VW Passat B6 Fuel Pump Control Module replacement (because of battery drain) … by David HK May 2017

Recently I have a battery drain problem with my 2008 Passat B6, the battery went dead overnight (the car has been driven the day before), zero voltage, even the door/alarm can’t be open, after manually open the door with the physical key, I inserted the electronic key, no light from the dashboard, nothing!

So I asked my local garage to bring and installed a new battery, tested for battery drain using a very small light bulb connected between the battery negative pole and the car negative thick wire (which was disconnected), it was very bright, from his professional experience, he said the light bulb should be very dim or no light at all after a minute or 2.  He advised me should take the car to the VW dealer to have a thorough dialogistic, they should have the equipment and computer and so on, for him to check, it may take days or even weeks and still find nothing, etc. so I interpreted that he was reluctant to take on this difficult job to investigate where and fix the battery drain.

Before he left, he disconnected the new battery’s negative wire to avoid draining the new battery, and told me to listen for any hidden noise inside the car when the battery is connected, such as small motors inside the car seats etc, or if something consumes electricity, there may be some ohm sounds.

So in the evening, I asked a friend around to help me to do the listening sitting inside the car, closed all doors (the Passat cabin itself is a very good sound insulator), while I connect / disconnect the battery negative wire.  In 1^st attempt, he immediately detected a sound inside the car when the battery is connected, because when the battery is disconnected, the Passat cabin is just dead silence!

But he was unable to pin point where is the sound come from, it is a constant low frequency sound.  So we switched our role, this time I sat inside the car, at the backseat (I was suspecting small motors inside the front seats, so I sit in the back to try to get a better hearing).  I closed the doors, give him the signal to connect the battery negative wire, immediately there is a sound, and it is coming from the back of my right shoulder level, so I put my ear at the right hand top corner of the back seat, there! Sounds like some liquid floating across while in the background there is a motor sound.  At that time, I have no idea there is a low pressure fuel pump and a fuel pump control module just under the back seat that I am sitting (on the right hand side).

So next morning I bring my car to my local garage, told him what I found, and he also listen and acknowledge there is something with the fuel pump, I asked is there some kinds of relay to control the fuel pump? He said yes, and also connected his OBD scanner, it reported the fuel pump voltage resistance is too high, and possible fuel pump problem (in Chinese! I suspected he bought his scanner from China, it is a dedicated box, with a small LCD display and the connection wire).  He quoted me fuel pump cost is around US$600, and the fuel pump control module is another US$200, excluding labour!

I was totally unexpected for such expensive parts, because I just spent more than US$1300 on 2 front wheel bearing, high pressure fuel pump, annual oil change, and the MOT.

Anyway, his small garage cannot take my car on that day as he has current jobs not finished yet, and I have my Easter vacation in 2 days time, so I bring my car back, disconnect the batter for the time bring.

Upon my returning from vacation, I disconnected the fuel pump control module for inspection, there are some oily liquid leaking out from the module, and there is a melted plastic ‘pop’, I suspected the oily liquid is coming out from the melted ‘pop’, and caused by over-heated electronic components inside the module, such as voltage regular, which tends to get very hot from my quadcopter building experience.

In the mean time I searched the web on all things related to fuel pump and the fuel pump control module.  Most people with a faulty fuel pump control module is that it causes the fuel pump not working, or working intermittently, there is 1 case that is causing the fuel pump continue spinning even with the ignition off! And his fuel pump control module has the exact melted plastic ‘pop’ on the exact location of the module.  It looks like there is a very hot heat point inside the module, causing the plastic to start melting at that point, until it melt through the plastic wall and some melted plastic started to form a small ‘pop’ on the outside!

I believe the fuel pump is working because it can provide fuel to drive the car, and continue spinning even after the ignition is off.  It is just that it won’t stop because the control module continuously provides current to the pump, causing by an ‘always’ on signal voltage inside the module somehow.

Since there are so many Volkswagen cars in China, I suspect I can buy this fuel pump control module much cheaper from China.  So I bought one from Taobao, the shop seems to sell genuine vw band spare parts, all with VW / Audi logo and part number and so on, so I give it try and bought the module.

And here it is, an enhanced version, with heat-sunk on top.  I replaced the module, check for battery leak, no more leak.  Also, I added 2 more small pieces of computer heat-sunk on top, to provide more heat dispersion.  

Further, I added 2 ‘air tunnel’ underneath the seat and on top of the module, these are the plastic tube from the litre size shampoo or shower bottles! Hopefully this will provide some air flow for the module to dispense the heat.    

So far the car has been running for more than 4 weeks, nothing unusual pops up, hopefully the module can last many years.  Fingers crossed! :)