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Engine will not turn off? Fuel cut off solenoid stays on. How to fix.

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Had this problem on a 1989 2.5 TD 90. There is a lot of info to be gleaned from lots of previous posts in the forum, although a lot of the posts do not have any pics or do not explain the outcome of the troubleshooting. I had a lot of help from members on this forum but it still took me a while to work everything out. Hopefully this will make it relatively simple.

Ok, in summary, the engine is running, you park and want to turn off the engine. You turn the key but the engine keeps running. Probably what has happened is one of these things:

1/ The rubber stopper on the end of the fuel stop solenoid is damaged or deteriorated and allows fuel to get past the plunger.

2/The plunger is jammed or there is a fault with the spring.

3/The plunger is impeded from dropping down and sealing the hole by some swarf in the hole.

4/The white wire attached to the cut off solenoid, has some residual voltage (often as little as 2 volts) which is holding the plunger in the up position.

If you carefuly remove the solenoid and plunger (careful not to lose the spring) look at the bottom of the plunger there is a rubber tip. If this is not in perfect condition it will not seal over the hole.

If you look into the hole where the solenoid screws in and you can see swarf then it is likely that you will need your injection pump rebuilt.

The third case above, if you measure the voltage at the top of the cut off solenoid with the engine off and the ignition key in the off position, and find voltage around the 2 volts mark. This is enough to hold the solenoid in the open position, and the vehicle will keep running. The reason for this is because the alternator is backfeeding voltage via the charge light wire on your dashboard instrument panel. Possibly because of a lazy diode in your alternator.

This is what was happening in my case anyway, and i have read about others with the same symptoms.

200TDI and on models have an external diode sited in the loom, in the location of underneath right hand vent handle, but underneath the dash tray insert. To access it, you must remove the top piece of the dash, the left hand end grab handle, the sloping grey vertical trim, and then you can pull out the dash tray. The loom is underneath it in the corner.

The picture below shows the diode incased in heatshrink. This is on my 1995 300TDi 110:


You can see how the diode sits in the circuit in this diagram:


Note that the white wire coming in from the bottom of the page heading right, is from the ignition live. The brown with yellow wire at the bottom of the page, that heads off the page to the right, goes to the alternator.

The diode is there to stop a backfeed to the fuel cut off solenoid. As far as im aware this is only present on 200TDi models and on.

My 2.5TD did not have this diode arrangement, so i added it, exactly like in the wiring diagram. This fixed the problem of my fuel cut off solenoid staying energised, and not being able to turn off the engine.


I purchased the diode from Maplins for about £0.40p.http://www.maplin.co.uk/1n4148-switching-diode-46386

There is also a resistor, shown in the wiring diagram above. This is located behind the speedo cluster. It can take some rummaging to find it. i would advise removal of the steering wheel, disconnect the speedo cable, and then pull away the instrument panel and put it to one side. The picture below shows what you are looking for on a 2.5TD. Its the opaque fuse holder in the centre of the pic. Inside is a resistor:


This can be replaced in the same way as the diode if need be, just by soldering it to the wires and then heatshrink over it. You can buy the resistor at Maplins again: http://www.maplin.co.uk/metal-film-2w-resistor-5729

I replaced mine as i already had everything out. The original resistors are no longer available as far as i know.

The purpose of this resistor as far as i can tell, is to provide a voltage drop for the charge wrning bulb.

Hope this helps.

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