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My car starts fine, but when I switch the ignition key to off, the engine still stays running. The accessories etc all turn off and even if I take the key out it still keeps running. The only way I can stop the engine is by pulling out the lead to the ignition coil. Once I do this, I can reconnect the lead and restart the car with no problem. The accessories all come on fine. The car has a new ignition switch, so at a loss as to why it won't switch off?

Car is a 1983 BMW 735i

  • Is there a permanent supply to the coil? Test and if it is, then you have the wires on the switch incorrect. – Solar Mike May 14 at 6:01
  • Did you install the new ignition switch due to the current fault you are experiencing? – HandyHowie May 14 at 7:14
  • Hi guys - I did install the new switch as I had this occur not long ago (about 3-4 months). Once we put in the new switch it all was fine until now. Will check for permanent supply to the coil. I will also check the main relay which powers the fuel pump relay, injectors & DME. – Frank May 14 at 8:32
  • Any aftermarket equipment installed? – Moab May 14 at 19:14
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Nice car, I've always liked that shape! Keep it rust free if you can.

There is a phenomenon called dieseling, which is where an engine runs without the ignition system being powered up due to auto-ignition of the fuel in a cylinder. However, this requires a mechanical ignition system, fuel pump and carburetor or mechanical fuel injection.

This can't be the problem with your car. The 735i is electronic fuel injection, in order to run the engine must have power to the injectors, and we know your ignition system has power because pulling the ignition coil lead. The ignition wire is still hot even with the key out. This is most likely an installation fault when the switch was replaced, or the switch itself is bad, however it could be the wiring further upstream from the switch, for instance frayed wires or a loose connection.

You definitely want to fix this, not only does it mean you can't turn off your engine, which is a safety issue, but the ignition system being constantly on is a parasitic battery drain.

If I was fixing this myself I'd get at the ignition switch first and look for the obvious. Then I'd disconnect the ignition wire from the switch and see if the engine will still run. If it does run then the problem isn't in the switch or the switch connection, instead it's in the wiring somewhere. If it doesn't run then the problem is in the switch somewhere.

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  • Thanks for your feedback ! It is a great car. I am also beginning to suspect electrical issues upstream. I am going to check the main relay as this powers the fuel pump relay, injectors & spark (as well as DME). Will also revisit the switch and perform the checks suggested. – Frank May 14 at 8:34
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    It may be worth just replacing the relay, I imagine it would fail open. – GdD May 14 at 8:38
  • An update on this as details may be useful for others who have air flow meter set ups in their car. Well the relay and others were not the culprit. It is the air flow meter ! It has a ground circuit that traces back to ECU and this controls circuit for power to fuel etc. So need to get my AFM repaired. Who would have thought ! – Frank May 21 at 0:48
  • That must have taken some tracing @Frank! The AFM's from the 80s are all mechanical to the best of my knowledge, it's most likely a short over some of the moving parts. I'd be tempted to take it apart but would probably just swap it out in the end. – GdD May 21 at 7:44
  • My '83 model is the Euro version with Motronic and the AFM has internal circuits & pins that supply inputs & ground to the ECU. Luckily we have a company here in Australia that completely refurbish these old ECUs, AFMs, circuit boards etc. So I have a totally remanufactured AFM on its way :) – Frank May 26 at 1:08

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