Why is a diode installed on a fuel pump relay?

RedSabre · #1 (**permalink**) 10-10-2007, 06:05 PM

My electric fuel pump has a Siemens relay in the power circuit and is getting really hot so I am replacing it since it causes my fuel supply to stop on occasion. Question we have is: the relay has a diode installed across the 85 to 86 terminals and was wondering why. Is it for back current power surges? I know this is not the way it was manufactured and have not seen this before. Will the diode need a certain amperage limit? Obviously needs to be 12-14 volt handling capacity. Any ideas?

Mike

autoxmike · #2 (**permalink**) 10-10-2007, 06:33 PM

Diodes are also used with relays to protect transistors and integrated circuits from the brief high voltage produced when the relay coil is switched off. Conduction only occurs when the relay coil is switched off, at this moment current tries to continue flowing through the coil and it is harmlessly diverted through the diode. Without the diode no current could flow and the coil would produce a damaging high voltage 'spike' in its attempt to keep the current flowing.

RedSabre · #3 (**permalink**) 10-11-2007, 03:43 PM

Thanks for the info Mike. Guess we will need to check the diode to see if it is good or not and if so, will install it on the new relay.

Mike