J538 Fuel Pump Control Module

Hi all,

I have a no-start condition on my RS6. I've narrowed it down to a lack of fuel. I don't hear the fuel pumps at all when I turn the key on. Before leaping into a fuel pump replacement project I started from the electrical side, which seems to have borne fruit, unlike the times on previous Audis where I hoped for an electrical problem only to have to get into the tank.

After some searching I finally found the J538 module behind the right side upper rear seat extension, behind some foam. (Were they trying to hide it??) With some difficulty I removed the connectors and I have confirmed that there's over 10 volts at the input during cranking. If I apply battery to the outputs directly I hear both pumps. So it appears it's the control module that's failed. (I say appears because I have not yet tried jumping the input to the outputs and verifying the car runs -- that's the next step.)

I'd like to take the module apart and inspect it, but I can't figure out how to get it out of the car. There are no visible mounting screws. Do I have to take the rear package shelf trim off to remove this? (If so, any tips?)

And of course if anyone has one sitting around gathering dust...

2 bolts/nuts on the back side:
https://c1552172.ssl.cf0.rackcdn.com/215738_x800.webp

Boy they didn't make this easy... thanks.
So it's under the package shelf trim then. Now to figure out how that comes apart...

Well, I made a jumper to bypass the module and I can now hear the pumps running, but the car still doesn't start. It's possible I just didn't hear the pumps when the seats were up.

Anyone have advice on where to hook up a fuel pressure gauge under the hood and what fittings I'll need? I guess it could be something as simple as a clogged fuel filter. It's been below freezing and I did run the tank almost to empty (oops), so it could even be ice related if there was any water in the tank. (I'm asking questions here rather than getting my hands dirty because the car is stuck in my driveway and it's cold outside :) )

Passenger side, wheel well/firewall. Can take the rubber line off the hardline, T there. Feed is the bigger one. Can use generic hose clamps and a T fitting. Pressure should be 58 psi with engine stopped, pumps running. At idle drops to 50ish.
Not RS, but looks similar: http://i1273.photobucket.com/albums/...ps1a5c17b3.jpg

Quote:

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Originally Posted by nubcake

2 bolts/nuts on the back side:
https://c1552172.ssl.cf0.rackcdn.com/215738_x800.webp

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Isn't that the control module for the auxiliary cooling fan (radiator fan #2)?

I was gonna say that. Cooling Fan control Module.

They're the same in shape, principle & function. And it's obviously easier to find a picture of a generic fan module, than rs6-specific fuel pump control unit.

Thanks for the tip on T-ing into the fuel line before it goes from hose clamps and nipples to the unknown fittings -- that is a good idea, especially since to get to the fuel rail connection you need to remove the airbox and MAFs.

The fuel pump module looks almost identical to the picture above, I guess they reused the case and connectors.

Well, it turns out the J538 was a wild goose chase: the actual problem was a plugged fuel filter. I put a new one in and the car runs fine now.

It's a little bitty thing, much smaller than in every other Audi I've had. I guess they had to switch to a tiny one for the RS6 to make room for the second fuel pump.

The vehicle applications for this filter are amusing: https://catalog.mahle-aftermarket.co....xhtml?eid=867

Lots of cars with < 75 HP on there. Nothing over 150 HP except the RS6.

One thing I did learn during this process is that there doesn't appear to be a fuel pump priming stage in this car. With the pump control module bypassed the fuel pumps are fully audible, and they don't start until the engine starts cranking. They are off at key-on.