I know we're not suppose to post question about automotive stuff but this is more of an educational question. I found an old relay red on the net how to test it pretty simple from the battery pin 85 and 86 to the continuity tester pin 30 and 87. My question is how come there's no voltage across pin 30 and 87? Aren't those 2 pins feeding the high amperage side of the relay?

 

Unless it's a special relay, the coil and the contacts are isolated. Those Bosch style relays are everywhere, connecting one coil terminal to one contact would severely hamper their versatility.

 

I'm trying to understand how can a relay power up headlights on a car if there's no voltage at the "output" of the relay pin 30 and 87?

 

It's a switch. You connect voltage on one pin and the load on the other.

 

I know but how come there's no voltage on pin 30 and 87 how would the light light up if there's no voltage?

 

There is no voltage on pin 30 until you apply it. A relay can be used to switch any voltage that it's rated for. Let's say you want to switch 24volts and have 12volts as your control voltage. Having 12 volts on pin 30 would make the relay useless to you, that's why the coil (control) and contacts are isolated from each other.

 

You do understand how a switch works, right? It simply opens or closes a connection. It provides no intrinsic voltage on its own. A relay contact is the same as a switch.

 

OK so on pin 30 and 87 when closed that's when the higher voltage would kick in the lower voltage is only used to magnetized the coil? All that from the same voltage source?

 

OK so on pin 30 and 87 when closed that's when the higher voltage would kick in the lower voltage is only used to magnetized the coil? All that from the same voltage source?

It can be from the same voltage source if both the coil and the load have the same voltage rating, or it can be different if not.

 

I know we're not suppose to post question about automotive stuff but this is more of an educational question. I found an old relay red on the net how to test it pretty simple from the battery pin 85 and 86 to the continuity tester pin 30 and 87. My question is how come there's no voltage across pin 30 and 87? Aren't those 2 pins feeding the high amperage side of the relay?

What is missing for you here is the car itself. When the relay is plugged into the car at the correct socket it gets power from the electrical system (battery and/or alternator) using some combination of pins 85 and 86. Somewhere else in the car is a switch that allows current to flow in the coil side of the relay.

Also in that socket is the power for either pin 30 or 87.

So all power, and all control, and all the loads get connected from that socket. They exist outside of the relay.

 

Maybe this will help

 

Perfect thanks