Functionality of our diaphragm pump

The electric fuel pump HARDI is a piston diaphragm pump, where the electric part is separated from the hydraulic part by the diaphragm. In the HARDI pump, a voltage generates an electric field in a solenoid coil, which causes an armature to be attracted to a core against a spring, causing the diaphragm to perform a stroke. This stroke creates a vacuum in the lower part of the pump, in the valve housing, which sucks the fuel into the valve housing through the intake valve.

At this very moment of the suction process, the push rod (a plunger), which leads from the diaphragm to the electrical system, actuates a microswitch, which interrupts the current to the coil, brings the magnetic field to a standstill and causes the conical compression spring to return the diaphragm to its initial state. As the diaphragm descends, the fuel in the valve body is forced out through the pump discharge valve, completing a delivery stroke.

However, as soon as the diaphragm is pushed down by the compression spring, the microswitch is again closed, the current flows through the coil, the magnetic field is built up and the diaphragm is again moved in the opposite direction.

So the whole control system is designed in such a way that by switching strokes one after the other, a conveyance is created.

Here a Video Youtube-Video

Technical knowledge / explanations

Response voltage

The response voltage represents an important quality criterion for electric fuel pumps. Due to weak batteries, especially in winter, the available voltage of the battery can drop to 10.5 V. The pump must start and deliver reliably even at this undervoltage and a temperature of - 30 degrees.

Even with this undervoltage and a temperature of - 30 degrees, the pump must start and deliver reliably, because the pump should deliver the gasoline to the carburetor before the starter even turns.

With the HARDI pumps a response voltage of max. 9.5 V is always guaranteed, in practice the value is even lower, e.g. 8.5 V with the 13312.

With 24 V pumps the response voltage is even lower, approx. 16 V.

A good response voltage must also be ensured for the 6 V pump. Due to the very low output voltage of only 6 V, each pump is set in such a way that a response voltage of 5.5 V is guaranteed in any case.

For which media is the HARDI pump suitable ?

Since about 2018, every HARDI pump is suitable for all commercial fuels, i.e. gasoline E10, premium gasoline, diesel, also methanol and biofuels. Since then, only a very resistant type of diaphragm (black) is used. Older pumps may still have white plastic diaphragms in use. If this is the case, we advise you to change the diaphragm, please contact us for the necessary service/replacement... .

For kerosene there is no official release, but we know from users that they also use HARDI for the pumping of kerosene.


The installation of the pump is possible without problems in many places in the car, but an installation near the tank is recommended, since the pump prefers to push as well as suck.

Since the pump has spring-loaded valves, it can be installed vertically or horizontally depending on the available space. It requires only an electrical connection to terminal 15 of the ignition, where it is powered as soon as the ignition is turned on and immediately begins to deliver and prime fuel toward the carburetor.

The pump is polarity protected, so if + and - are mixed up, the pump will not be damaged but will not run. On vehicles with positive ground, only the electrical wires must be reversed when connecting to the pump.

The pump must be installed in such a way that no resonance floor is created. This can be achieved by additional rubber buffers.

In addition, the pump must be installed tightly, the lines must be secured with hose clamps or, in the case of screwed connections, must be countered; this is particularly important for the 9912 pump.

Only if the pump is installed tightly, it will run quietly and only a slight click will be heard.

During the priming process the pump is much louder.

All pumps are leak-tested on the test bench and leave the factory leak-tight.

During operation, dirt or dirt/threads/chips can of course settle in the valve, so that the valve no longer closes completely and the pump sucks in very poorly and is very loud.

For safety reasons, we always recommend the use of a safety shutdown when installing an electric fuel pump.

Basically, we believe that we have the right pressure for each carburetor due to our three pressure ranges. Thus, a pressure reducer would not be necessary from our point of view - if nevertheless one is desired (for safety 🙂 ) it is available in the store.

Topic Filter

In recent years, the subject of filters has become increasingly important and is certainly one of the most important reasons for a functioning system with the HARDI pump. Basically, for us the correct line away from the tank is as follows....


Thus, our preference is no filter in front of the carburetor. Our HARDI pump works with the back pressure of the carburetor and therefore obstacles (like the filter) are counterproductive regarding the proper function of the HARDI pump. Through many conversations with customers we know that systems can still work (i.e. even with a filter in front of the carburetor). However, we want to talk about the optimal installation here....

We have tested about 15 different small filters in the last 7 years and can only really recommend the following filters with a clear conscience. The experience is basically with all "bad" filters that too much power and thus flow rate is taken from the pump. This in turn can lead to under-delivery of fuel in some cases.

Likewise, we always point out that the HARDI pump should also be protected from dirt. However, this does not mean that a filter is installed before and after the pump. This would also be counterproductive and the system with the HARDI pump would probably not work for long.

Now here are the filters that passed our test as successful. The loss of flow rate was about 5-8L/h

Fuel filter 8mm from Pierburg 4.00030.80.0

Fuel filter 6/8mm from Mahle Knecht KL 63 OF

Fuel filter 8/10mm from SF SK3759

From our point of view, changing these filters once a year/season is enough and everything should be don't need anything more, not even anything that is more expensive.... 🙂