Megane firing on only two cylinders - knackered ECU?

[anorak]

1999 Megane with K7M engine and Fenix 5 fuel injection system - 2 ignition coils combined in one block.

Having eventually found the break in the wiring harness that was stopping my Megane from starting, I have got it to run, but only on cylinders 1 and 4.

I have checked the ignition coil resistances, and these seem okay, and have checked the continuity of the wires from the coil connector to pins 28 and 29 on the ECU. However, when the engine is running, I'm getting 14+ volts at pin 28, but only 0.6 volts at pin 29.

I fear that this suggests that the ECU has got damaged during all of my previous investigations. The running on two cylinders seemed to happen immediately after connecting a timing light on cylinder 1 to check for spark. This method was suggested in the Haynes manual - could this really have damaged the ECU?

Is there a definitive way to test the ECU other than replacement?

Any other suggestions to why cylinders 2 and 3 are not firing are very welcome.

	ECU Electronic Control Unit In automotive electronics, an electronic control unit (ECU) is an embedded system that controls one or more of the electrical subsystems in a vehicle. Some modern cars have up to 70 ECUs.

	Ignition Coils Ignition Coils An ignition coil (also called a spark coil) is an induction coil in an automobile's ignition system which transforms a storage battery's 12 volts to the thousands of volts needed to spark the spark plugs. This specific form of the autotransformer, together with the contact breaker, converts low voltage from a battery into the high voltage required by spark plugs in an internal combustion engine. In older vehicles a single (large) coil would serve all the spark plugs via the ignition distributor. In modern systems, the distributor is omitted and ignition is instead electronically controlled. Much smaller coils are used with one coil for each spark plug or one coil serving two spark plugs (so two coils in a four-cylinder car). These coils may be remote-mounted or they may be placed on top of the spark plug (coil-on-plug or Direct Ignition). Where one coil serves two spark plugs (in two cylinders), it is through the "wasted spark" system. In this arrangement the coil generates two sparks per cycle to both the cylinders. The fuel in the cylinder that is nearing the end of its compression stroke is ignited, whereas the spark in its companion that is nearing the end of its exhaust stroke has no effect. The wasted spark system is more reliable than a single coil system with a distributor and cheaper than coil-on-plug. Where the coils are remote mounted they may all be contained in a single moulded block with multiple high-tension terminals. This is commonly called a coil-pack.

[anorak]

Forgot to say; there is no warning light showing on the dashboard.

[philbirkett]

Hi anorak, i am also having the same problem, same engine, only seems to fire on 1 and 4. Put new coil, leads, plugs in, only 1 and 4 are showing blackened, ie trying to start, 2 and 3 are as new. Spark is there on 2 and 3, but seems weaker. Where was the break in your wiring harness ? thanks, i'll let you know if I progress. Phil.

	Ignition Coils Ignition Coils An ignition coil (also called a spark coil) is an induction coil in an automobile's ignition system which transforms a storage battery's 12 volts to the thousands of volts needed to spark the spark plugs. This specific form of the autotransformer, together with the contact breaker, converts low voltage from a battery into the high voltage required by spark plugs in an internal combustion engine. In older vehicles a single (large) coil would serve all the spark plugs via the ignition distributor. In modern systems, the distributor is omitted and ignition is instead electronically controlled. Much smaller coils are used with one coil for each spark plug or one coil serving two spark plugs (so two coils in a four-cylinder car). These coils may be remote-mounted or they may be placed on top of the spark plug (coil-on-plug or Direct Ignition). Where one coil serves two spark plugs (in two cylinders), it is through the "wasted spark" system. In this arrangement the coil generates two sparks per cycle to both the cylinders. The fuel in the cylinder that is nearing the end of its compression stroke is ignited, whereas the spark in its companion that is nearing the end of its exhaust stroke has no effect. The wasted spark system is more reliable than a single coil system with a distributor and cheaper than coil-on-plug. Where the coils are remote mounted they may all be contained in a single moulded block with multiple high-tension terminals. This is commonly called a coil-pack.

[anorak]

Originally Posted by philbirkett

Hi anorak, i am also having the same problem, same engine, only seems to fire on 1 and 4. Put new coil, leads, plugs in, only 1 and 4 are showing blackened, ie trying to start, 2 and 3 are as new. Spark is there on 2 and 3, but seems weaker. Where was the break in your wiring harness ? thanks, i'll let you know if I progress. Phil.

Phil,

The break in the wiring harness was stopping the engine from running at all. It was on a very thin red wire that comes from a white connector block underneath the engine compartment relays. The actual break was on the bend where the wiring harness goes from being horizontal to vertical (where other wires branch off). This wire eventually ends up at pin 32 on the ECU.

I've got no spark at all on cylinders 2 and 3, and am pretty sure that attaching a timing strobe light has killed the ECU, and that was following advice in the Haynes manual. To say that I'm ****** off after all of the time spent on finding the underlying cause is an understatement!

	ECU Electronic Control Unit In automotive electronics, an electronic control unit (ECU) is an embedded system that controls one or more of the electrical subsystems in a vehicle. Some modern cars have up to 70 ECUs.

[anorak]

Well thank goodness it wasn't the ECU! It was a bad connection where the connector plugs into the ignition coil. With all of the pulling off and pushing on connectors over the past week, it must have become loose. So basically the connection to the coil pin for cylinders 1 and 4 was sound, but that for 2 and 3 wasn't.

I've managed to slightly bend the pins to force a good contact as a temporary measure, but I'd like to replace the connector block. Renault only sell it with the complete wiring harness - does anyone know where I can get one?

My eternal thanks to Graham at Bluestreak ( an ECU repairer) for suggesting this; apparently it is a common problem with Meganes.

	ECU Electronic Control Unit In automotive electronics, an electronic control unit (ECU) is an embedded system that controls one or more of the electrical subsystems in a vehicle. Some modern cars have up to 70 ECUs.

	Ignition Coils Ignition Coils An ignition coil (also called a spark coil) is an induction coil in an automobile's ignition system which transforms a storage battery's 12 volts to the thousands of volts needed to spark the spark plugs. This specific form of the autotransformer, together with the contact breaker, converts low voltage from a battery into the high voltage required by spark plugs in an internal combustion engine. In older vehicles a single (large) coil would serve all the spark plugs via the ignition distributor. In modern systems, the distributor is omitted and ignition is instead electronically controlled. Much smaller coils are used with one coil for each spark plug or one coil serving two spark plugs (so two coils in a four-cylinder car). These coils may be remote-mounted or they may be placed on top of the spark plug (coil-on-plug or Direct Ignition). Where one coil serves two spark plugs (in two cylinders), it is through the "wasted spark" system. In this arrangement the coil generates two sparks per cycle to both the cylinders. The fuel in the cylinder that is nearing the end of its compression stroke is ignited, whereas the spark in its companion that is nearing the end of its exhaust stroke has no effect. The wasted spark system is more reliable than a single coil system with a distributor and cheaper than coil-on-plug. Where the coils are remote mounted they may all be contained in a single moulded block with multiple high-tension terminals. This is commonly called a coil-pack.