Yanmar fuel pump 129612-52100: test and prime

The Yanmar fuel pump 129612-52100 is a 12 V electric lift pump, also called a feed pump. It moves diesel from the tank to the fuel filter and the injection pump on Yanmar TNV engines in compact excavators such as the VIO20-3, VIO25-3, VIO45, VIO55 and SV100. A failing pump causes hard starting and power loss. Check the power supply, strainer and filters before you replace it. The part is on our Yanmar 129612-52100 fuel pump page.

What the Yanmar fuel pump does

On a Yanmar TNV engine, fuel leaves the tank and passes through the fuel filter / water separator first. The electric fuel pump then pushes it to the fuel filter and on to the fuel injection pump. The injection pump receives more fuel than it needs, because fuel also cools and lubricates it. A relief valve returns the extra fuel to the tank. This order comes from the Yanmar TNV DI service manual.

The same manual describes the electric fuel pump as electro-magnetic, running on 12 VDC. Its job is to keep a constant supply of diesel at the injection pump. It does not create injection pressure. That is the injection pump's job.

According to the Broken Tractor listing for 129612-52100, the pump is used on these Yanmar machines:

• B25V-1 and SV100
• VIO20-3 and VIO25-3
• VIO45, VIO45-5B, VIO50 and VIO55

Our part page also lists the 4TNV88, 4TNV94 and 4TNV98 engines. Always confirm with the engine serial number or the old pump label before you order.

Signs of a failing lift pump

A weak or dead lift pump starves the injection pump. Typical symptoms are:

• The engine is hard to start, or needs long cranking, after it has stood for a while.
• It runs rough or loses power when the excavator works hard.
• Fuel pressure is low, or no fuel reaches the injection pump.
• In the end, the engine will not start.

The same symptoms can come from other faults. The Yanmar troubleshooting table lists a clogged fuel filter, a cracked or blocked fuel pipe, air in the fuel system and a clogged strainer at the feed pump inlet. Rule these out first, so you do not replace a good pump.

How to check the Yanmar fuel pump

Work through these checks in order. Most need only a test lamp or a multimeter.

1. Turn the key to ON with the engine off. On many machines you can hear or feel the pump click or hum. Silence points to a power or pump fault.
2. Measure the voltage at the pump connector with the key ON. If there is no 12 V, check the fuse, the relay, the wiring and the earth before you blame the pump.
3. Look at the fuel filter / water separator. Drain any water and replace the element if it is dirty. Yanmar fuel filter elements such as 41650-502320G are on our parts list.
4. Clean the strainer at the pump inlet. A blocked strainer stops flow even when the pump runs.
5. Check the hoses for cracks and air leaks. Yanmar says to replace rubber fuel hoses every two years or 2,000 hours, whichever comes first, because they dry out and become brittle.
6. Loosen the air bleed port with the key ON and catch the fuel in a container. A steady flow without bubbles means the pump is delivering.

If the pump has 12 V and a good earth but delivers little or no fuel, replace it.

For more on how the two filters protect the pump and injectors, read our guide to primary and secondary fuel filters.

Fitting and priming a new pump

Disconnect the battery before you start. Plug the fuel hoses while they are off, and catch spilt diesel in a container, not a rag. Note the hose directions so the new pump is not fitted backwards. Fit a clean strainer or filter with the new pump, because electric pumps are sensitive to dirt.

Then prime the system. The Yanmar TNV operation manual gives the method for engines with an electric fuel pump:

1. Turn the key switch to ON for 10 to 15 seconds.
2. Keep it there until the fuel from the air bleed port has no bubbles.
3. Close the air bleed port.

Never crank the starter motor to prime the system. Yanmar warns this can overheat the starter and damage the coils, the pinion or the ring gear. Wear eye protection, because the fuel system is under pressure.

Ordering the right part from FDA

The 129612-52100 belongs to our Yanmar parts range. We also quote a Yanmar 12 V fuel feed pump with part number 119225-52102. It is a different number, so do not treat the two as interchangeable without checking your engine's parts book.

Yanmar parts sit alongside the truck, trailer and equipment parts on our trailer and truck spare parts page. You can also search any number in our parts search.

To get a quotation, send us:

• The part number, 129612-52100, or a photo of the old pump label.
• The machine model, for example VIO55, and the engine model and serial number.
• The quantity, and whether you also need filters or hoses.

Send these through our quotation request form and our team will confirm the part before we quote.