There are 3 pages of posts that come up when you search for "fuel pump" in our forum, so I thought it would be nice to collect the pertinent info and put it in one thread.
First off, the problem: The motor turns over but doesn't start.
Questions to help diagnose:
1) when you open the driver door, do you hear the buzzing noise from under the car? If you hear that, your fuel pump is working. Your problem is not the fuel pump, fuel pump relay, or fuel pump fuse. Stop reading this thread, it does not apply to you.
If you do not hear the buzzing noise when you open the driver door, the fuel pump is not priming. Continue to step 2.
2) Does your interior dome light turn on when the door is open? Does the door chime sound when the key is in ignition and the door is opened? If the answer to both of those is "no", then your door latch microswitch is not working correctly. Go search for threads on how to fix that problem.
If the dome lights turn on with the door open, continue to step 3.
3) Is fuse #28 blown? That is the fuel pump fuse.
If it is blown, try replacing it (15A fuse) and see if normal operation resumes. Warning: blowing that fuse is usually indicative of a relay and/or pump going bad! If it blows when you put a new fuse in and turn the ignition on, continue to step 5. If nothing happens when you replace it, go to step 4.
4) Test the fuse location with a test light or mutimeter to see if there is 12V+ when the key is on. If there is 12V+, go to step 6. If there is not, go to step 5.
5) Check fuel pump relay. It's the 4th from the left on the bottom row of relays, under the dash. The label next to it says "diesel F". It's relay #409, part # 1J0 906 383 C. Costs about $43 online, and $66 at the dealer.
I'll let the experts weigh in here on how to "properly" test the relay.
Here are two possible methods that don't involve using jumpers in the fuse block or trying to wiggle a multimeter in there:
1) Borrow a 409 relay from anyone else who has a Mk4. Swap it in. If your fuel pump works with their relay in there, you've found the problem. This is by far the easiest way to test. Everyone knows someone else with a Mk4 - doesn't matter what model.
2) Go to step 6 & test the fuel pump itself, thus determining the relay was faulty.
6) Test fuel pump & connector. It's located under the right rear passenger seat. Remove the lower cushion, and fold the square flap forward to access the panel. Remove the 3 screws and pry up the cover panel gently. Check to see the main connection is in decent shape. Some of them are defective and come loose. Use a multimeter and see if it's getting voltage at the outer connections on the terminals. If there is no voltage, the relay may be the problem.
If you have a small battery (RC car battery works nicely) for testing, use the outer connections on the cover to the fuel pump to see if the pump operates when you apply current. Alternately, you can use a square 9V battery with a couple of leads from it. That should give enough current to see if the motor at least turns with current applied. It's a loud little thing, will be very evident whether it works or not.
If the motor works when you apply current, the relay may be faulty or the connector may be faulty. You'll have to do some more circuit testing to see what the deal is.
If the motor does not work when you apply current, the motor is toast. Replace it.
germanautoparts.com has the motor for $201.48 as of this writing.
More to be added to this posting as the experts weigh in...