Ignition coil resistance

[Diver300]

This is on a Ford Zetec engine. There's a coil pack, with four HT leads and three LT connections. The coil pack is two double-ended coils as can be seen from the construction.

It's much like this:- https://www.ebay.co.uk/itm/316432985538 but it's made by NGK

The three LT connections are two primary coils of about 0.5 Ohms each and a common connection.

The HT connections for cylinders 1 and 4 have a resistance of about 8300 Ohms between them. The resistance between the HT connections for 2 and 3 is about the same.

That all seems fine. However, between the two HT coils there is a resistance of about 2.3 MOhm, which I'm fairly sure is wrong as it should be open circuit and indicates that there is a significant fault. There is no connection between the primaries and the secondaries.

Am I correct that there should be no connection at all between the two secondaries?

 

[MaxHeadRoom78]

The common arrangement now is the waste-spark method of ignition, where each coil feeds two cylinders with the primaries being isolated from the secondary now.
The may be a form of this , but a pair in one unit.?

 

[augustinetez]

Sounds like internal leakage in the coil pack = faulty - I'm assuming your getting misfire or low power problems.

 

[Diver300]

The common arrangement now is the waste-spark method of ignition, where each coil feeds two cylinders with the primaries being isolated from the secondary now.
The may be a form of this , but a pair in one unit.?

I've seen cars where there are two separate double-ended coils, but this car has two coils in one unit, like this:-
https://www.rsshop.com.au/product-page/zetec-coil-pack-ze001

I also found three coils in one for V6 engines.

 

[Diver300]

Sounds like internal leakage in the coil pack = faulty - I'm assuming your getting misfire or low power problems.

I am getting a misfire. It's intermittent. I just wanted confirmation that any conduction between the two secondaries is a fault.

On another car, there were two separate coil assemblies, so no possibility of any conduction between the two secondaries and they were in different assemblies. When one of those failed, there was conduction between the secondary and the primary. That was easy to confirm as I wasn't getting any spark on one cylinder, and I could compare the two assemblies.

I looked at several online videos about testing coil packs, and I didn't find any that checked leakage between the primary and the secondary, or between secondaries. They only checked the resistance of the windings. In this one, which I think has failed, and the previous on which I know had failed, all the winding resistances are correct and the failure is the insulation between the windings that should be separate. Checking the resistance of the windings would not show the faults that I have experienced.

 

[rjenkinsgb]

It's a common problem with those, I've had to replace one on a previous Mondeo.

The underside of the old one on mine had visible cracks in the encapsulating resin. It was causing an intermittent misfire, especially during acceleration.

Those encapsulated block types can sometimes also be damaged by someone doing a misfire test by pulling a plug lead - the open circuit HT voltage can be high enough to cause the insulation to break down.

 

[ahsrabrifat]

This is on a Ford Zetec engine. There's a coil pack, with four HT leads and three LT connections. The coil pack is two double-ended coils as can be seen from the construction.

It's much like this:- https://www.ebay.co.uk/itm/316432985538 but it's made by NGK

The three LT connections are two primary coils of about 0.5 Ohms each and a common connection.

The HT connections for cylinders 1 and 4 have a resistance of about 8300 Ohms between them. The resistance between the HT connections for 2 and 3 is about the same.

That all seems fine. However, between the two HT coils there is a resistance of about 2.3 MOhm, which I'm fairly sure is wrong as it should be open circuit and indicates that there is a significant fault. There is no connection between the primaries and the secondaries.

Am I correct that there should be no connection at all between the two secondaries?

The 2.3 MΩ resistance is not normal, and the coil pack is likely faulty. This leakage could cause weak sparks, misfires, or ignition instability under load. Replacing the coil pack.