A short time ago I was travelling deep in the Mozambican bush in my 1998 Tdi Defender when the engine abruptly lost all power and the vehicle stopped dead in the middle of a road that is not marked on any map that I have ever seen.
A petrol engine will keep on running as long as it gets fuel and sparks, but a diesel engine needs only fuel. Troubleshooting is, therefore, simplified but the source of the problem is often that nightmarish device, the injector pump. I quickly established that there was fuel in all the important places all the way to the inlet of the injector pump but found that there was no spitting of droplets of fuel from a loosened coupling at an injector nozzle.
The way a diesel engine is stopped is by removing the fuel source, either by way of a mechanical pull-wire such as on a farm tractor, or by an electrically powered valve located in the injector pump body called the shut-off valve. The shut-off valve, being activated by a magnetic solenoid, is a convenient place for an immobiliser to disable the engine, the immobiliser controls a relay interposed between the vehicles battery power and the solenoid of the valve and can prevent the valve from passing fuel into the engine.
When the injector pump stops injecting suddenly it's probably because the shut-off valve is shut, and that can be either because the immobiliser has invoked its considerable authority, or because the coil of the solenoid is open-circuit and no longer passes any current. In the absence of a multi-meter the state of the immobiliser can be determined by scratching the wire to the shut-off valve on the engine metal and checking for sparks, if there are sparks there is a voltage at the solenoid. The normal current through the coil of the solenoid is about 1.5 amperes and this is sufficient to make a small spark if the electric lead is touched intermittently to the terminal of the solenoid. No spark means no current, and this is bad news. If the immobiliser was the culprit then the solenoid could be hot-wired to a permanent current source and the engine would run normally, until you wanted to shut it off by pulling out the hot-wire.
In my case the problem was obviously with the solenoid being open-circuit, which in the event was a huge problem because, firstly, the injector pump was cleverly designed by Bosch to make removal of the shut-off valve in the field extremely difficult. There is no normal spanner that can unscrew the solenoid from the pump body. There is insufficient clearance for a ring spanner to encircle the 23mm nut size, access for even the thinnest-walled socket is prevented by the pump body, and an open-ended flat spanner must be savagely trimmed to clear the nearby throttle-cable bracket, which can also not be removed because its screws are inaccessible while the pump is attached to the engine.
Secondly, hauling the vehicle out would require a long hike to a point of cellphone contact and the assistance of something very powerful, like a 4wd tractor or a 6-ton 4x4 truck because the road in both directions featured steep inclines of deep, soft sand. The injector pump can be taken out in the field but special tools are required to set a timing point before removal and a special device is required to hold the timing belt pulley when the support of the pump is taken away. A 24-hour guard would have to be hired to camp next to the vehicle and its contents from the depredation of hungry locals.
Faced with all of these unattractive alternatives and with little left to lose I found a cold-chisel and a light hammer in my toolkit and proceeded to gently tap the nut-head anti-clockwise with the hammer and the chisel. After ten or twenty whacks of the hammer on the chisel, and the chisel very near a corner of the nut-head the solenoid started to turn and was out within a few minutes bearing surprisingly little damage. With the solenoid unscrewed it is a simple matter to pull the plunger with its rubber tip from the solenoid body and return the solenoid to its hole in the pump body.
With the plunger removed there is nothing left to impede the fuel flow and the engine starts normally. To stop it you just pop the clutch with a high gear engaged and a foot on the brake pedal, remembering to turn the key to off before leaving the vehicle. The whole repair took less than an hour, most of which was used to curse Bosch for putting a fragile part (unnecessarily fragile) in such an awkward place. The alternative would have been hundreds of kilometers of towing, weeks of waitng for parts from SA, and and massive expense.
It is not possible to abuse this solenoid in normal use and nor is it possible to treat it with special care. How long your solenoid will last is, therefore, written somewhere in the stars. If mine lasted for ten years and 220,000km yours may last this long, or not, but there is a potential for failure in every one of them. If yours fails in an awkward place you will need only a hammer and a chisel or a long, strong screwdriver to get out of trouble. And an 8mm spanner to take the spade terminal off the head of the solenoid. And a mat to place under engine to catch whatever you drop. And a sense of humour.