RE: Relay problem in 74 GTV

["John Fielding" <johnf@domain.elided>]

Ralph DeLauretis in AD V8 #1050 asked about relay heating feeding a Spica fuel pump and
whether the relay should get so hot.

It depends on the specification of the relay!

Some relays are rated for continuous operation others are only rated for intermittent
operation.  Examples of intermittent operation are those intended to power horns, high
beam flashers, starter solenoids etc.  Other relays are intended for continuous operation,
the master relay in the Bosch EFI  systems which contain the diode in series with the
relay coil (0332-014-112) are rated for continuous duty.

The vast majority of the heat generated by a relay is due to the resistance of the coil,
the contacts have to be really bad to cause any heating.  Modern automotive relays
designed for continuous operation are safe to at least 100C, which is quite common under
the hood of a car.  Intermittent relays tend to have relay coil resistance of approx.60
ohms = 220mA, those for continuous operation tend to be in excess of 80 ohms = 165mA.  The
Bosch type with the diode can't be measured directly with an ohmeter but typically draw
approx.  200mA , the other common Bosch relay which doesn't contain the diode but is
otherwise identical externally (0332-014-113) draws approx. 150mA.

Not sure how much current a Spica fuel pump draws but doubt if it would be as much as 15A,
the Bosch EFI high pressure pumps typically draw approx. 8A and the standard installation
uses a standard 30A relay and a 10A fuse.

John
Durban
South Africa
Alfetta 1.8L turbo
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