Fuel Pump Relay with Connections to Coil


While converting a type IV motor to run in the '60 Double Cab, I found that I need to add a relay to control the fuel pump. I have found numerous solutions on the net, but decided to use the following circuit. I pulled two fuel pump relays at the local junkyard, one from an Audi and the other from a VW Rabbit. Both of these cars had Bosch CIS fuel injection. It turns out that both of these relays have the same VW/Audi part number. The Audi relay was made in Austria and the VW was made in Germany. Both relays were made by Siemens.

 Part number: 321 906 059 C

 I checked at the local parts house and a new relay costs around $45. I picked up both of these relays for $1.00/ea and with all the taxes was out of the junkyard with a total cost of $3.06

 I opened both relays up and looked inside and found that the basis of the circuit is a NE555 timer chip operating in the One-Shot Mode. As long as input pulses are received by the relay faster than the RC time constant, the relay will stay on.

 Calculating the time constant based upon the resistor and capacitor values results in a time of 0.52 Seconds. This allows the slow rotating distributor during startup to cause the fuel pump relay to turn on.

 One interesting note that I found out while experimenting with the relay, is that the circuit requires that the coil be in the circuit. Just applying 12V to the trigger input is not enough. The circuit relies on the higher voltage pulse that is generated by the coil when the points open.

 I hooked up a coil in the circuit and closed the coil circuit by hand (simulated points) and the relay operates exactly as expected.



Relay Terminal


Connect To


Switched +12V

Coil (+)



Chassis Ground


Fuel Pump Power

+12V Directly from Battery


Switched/Fused Power

Fuel Pump (+)


Trigger Input

Coil (-)


As you can see by the connections, this is really quite a simple circuit to use. When the motor stops, the relay will open 0.5 seconds later and the fuel pump will turn off.

Now for all you that want to see what is inside of the relay, I reverse engineered it and put together a schematic showing the relay and the connections to the engine and fuel pump.

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