Testing Coil Packs [Modern]

[Ryannn]

Just after some info to help with a problem I'm having please!

I need to check the coil pack wiring on an S-Type, each plug has a separate coil and introducing a new coil hasn't cured a mis-fire. I'm suspecting a fault in the wiring.

So, if I'm testing each plug with a multi-meter, what am I looking for? Will there be a constant voltage when the engine is running or will it pulse with the timing?

Thanks!

[cobblers]

A consistent misfire, all the time no matter what? Have you read the codes off the ECU?

Is it practical to pull the individual coils out of the head and visually check for a spark on the plug?

The coils will probably have three wires - Ground, switched ignition 12v and a pulsed signal.
Ideally you'd first do a static test with the engine off.

Get a multimeter and work out the two pins which are common on all 4 coils, these are the ground and 12v. I'd start with guessing the outside two are ground/12v and the middle is signal, then take it from there.
The pin which isn't common between the coils is the signal which will come direct from the ECU.

If the ECU is accessible then just take the plug off it, stick one end of your multimeter probe in the signal wire of a coil and then test each pin on the ECU connector til you find continuity - 0.00 on the screen, do the same for each coil connector to be sure.

If they all check out OK, then you need to see if there is correct pulse getting to the coil in question.

Ideally you need an oscilloscope and a dummy load but in the absence of that just set your multimeter to DC volts and with the engine running, unplug each coil and probe the signal wire. Do the same with it set at AC volts.

The signal is too slow for the meter to really comfortably measure it at AC and too fast for DC so the readings are impossible to predict, but if everything is OK then you'll get four very similar readings of about 1-4v.

[ChasR]

With dummy leads you can generally tell what the condition of coilpacks is like. The above also works.

However, when accessing the quality of a coilpack I have found it almost impossible to determine. I installed an Intermotor coilpack onto my sister's Mondeo and a friend's car once (I was far too tight to pay double for a Ford item and I probably am now too when the prices were comparable to pattern ones not too long ago!). Both cars started to backfire and misfire badly at low rpm and high load. Despite changing the leads (to Magnecors on my sister's car and pattern on a firend's) and the plugs the backfire was less but still present. My sister's car was sold. It was only when my friend changed his leads and coilpack to genuine Ford items that the misfire completely went. To quote him the car felt like a new one in comparison the mystery backfire he had. I do now have a device which measures the voltage of the HT system but even that is not entirely foolproof. I probably could also do with more knowledge to use it effectively:

buy1.snapon.com/products/diagnostics/dis.asp



EDIT: Which Jaguar engine is it? If it is a V6 and it has overheated in the past I would personally check the compression. They are a sturdy engine but in my experience they really do not take well to overheating.

[Ryannn]

Thanks guys, it's the 3 litre V6 yes.

The car is showing a fault on both Lambda sensors and I can tell from the rough idle and low power tha it has a misfire. I ordered a new coil and tried it in each position but the misfire was still present. One of the coil packs has a cable tie round it holding the wiring plug in, I'm tempted to just wire a new plug onto that one for a start but I wanted to know if I could check for a voltage first.

It's a very recent problem in a car that hasn't caused any issues over the last 2 years!

[squonk]

Stupid question I know, but have you done the first basic - fitted a set of new spark plugs?

[ChasR]

Putting on an new plug is not a bad shout . IIRC a friend of mine managed to go with new Bosch connectors via a certain website I can no longer remember the name of! Is it a constant misfire or intermittent? New plugs is also not a bad shout even if they do cost a few bob on those (they are platinum plugs IIRC).

How many miles have you done in the car over that time? I do hope I am wrong but I would not rule out a compression test for the time it takes. My old Mondeo V6 2.5 had a slight misfire but the news was not good!

[froggy]

Plugs are often overlooked as some curse word needs removing to replace them . They also run a returnledd fuel system and the sensor on the rail can act up , my dad runs one and we had some lambda issues which turned out to be fuel pressure related . New plugs would be the first step as the coils are pretty reliable

[rapidyellow]

past experience on a variety of cars with coil packs is that they either work or they don't, if they don't connect directly to the top of the plug then the short HT lead or the contact spring inside the "on plug" coil pack can be weak, I have stretched these out on one of our MG's so that theres a good contact, and then there have been no other issues

[froggy]

Not seen it for a while but I've found pin holes in the rubber boot in pencil coils where the spring meets the top of the plug .

[ejenner]

I'd be doing a compression test first. Everything else is a waste of time until you know whether the engine is physically sound.

[Ryannn]

It's not my own car so I'll try to convince it's owner to whip the plugs out. The only downside is having to take the inlet manifold off