V40 T4, 1.9 147 KW: Ignition issues, won't accelerate

[ww1dm1]

Originally Posted by loely

No, becouse it is a transformer it is called inductive and the current is always later then the tension is .(weerd but a electrical law).When interrupted the current resists and flows in the other way round in the coil ( for a very short time that is why they put a diode in a ralay over the coil to protect it)this make the current very high in the secundary of the transformer and can destroy components.
You have seen that sometimes when disconecting the powerplug from a home circuit you see a spark at other times you don't. That is the same reason.
Not that easy to explain in simple words.
Hope you can understand a bit of this hokus pokus.

That's a bit more clear. Though why does current increase and flow backwards if you disconnect the secondary coil?

Originally Posted by vlitavsky

I drove my car only twice from that time but it happened only when it warmed up as I remember

In that case I'd guess vacuum leak, otherwise maybe a dodgy front O2 sensor. Difficult to diagnose such problems blindly, I'd get some live readings via OBD2 if I were you.

[canis]

Can't quote, I'm in bed, but:

Yes, I was going to mention the relay diode as well. It's important to remember that induction only occurs with alterneting current. This was traditionally achieved by the points opening and closing, by means of four cams on the distributer shaft. The points close, giving the primary coil 12vDC, then they're opened (0v), and it is this alternation which allows inductance into the secondary coil, desperate to get to ground it jumps the gap in the plug producing the spark. Today all the on/off business is managed electronically, but the same problem exists: When the coil is off, it already has a charge it needs to let go.

If the HT has no route to ground, perfect isolation, it'll be fine because it's not the voltage that's the enemy here, it's the change of volts. The surge of energy trying to escape through the plug suddenly finding itself cut off, leaves a high potential. If it can find a route to ground through the puny wire on the circuit board, it'll take it, and that can't be good.

Now, I don't want to stake my reputation on this, I'm at the edge of my experience here. Although I did a C&G in electronics, it was 20-odd years ago and I didn't exactly give it my best shot even then. Mea culpa.

I believe starting the engine with a plug lead disconnected will have no effect. The danger is removing a lead when the engine is running, you won't be able to move fast enough to go from connected to disconnected instantly, leaving surge potential a possibility. It could be that Volvo have somehow protected against this, I dunno.

Hopefully some electronics genius will appear and put us all straight!

[ww1dm1]

Originally Posted by canis

Can't quote, I'm in bed, but:

Yes, I was going to mention the relay diode as well. It's important to remember that induction only occurs with alterneting current. This was traditionally achieved by the points opening and closing, by means of four cams on the distributer shaft. The points close, giving the primary coil 12vDC, then they're opened (0v), and it is this alternation which allows inductance into the secondary coil, desperate to get to ground it jumps the gap in the plug producing the spark. Today all the on/off business is managed electronically, but the same problem exists: When the coil is off, it already has a charge it needs to let go.

If the HT has no route to ground, perfect isolation, it'll be fine because it's not the voltage that's the enemy here, it's the change of volts. The surge of energy trying to escape through the plug suddenly finding itself cut off, leaves a high potential. If it can find a route to ground through the puny wire on the circuit board, it'll take it, and that can't be good.

Now, I don't want to stake my reputation on this, I'm at the edge of my experience here. Although I did a C&G in electronics, it was 20-odd years ago and I didn't exactly give it my best shot even then. Mea culpa.

I believe starting the engine with a plug lead disconnected will have no effect. The danger is removing a lead when the engine is running, you won't be able to move fast enough to go from connected to disconnected instantly, leaving surge potential a possibility. It could be that Volvo have somehow protected against this, I dunno.

Hopefully some electronics genius will appear and put us all straight!

This does make sense, if the circuit is not isolated properly against exactly this scenario - it will definitely fail. Though I'm guessing it is in the case of T4 coil packs, otherwise they wouldn't last long with open circuit.

[loely]

WW1DM1,
for the explanation of "why" ,you have to dig in the lessons of "magnetism "and the its behavior by "ohms,inductive and capacitive loads".
It is not a enjoyable material and if you don't realy need it ,leave it for the teachers and co.
The rule in the school books says"The current fights its reason off existance or disappearance"but I can not explain it properly so I am also on the Ignorant side for this question.I am not bright enough for this.
A open circuit can have the tension go up to 30 000 v and there is always somthing that can't resist such a high voltage and the spark may jump to the earth (not always) an ruin the HT coil or electronics.

[vlitavsky]

Ok, so problem solved once again, that guy have put there some kind of low cost MAF sensor which was causing these issues, I replaced MAF sensor with old one and it is working. It is not idling badly but I think old MAF is not good either. I have opportunity to buy MAF Siemens 5WK9 604 what do you think?

[canis]

It sounds like improper fuelling, probably because of a sensor of some kind. Could be MAF, could be the Lambda, could be one or two others, there's really no way to tell besides guessing.

As ww1dm1 said, the only way to be certain is to get the OBD codes read with a code-reading device. The engine management will be attempting to guess at whatever duff information it's recieving, and that's why the idle is fluctuating as it tries to correct it's own decision making. The good news is whatever information it has detected to be flawed in some way, it will report to the reader, which saves a lot of confusion. The bad news is, code readers are expensive.

There are a lot of workarounds. The cheapest seems to be using a laptop and an OBDii-to-USB lead, and downloading some software to interpret it. Another option is to ask someone in the forum near to your location kindly. Some (most?) garages can read codes, it cost me £20 when I did that. Otherwise, it's back to continuing changing sensors until you stumble upon the duff one accidentally, which is what I do, unfortunately.

[vlitavsky]

I will I just need to find somebody competent in Prague for volvo cars, because going to not specialised garage is expensive..