Is there a difference in the round relays (red vs black)

[Tidybuoy]

I was jut wondering if there is a difference in the common round relays that are in our cars. I sometimes see red relays on some cars. Mine (all 3 of them) are black. I also noticed that my power window relay gets much warmer that the other two relays - fuel pump and rear defogger. My car is a '74 so it has far less relays than the newer cars.

[jimtweet]

yes there is a difference, i believe the red ones have a exra diode or something of some sorts, there is a whole thread on this if you search.

[spuggy]

+1 what jimtweet says.

There's extra "stuff" in the red relay to prevent the contacts arcing inside the relay when the field collapses as it makes and breaks. Red relays are normally used for the fuel pump, which can carry a significant amperage (Bosch rate my 044 fuel pump over 13 amps, although it seems to run fine on a 10A fuse).

On a later CIS car, the fuel pump is controlled by the air flapper and should switch off when it goes back to rest - e.g. when the motor isn't running. If this switch is badly adjusted at idle, it could cycle pretty frequently.