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Engine Running On

For those of you that have experienced their cars running on (often referred to as dieseling) even when the ignition is switched off and have tried all the usual remedies for this such as adjusting timing, fuel mixture or removing carbon build up in the combustion chamber

but still didn’t manage to cure it. There is one cheap and easy thing to try, particularly if you have converted to an alternator and/or have had other modifications on the wiring.

What can happen is that when you switch off, your alternator pushes power back down the wire to the ignition warning light, then on to the ignition switch, keeping the circuit live momentarily or in more extreme cases permanently, until such times you pull a wire off the coil to break the circuit.

Image of 1N5822 Diode1N5822 Diode

To do this fix you will need a diode. I bought a 1N5822 diode from Ebay, just a few quid for a bag full. The diode is placed in-line on the thin cable to the alternator and allows power to travel in one direction only, towards the alternator and not back the other way.

Here’s a little step by step guide:

Step 1 – Pull off the connector from the back of your alternator, strip back the black sleeving and you should see either one or two thicker cables, these take the charge back to the battery, and one thinner wire which is from the ignition warning light, it’s the thinner wire you want.

Diode Soldered In-Line
Diode Soldered In-Line

Step 2 – Cut the thinner wire, trim back enough to allow the diode to fit neatly in-line, solder the wire to diode and protect with some heat shrink tubing. 

Step 3 – Use harness or insulating tape to tidy up the cables then refit the plug to the back of the alternator.

Diode Soldered In-LineDiode Soldered In-Line

It’s not always easy by looking at the diode as to which way the current flows, so you can quite easily fit the diode the wrong way round. If you do this it won’t do any harm, but it will stop the ignition warning light from coming on, don’t ask me how I know this!

Written by: Darren Groves