Using M800 to fire Saab Direct Ignition Cassette

[RossB]

I have been looking in to this and have a bit more information now. The DI casette triggers off the falling edge but the capasitor needs to re-charge between firing events (there is only 1 capasitor for the 4 coils). The MoTeC Falling edge trigger is preceded by the Dwell time during which the voltage is held high, after the trigger the voltage is kept low until the begining of the Dwell period for the next firing event. This is not allowing the capasitor to re-charge for the next cylinder. So this is not going to work as you have found out. To make this work you would need to use ignition mode 2 (rising edge) and invert the output with an external circuit.

[ejenner]

I think I know this. But the detail I'm not aware of would be this bit about inverting the output with the external circuit. I'm certianly no electonics wizard, more a 'jack of all trades' - so I understand bits and pieces but I would not know how to find the right kind of inverting circuit to make this work.

Do you have any recomendations for where I might find this inverter or if I can build one following a circuit design you know of?

If you use ignition mode 2 by iteslf the cassette sits there in a state of constant stress, as if the charge is constantly building up without being released so I have a feeling that is damaging for the cassette and ignition mode 2 by itself is not good enough.

[ejenner]

p.s. what are the chances of writing some code for a special ignition type that would control this from inside the Motec without any need for an external inverter?

[RossB]

Ignition mode 2 would not be suitable without inverting the output. I don't have any specific recommendations as to where you could get an inverter from, it would probably need to be made specifically for this.

I have looked in to the possibility of creating a specific mode for this but it would involve creating a new waveform generation mode which is a big job. Unfortunately the demand for this would not justify this.

[ejenner]

I did actually buy what I thought was the best ECU available at quite a cost. So really nothing should be too much hassle. It's not good for your reputation to propose long-term resolutions of this kind. But you might do it anyway. One very simplistic overview of the situation might be that Megasquirt ($50 dollars for a bag of components and a circuit diagram) is more configurable than a Motec M800. So why would you buy an M800 instead?

I have spoken to a couple of different electronics experts and they both said that the best place to begin when creating an inverter circuit to change the output signal from the Motec would be to learn something about the chips used for the ignition outputs. This is so you know what kind of technology would be capable of properly reversing the signal.

"either open up the ECU you are working with or ask the manufacturer to provide you with the part number for the output transistors they are using for the ignition system. Since the current system switches to ground it can be assumed that it is either an NPN transistor or an N channel mosfet. Once you know the part number, you can reference it to find either a PNP transistor or a P channel mosfet."

[SprinterTRD]

Ejenner,

Its not as simple as inverting the signal. The Motec does this anyway by selecting type 1 or type 2. The problem is that 99% of ignition modules require the Dwell time to occur before the fireing edge. The module you have expects the firing edge to occur before the dwell time.

I will try to explain the operation of Ignition mode 1 and 2
Mode 1
The ECU output is normally grounded stopping power from flowing through the coil
The ECU output then goes high to start the dwell time (to keep it simple this time charges the coil)
The ECU output then returns to the grounded state. At this point the spark is generated.

Mode 2
The ECU output is normally High (around 5Volts) stopping power from flowing through the coil
The ECU output then grounded to start the dwell time
The ECU output then returns to the High state. At this point the spark is generated

As you can see mode 2 is inverted version on mode 1

Now what your system needs
The ECU output starts high
The ECU output goes to the grounded state. At this point the spark is generated.
The ECU output stays low for around 3ms to discharge the internal capacitor in the unit
The ECU output returns high

As you can see mode 2 is what you need however your module would fire on the dwell edge not the firing edge.

From Ross's description this would mean a total rewrite of the ignition system code to control this module.

As a solution you could get someone to write some code in a AVR or PIC processor to change the mode 2 waveform to suite your cassette or just get a CDI system that requires conventional input signal to operate.

[RossB]

Sprinter has described the modes very well as well as the Saab DI input requirement. Unfortunately this ignition system is unique in its requirements for this type of input, it is also a system that has been in existance for more than 10 years and in that time we have not had a request to provide a mode to run it, until now. If the system was more popular with MoTeC users we would have probably already created a mode for it. The amount of work involved in doing something like this for a single application is not viable, especially when there are other ignition systems available which can offer the same (or better) performance.

[ejenner]

How many more people would need to ask in order to get it working?

Truth is, many people will have asked before but there are several hoops to jump through before you get in contact with the people who decide if there is a case for modification or not. Many people will have not perused to this extent and just given up earlier in the process.

First your local dealer, then your local Motec office, then possibly this forum and you could phone Motec in Australia as well... but most people don't have the time or patience to take it that far and that will be why you have not had the request before.

Another way of looking at this situation is that the Saab CDI cassette has been around since 1984 and you've never written a mode for it. So there has been 25 years to get something sorted out and nothing has been done. They still use this cassette on the current Saab 9-5 so it won't be going anywhere for a while yet. The Saab engines are also being installed into all kinds of different cars so it isn't all about Saab owners either.


Caterham:






Opal Ascona:






BMW E30 Touring:








Astra Van Sleeper:

[RossB]

We get a lot of feature requests for all of our products. A lot of these come from our dealers but we also get enquiries every day from all over the world. We have always made it easy for users to contact us directly via e-mail and, more recently, via this forum so we have a pretty good idea of what our customer's requirements are. When we recieve a request for a new feature we have to look at what it adds in terms of value to the product, how it would impact on product sales and how much work is involved in development and testing. The M800 is one of the most configurable ECUs available but we do not have a mode for this ignition system simply because we have never been asked to do it. The M800 has been around for a long time and so has this ignition system so we have to assume from this that the demand isn't there for this application (I'd be happy to be proven wrong here).

We have recognised that there are some applications which require special control modes which is why we are developing the M1 range of ECUs. These will allow special control modes (such as the Saab DI) to be created without the need for a full firmware upgrade. It will also be able to control Direct Injection and Diesel in keeping with current and future engine trends.

[LOLA-Christian]

M400 and SAAB DI, not a good idea, then?
Or has there been some development during the years for SAAB DI and Motec hundred series?