Topic

There are lots of posts on the forum about the Amber engine Malfunction Indicator Light (MIL) illuminating (which is a Major Fault leading to an MOT failure). My problem is that the engine MIL light is not coming on (which is also an MOT failure, presumably as a safeguard against unscrupulous owners talking the MIL bulb out to hide a major engine fault). My 2004 MGTF Spark has been running sweet as a nut with no warnings or malfunctions on either the Dashboard or OBD and I assumed it would fly through its MOT – it failed due to the MIL Light not illuminating. When I picked the car up after the MOT, the MIL light came on for me. I dragged out the Inspector and point out his error, whereupon the MIL light would not come on during repeated ignition on – start-up events; the car remained an MOT failure. Since then, a 100% repeatable pattern has emerged. For the first start after the Alarm/Immobiliser has been deactivated, the MIL light comes on (albeit it flickers on, goes off for a quarter of a second, and then comes back solid and goes off when the engine starts (i.e. normal function). For all subsequent starts until the Alarm/Immobiliser has been cycled, the MIL light remains off. Does anyone know if this is a known behaviour or an interface issue between the Engine Management Computer and the security system? I don’t fancy my chances of telling the MOT Inspector that he has to cycle the Alarm System before he checks the MIL light operation in the re-test!

That's a new one on me. I would first look at the bulb and the live and earth connections to it.

As a matter of course with strange electrical gremlins, I always suggest examining the boot hinge loom for internal damage.

Hi and welcome to the forum

I would go through the alarm sequence with the tester.

James

The good news is that the engine MIL problem is sorted and the car rushed through a successful MOT. I wish I could claim full credit for analysing the system and explicitly tracking down the fault - but I cannot.

Given the scenario described in my original narrative, it was clearly linked to the alarm system. What I did not say in my narrative was that, as the car only comes out of the garage occasionally, I have fitted quick connectors to the battery and disconnect it when it goes away (might be a drain to earth somewhere as the battery fully discharges within a 1-2 weeks). I know disconnecting the battery can cause some issues with the alarm / immobiliser system so I left the battery connected, used the OBD to look at any fault codes (of which there none), but did a 'Clear all Codes' anyway. The engine MIL operation was fully compliant with the design thereafter, so the answer seems to be don't go disconnecting the battery!

Good it's sorted.

However, I use a battery isolator on the earth for the same reason.(ie Parasitic drain), but I never have an issue with the MIL. Mind you, my electrical system is an earlier mk2 F version.