VW (Bosch) TDI injection pump fuel shut off solenoid functions

Question

The Bosch diesel injection pumps used by VW in their ALH TDI engines have a fuel shutoff solenoid (N109) which is supposed to cut off fuel to the engine. The engine has been setting a DTC

17946 - Fuel Shutoff Solenoid (N109)     P1538 - 35-10 - Open or Short to Ground - Intermittent

This has been a real puzzle to me, I'd assumed that the fuel shutoff solenoid was used to shut down the engine, and the engine has always shutdown reliably. Today I was setting the IQ (injection quantity) in my on going effort to solve the engine's smoking problem (nicotine patches don't help…). As I watched the IQ drop to 0 when coasting in gear it got me thinking – is the fuel cutoff solenoid used to implement the coasting fuel cutoff? Does anybody know?

Answer 1

Since no one has chimed in, I will at least give it an answer.

I don't think that it would. The control unit for the diesel injection can limit fuel through the unit injector solenoids directly. The N109 takes a signal from the J491 crash control unit, although it also takes a signal from the J248 diesel injection control unit as well.

It would seem to certainly be there so that the fuel can be cut off in the event of a crash. It may also provide the function of a coasting fuel cutoff, but I can't be sure.

However, if the idea was to physically cut off fuel in a crash.. then I would say you would not want to do this in a coasting situation, as you would want to keep the fuel flowing and pressure in the fuel rail for the instant moment that you start to inject fuel again.

So I think it is a simple assumption that this solenoid is only used in a crash and to physically cut off fuel flow.

If you're not sure, you can always rule out the possibility by replacing it.

Keep an eye out for the VW pdfs that can be found online on their engines, they are a great source of information about the design and function of the engines.

1.9 Diesel: http://www.myarchive.us/richc/VW_TDI_with_PumpeDuse.pdf

Answer 2

My B4 Passat 1996, suddenly, would idle but not respond to the accelerator pedal. It would not go over 5mph before it would quit.

Answer: Bad N109 relay that was partially working and supplying a partial voltage to the shut-off solenoid on the injection pump. Just enough opening of the solenoid to allow idle. The solenoid has one wire to the top of it that should be 12 volts when solenoid is open, letting fuel flow. It grounds thru the engine block. Jumping a wire from battery to solenoid allows you to hear the "click" of it working and it being open to fuel flow. It allowed me to get home.

Stuck in parking lot, none of the mechanics I called had any idea what could be wrong. Went on-line and there were many posts on possibilities and how to get around it. Glad I had my cell phone. None of the parts stores had a N109 relay and it was an order item.

Answer 3

On the ALH engine the fuel shutoff solenoid does only that: shut off fuel in the VE pump.

In the 1980's I went on a family vacation with a pre-ALH engine IDI engine with a similar pump. The alternator died 700 miles from home. Removing the solenoid from the injection pump, and not having an ECU, the car made it home on just a charged battery, and a couple of bump starts.

That would not be possible today with the ALH, because of the additional electronics necessary, but I can assure you that the only function of that solenoid is to shut off or to enable fuel flow to the injection pump. If it fails, you can remove it, plug the hole, start the engine and get home. There are several ways to shut the engine down when you get there.