Diode In Starter Solenoid?

[imp]

My nephew has installed a 1995- 5.0 HO in his '65 Galaxie, opting to go with the efi instead of carbureting it. Somewhere long ago, I heard, read, somehow was told, Ford had started placing a diode INSIDE the fender-mounted starter solenoid to protect the PCM against voltage spikes generated when the starter is switched on and off. The solenoids were identifiable by having only 3 terminals, instead of 4. So, I told him get rid of the '65 solenoid. This morning he calls from O'Reilly; he's specifying EEC-IV solenoid for 1993 Mustang; they showed him 3 or 4 different brands, ALL having 4 terminals. So, could the suppliers be putting the diode in ALL the product, to cover old and new starting systems?

Or, am I wrong on this? O'Reilly finally found a Motorcraft specced for Mustang: 3 Terminals! My later-year schematics show a power diode mounted directly in the underhood junction box (fusebox) feeding power to the PCM Power Relay. Ideas? imp

 

[jrichker]

The diode is a cheapy, easy thing to add. Use a 1N4004 or 1N4005 available from Radio Shack or other electronic suppliers.

Using a diode as a dampener in automotive circuits:

Theory of operation:
The diode acts as a surge absorber or one way gate. Anytime you turn the power off an inductive load (coil, motor, solenoid, etc) the collapse of the magnetic field creates a voltage spike of opposite polarity to the voltage applied. The positive wire sees a negative spike and the negative wire sees a positive spike. The diode is wired backwards so that it does not conduct during normal forward operation. When the magnetic field collapses, the diode sees the reverse polarity spike and clamps it. This prevents the spike from possibly damaging the rest of the electronics in the car.

When used to clamp spikes in automotive circuits, connect the lead that comes from the end of the diode that has the band on it to the positive side of the circuit. Connect it the other way and you will make smoke.

A properly functioning diode will measure very high resistance or open circuit in one direction. Reverse the meter leads and it will measure a lower resistance.