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RPM Relay

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The RPM relay is responsible for running the fuel pump while the car is running, as well as providing power to the warm-up regulator and turning on the main fuel relay[1]. It is #34 in the relay compartment diagram[2].

Note that this is a special relay, not a common automotive relay. It has a larger, rectangular shape with six pins. The stock Bosche relay is black, but green Volvo ones are also available.

Solid State RPM Relay Edit

Ozzie H. has posted a detailed review of a new solid state RPM relay on his blog. The new relay, designed by Dave McKeen (aka bitsyncmaster on DMCTalk), replaces the original relay, which was built of discreet components, with a new microcontroller-based design featuring surface mount components.

The new relay not only improves reliability, but includes a "hot start" feature that will prime the fuel system for one second at one hour intervals while the car is parked, ensuring that the fuel pressure remains at optimal levels and minimizing cranking when starting. It also draws significantly less power than the original relay, emits less heat, and has no mechanical parts that can wear out. Dave has also commented on criticisms about the fail-safe ability of power transistors versus the conventional relays used in the original design.

The new relay can be purchased directly from Dave. He can be contacted at: davidmckeen@netzero.net, or via a private message to Bitsyncmaster on DMCTalk. The cost of the new model includes a core swap for your old RPM relay.

Manually Running the Fuel Pump Edit

There are a few cases where it is useful to manually operate the fuel pump, such as assisting with emptying the gas tank, or to test parts of the fuel system. It may also be useful in an emergency when the RPM relay has failed on the road. Note that this bypasses the ECU and will affect the fuel mixture, and as such a car should only be driven in emergency situations with this bypass in place.

To run the pump manually, you can remove the RPM relay from the relay compartment and short pins 30 and 87 of the relay socket together with a suitable jumper.[1] You should be able to hear a faint hum or buzz from the front of the car as the pump runs.

Special T Auto has a photo of such a jumper and how it looks installed.

See Also Edit

External Links Edit

References Edit

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