No voltage feed at lift pump.

[Ratty1972]

Hi, I have a 2005 Audi A4 (B7) 2.5 TDI (V6) cabriolet. It will not start so I've set about fault finding. There are no fault codes showing and no fuel was coming from the tank. I have checked the fuel pump fuse (28) and the relay (fitted a new relay) and all appears well. I've tested the lift pump in the tank and it is working. I've checked the feed to the lift pump and I'm only getting around 3 volts?? The voltage does not increase when cranking the engine. Any ideas??

[VAG-Abound]

High resistance point in the supply or earth paths? Disconnect the earth side and see if the voltage lifts to 12V. If so supply is good but earth side is bad. Do you see 12V at fuse 28?

[Ratty1972]

Thanks for the reply. I'll check feed at the fuse, I only pulled the fuse to check if it was blown. Any idea where the earth point is? I did check the feed at the pump with a multi-meter and used the body to earth but still got no feed.

[VAG-Abound]

Not sure where that earth runs to at the mo. See what you find next and we'll loop back to it if needed. Check resistance of the pump connector earth pin to body (eg unpainted seat mount nut) - should be low of course.

[Crasher]

So the lift pump does not run if you hot wire the relay? The most common reason for these engines not running is failure of the distributor injection pump.

[Ratty1972]

I've not tried to bridge the relay yet as I'm not sure how to and wouldn't want to fry anything. Is it possible for the injection pump to kill the feed to the lift pump if it's detected a fault?

[Crasher]

There are 30 and 87 terminals on the relay which is live in and live out respectively, study how these plug into the carrier and bridge the terminals, add a 10amp fuse to your bridge wire for safety.

[Ratty1972]

I’ve checked the fuse again today. The fuse is fine. I’ve run a test with the multimeter and there’s no ignition feed to either side of the fuse. When the engine is cranked, it shows 12v+ but still not starting. I’ve run a further test at the lift pump end, this shows zero on ignition then 12v+ when cranked. The reading then cuts off after around 3 seconds and goes back to zero. I’m guessing that this means both fuse and relay are fine? There are still no codes displayed.

[Ratty1972]

So currently I have now got a feed going to the lift pump when the engine is being cranked. This is active for around 3 seconds then cuts off. I have disconnected the feed pipe from the side of the main pump (vp44) and fuel is being fed during the 3 seconds that the lift pump is active. This then in turn shuts off. Would this point to a mechanical failure of the vp44 pump? (I would assume I'd get a fault code if it was the ecu on the pump.)

[Crasher]

If the VP44 fuel pump diaphragm splits you don’t get a code. I will have to check at work but from memory the in tank pump just fills a swirl pot and the VEP pulls the fuel through, it certainly does on the A6 quattro such as the All Road with the same engine.