Re: Opened DI Casette - For those interested

I pulled my NG900 DI apart too and found the same thing... everything epoxied into place. I have had two other cars (Nissans) with DI systems and they had individual coils so I was thinking of just swapping out the Saab individual units from new to old as the old ones went bad. However, the epoxy prevents any such plans.

FWIW: On the other engines where there are individual DI coils they often have a separate ignition module which I think Saab has embedded in the epoxy. There is a little electronic stuff on top of the each coil, usually similarly sealed in tight. These cars also have coils go bad at 60K miles likes Saabs - pretty much all of them have the same problems. The ignition modules rarely go. That leads me to believe that it's the coil itself going bad... or something else in that direct area.

I think the overall issue is that you can't put electronic components into the top of a cyl head, heat them up to a couple hundred degrees day after day, bounce them around on the road, and don't provide any cooling via fans, fins, or heat sinks - and expect them to last. You've built a system with parts that *will* knowingly degrade and fail. Of course, you may not care when you sell them at $75-$100 each.

If the coil itself fails it looks like they could be swapped out between modules with the DI cassettes, it's certainly not unheard of for insulation to break down and arc over on HV transformers, though testing them on the bench to know which ones are bad could be an issue, and the arcing could certainly kill the electronics driving them.

Agreed...but is there a way the engineers could have designed the placement any better? That is, is there no other _feasible_ spot where this module could be mounted, where it would be subject to less intense heat and other harsh enviromental conditions?

How does Nissan mount these? AFAIK, Nissans apparently are not known to have DI issues.

- tex

The coils have to go in the head if you want to get rid of one of the biggest reliability issues on a conventional setup - HT leads. The story for the electronics is a bit more complicated.

The last Nissan I owned had only the coils in the head. The drivers were mounted on an inner wing. They could do that because they didn't try to do any fancy stuff using the spark plugs as sensors. At least they didn't at the time - I think it was a 1993 car.

Saab on the other hand, do use the spark plugs as sensors, so even if the driver electronics are remote, the sensor electronics need to connect to the HT side of the coils, so have to be in the head along with the coils.

IMHO, the system has achieved it's main aim - to reduce the number of failures which leave you stranded at the side of the road. HT leads always were a real headache. The problem is that when it does break, instead of 10 quids worth of leads, you have to shell out hundreds for a new module.

Cheers,

Colin.

Hall effect in '95? Are you sure? My '92 9000 2.3L had DI.

Absolutely positive. I think I would notice DI casettes when changing plugs. Maybe they kept Hall effect cars for the more discerning buyers!

Might depend on what part of the world you're in, I know Volvo offered various configurations depending on market, for example carbureted 240's and

740's right up into at least the late 80's while over here in North America every car they sold from 1975 on was fuel injected.

Saab didn't use DI on the low end non-turbo engines manufactured during the 1990:ies. These non-turbo engines used a simple Bosch Motronic ECU with an ordinary distributor-based ignition system. The turbo engines used the advanced Saab Trionic ECU and the DI system.

Oh, thanks. I just figured they switched everything over at the same time.

-Fred W newly edumacated on DI history ;-)

Car bought from Saab City in London, England.. Apolgies if I don't post correctly, I'm new to this sort of thing. At least Dave H. hasn't pulled me up (yet).

Scuse me?

Re-posting since this hasn't turned up on my server for about 24 hours. Sorry if anyone ends up getting this twice.

What makes you think you're missing out on breakdowns? I've had coil-on-plug ignition on my last four cars (a Nissan, an Alfa and two Saabs) and I've never had an ignition related problem on any of them. Before that, I certainly had to replace a few dizzy caps, leads, coils, condensers etc. All cheap stuff, but they were still all breakdowns. Like I said, DI tries to trade off the number of breakdowns against the cost of each one. Maybe not everyones idea of the best compromise, but that's what they went for. You'll get a very jaundiced view of DI by reading newsgroups like this one. People won't post about it unless it breaks.

I'm sure there are other benefits too. Cost maybe?

Cheers,

Colin.

Well, DI is a lot more complicated than just a replacement for points, condensor, rotor, coil, cap, and wires. It's an integral part of the engine instrumentation and management system. The spark plugs are used as combustion chamber sensors to determine the impedance of the fuel:air mixture, to decide (among other things) optimal boost and spark timing on a per-cylinder basis. These things just aren't possible with a distributor, cap, and rotor type system.

I doubt it's cheaper than old-style ignitions systems, but emissions and performance can be attained with DI that aren't possible without it.

Dave Hinz

Lets change that. I have *never* had to replace a DI cassette. The one in my current Saab, a 9-5, was installed at the factory in 1999. That was 116000 km (72500 miles) ago. I do *not* carry a spare DI in the trunk.

I wouldn't say it's impossible to get the same level of control without the DI , but the Saab setup does do the job neatly and I suspect more cheaply than using extra sensors.

Cheers,

Colin.

Check out this article:

I'd venture to say much of what they're using the DI for can't be done in other ways, since the spark plugs are the only thing that can be used as sensors in the combustion chambers. There's just no other way to get that information.

Dave Hinz

That's an interesting article. Cheers.

It looks like the basic parameter that they are trying to sense is combustion chamber pressure. The goals of sensing knock, misfire, ppp etc. are all achieved by interpreting the pressure waveform. The trick then, is to find a fast-response pressure sensor that's reliable and rugged enough to be used inside the combustion chamber.

Using the spark plugs as ion sensors is really compelling since the plugs are in there anyway, but it's not the only way and I bet it has a few problems. For a start, the plugs have to be designed primarily to ignite the charge. That must make them far from ideal as ion sensors. Then they have to break off their role as sensors every cycle to produce a spark.

One way round this would be to fit a second plug in each chamber. The second plug doesn't need to do any sparking so it can be designed to be a much better pressure sensor than a spark plug could ever be, and it can operate over the whole cycle. It doesn't have to measure ionization either. I've just done a quick Google for "combustion chamber pressure sensor" and got quite a few interesting hits. Most seem to be peizo-resistive but there's also fibre-optic ones. Some are even built into spark plugs or diesel glow plugs so you don't even need an extra hole in the head, so to speak :o).

Using a dedicated pressure sensor would probably allow better control than Saab currently get with their spark-plug ion sensing system, but I bet they're more expensive and the emissions regulations mean we don't need to resort to them yet, so I reckon Saab have the right idea, at least for now...

Cheers,

Colin.

Well, conductivity, which is a function of a few things, but yes.

Right, which would be the "combined sensor/actuator" which looks very like a spark plug.

Well, the timing of the cycle helps a lot. When they're measuring, they're not firing the spark, and the other way around. They know _about_ when these things will happen, just not _exactly_ when. Measuring the conductivitiy of the air:fuel mixture gives them that.

This is the reason that using a different flavor of spark plugs in a modern Saab is a bad idea.

I'm not convinced that pressure is the only variable that they care about. You could have the same pressure with wildly different air:fuel mixtures, just by changing charge air temperature, humidity, and probably another handful of things. A second sensor, of any type, would also change the head considerably, and possibly screw up the combustion chamber geometry, flow patterns, and a bunch of other things I can't even imagine.

Yeah, 'cuz we'd need that like we'd need a hole in the head. (had to do it; you understand I hope).

Goes a long way to explaining why they're not using a dizzy and coil any more, eh? Dave Hinz