So I'm going to show some ignorance and laziness by just flat out asking...

What is a relay and how can I check one?

Now I'll tell what I think the answer is and why I'm asking.
Wikipedia (http://en.wikipedia.org/wiki/Relay) says that a common application of a relay is to control a high-current circuit with a low-current signal. I'm guessing that is the application of a relay for the automotive "computer".

The real reason for this post is because my '96 351 was having trouble starting. When I tried to start it would act like there was no gas. I could turn the key to power and not to start and then hold the gas pedal down for about 10 seconds. Then on the next turn of the key the engine would start. I thought it was a fuel problem and had a PO320 code.

I took her to a new mechanic on recommendation from a friend. After some diagnosis he found it was a relay to the "computer" that was bad. I don't like to use words like bad or broken or fixed. I crave the details. Why does the armature no longer mechanically move to a contact. In my mind if the spring was the problem it would stay in the on position and I would never be the wiser. Any thoughts? Maybe I should just be quiet and be happy with a $14 part and $65 diagnosis.

Thanks,
Bobby

 

The relays employed in our trucks are (for the most part) standard "normally open" relays with a 12VDC (Volts Direct Current)coil. The coil must be energized to move the armature (set of contact points on a pivot and held "open" by a spring). The coil must be supplied a positive feed and a negative (ground in an automotive application). This is known as the switch circuit for the relay. When positive is supplied the coil is energized creating an electromagnet field. The magnetic field generated, overcomes the force of the spring and the contact points "close" which energizes the load circiut of the relay. This is the simplest type of relay I have described. There are many other types but the principle is the same. A relay is basically a remote switch that is activated by another mechanical switch of some kind. It saves manufacturers from having to run heavier wiring and heavier duty switches throughout the vehicle. Notice that the relays are found near the main power feeds. If you look closely at the side of one of the relays in the truck, you will see a small diagram that deliniates all of the parts I am referring to.

When a relay fails it can have several things happen. Most often, the coil winding snaps due to old age, deterioration from corrosion, or a short circuit from an outside source. Occasionally , the spring will fail but this is easy to determine since the absence of the spring function will allow the points to rattle back and forth on the pivot point causing intermittant operation of the relay load circuit even when the switch circuit is NOT engergized.