Originally Posted by
Bluegrass 7
Half voltage is too low indicating there is to much resistance in the pump circuit feed or ground return.
Most pumps draw an average of about 7 amps.
At 12 volts cranking voltage the total circuit resistance would be close to 1/2 ohm.
How this is is 12v / 7a =.58 ohms.
If the circuit resistance rises for some reason, using the voltage you see as 6.83, the resistance would be .98 ohm or nearly double.
Taking the sum of the voltage drops for 12v - 6.83v would mean the circuit fault is dropping 5.17 volts. 6.83+5.17 = 12v.
This amount of drop is not tolerable in that circuit same at it would not be for lighting at any point on the truck.
This is all new to me so bare with me... I take it the voltage isn't supposed to drop at all after two seconds? Would this drop indicate something is "turning on" at that time causing the resistance and voltage drop? If not and say it's just a bad ground, wouldn't the resistance be immediate and always show 6.83 volts where I tested?
Originally Posted by
Bluegrass 7
Another item to keep in mind is if the return line is blocked, the pump is trying to pump against it and may draw excess current that will drop the voltage if there is any substanical resistance in the circuit.
Once the truck is started everything is normal at any speed, even quick starts and stops so I'm
assuming there is no blockage.