit. Voltage drop can manifest itself as dim headlights, one circuit malfunctioning when another is switched on (a common symptom is that the heater fan slows and dash lights dim when the brake pedal is pressed) and all kinds of other random weird issues.

 

[df2x4]

I'm going to guess corrosion in the cables somewhere.

 

[Eveready]

You probably have this sorted by now, but I noticed a trick recently to try if no.Just use a jumper cable and jump battery ground to a nice shiny frame ground somewhere. If it clears up the problem then you know its time to replace the cables and buff all the grounds .