Ranger wiring question, diode?

G.A.S.

Well-Known Member
I am working on cutting out all the BS on my harness for my race truck. The engine compartment fuse box has a diode for the PCM. 1st off, what is a diode? MY buddy told me it was to direct current in one direction. 2nd, do I really need it? I'm sure Ford put it there for a reason, but I just want to make sure if it is completley neccessary because I would like to get rid of as much as possible. Thanx.
 

partybarge_pilot

Well-Known Member
Diodes let electricity pass in one direction only. If theres one in there, it's there for a reason. On some vehicles you need one between the alternator and the ignition to keep it form back feeding. Without it, the vehicle would not shut off.......
 

_

Well-Known Member
Some applications use diodes to prevent damage in a reverse polarity situation. Meaning, the diode may be there to protect the ECU if Pos/Neg terminals of the battery were to be reversed.
Also as PB-Pilot mentioned, it is not uncommon for diodes to be installed in a circuit to prevent the alternator from back feeding power to the ignition (not sure how many and/or if any OEM circuits utilize diodes). In a race application, this is a bandage and diodes should be avoided if possible. There are other more reliable methods to resolving the back feed issue without the liability.
If you post a pics of your schematics, it may be easier to evaluate the OEM reasoning behind the diode.
 

_

Well-Known Member
Diodes have been known to fail, legs break off from physical stress and/or they fail internally from application/environmental reasons. When they fail, you're charging may become non-operational (note: different alternators have different characteristics). In a race application, the proper method to wire your alternator sense and excite leads is to wire them through an isolated pole on the ignition switch. Wiring in this manner will insure there is no feedback from the alternator to the remaining circuits on the vehicle when the switch is in the "OFF" position and will provide the greatest reliability.

Additional;
In the use of diodes to prevent feedback, some persons have installed them inline of the sense lead. This is not favorable as it may cause your alternator to overcharge. Vehicle electrical components are designed to run in at approx 13.8V. The alternator sense lead is there to insure the vehicle electrical system is maintained at 13.8 to 14.1 volts (depending on the alternator). Diodes have a .5V drop across them meaning that if your electrical system is at 14.1V, your alternator will sense 13.6V (with a diode inline) and increase the charge output voltage by .5V. This now means your electrical system is operating at 14.6V. This is not a problem for any of the vehicle's electrical components except for the battery. Lead acid batteries commonly experience premature failure when continuously subjected to voltages in excess of 14.1V to 14.2V. Lead acid batteries begin to gas when they exceed 13.8V to 13.9V, and the gassing becomes greater with increased voltage. The detriment in gassing outside of the obvious loss of electrolyte is, the gas is highly corrosive. The gas will eat away at the internal battery plates, dissolving lead and potentially causing premature failure. Many times this situation will go un-noticed until the time of failure. A common mode of battery failure occurs when your vehicle is running, you shut the engine off briefly, then attempt to re-start. At this time the reduction in physical size of the lead conductors within the battery (reduced in size due to corrosion from gassing (over voltage)) "fuse". This fusing is due to the high current draw through the already compromised lead conductors under starting conditions. Most often there are no previous symptoms prior to this type of failure.
 
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