Replaced my DI cassette. Now getting misfire on 4... Help!

Most likely the new cassette was duff. Send it back for a replacement. I assume it's not too obvious that you've had it apart?

Cheers,

Colin.

replacement.

I don't think it's the cassette. I took the OLD cassette apart and the Cyl. 4 coil was gone. With the new cassette in, the DTCs report misfire on Cyl 4. So I figure somthing (Trionic?) caused cyl. 4 to misfire and that is what caused the cassette to go in the first place. I have a local independant Saab mechanic who comes highly recomended. I guess I'll have it towed to his place. :-(

Thanks, John Hooven

Oh, I see. I thought you'd taken the new one apart. Doh!

In that case, Maybe you were unlucky enough to have the old one blast the ECU as it failed. The dumbed-down diagram in the manual makes it look like the ECU outputs directly drive thyristors in the DI cassette without any buffering, so if one of those failed, it could have sent

400V back up the ECUs output. I'd say you're right. It's time to admit defeat and get the car looked at by someone with a box of bits to swap in and out to pin down the problem.

Incidentally, does the car feel like it's mis-firing when off-boost? If the ECU output is knackered, number 4 would never fire, so it would be quite noticeable.

Cheers,

Colin.

No, it idles fine. When I put the accelerator to the floor, it only reaches < 4000rpm and starts to buck. What is the ECU?

Thanks again, John Hooven

That'd be the Engine Control Unit which is an essential part of the engine management system for almost any modern engine due to the super-tight tolerances on emission control, fuel usage, etc.

I like Saab's 8V engines because they don't need any ECU, but the flipside is they're nowhere near as high-performing and less efficient overall. I like the reliability but 16V engines and later designs have a lot of good points which the older engines didn't have.

Regards,

Craig.

[...]

It may be that the condition of the old DI is still registered on the ECU and it need to be cleared. If this is the case, then the condition should improve after several engine starts. The ECU should be the last to suspect; it is (1) well shielded and doesn't 'wear and tear', and (2) it's very expensive.

yeah all u need to do is disconnect the battery for like 5min and reconnect it the ecu will reset everything.

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If you disconnetc the battery, remember that you should only disconnect the ground cable, not the + (plus) wire.

Cheers!

Yep. That'll be the limp-home mode because the ECU thinks there's a miss-fire. It's still a bit of an unknown whether the miss-fire is real or not.

"Engine Control Unit". In your case (I think) the Main Trionic control module.

If you have a scope, you could check for the trigger pulses on pins

2,3,4 and 5 of the DI cassette. Pin 5 is the suspect one.

Here's the diagram from the manual, if it's any help.

Cheers,

Colin.

Why?

-Fred W

I've always thought of it as good practice ever since I nearly welded a spanner to the engine of an Austin Allegro in my younger days...

Cheers,

Colin.

Ah, so he's saying disconnect ground _first_, not "don't disconnect + ever", then?

It's a safety procedure, touch the bodywork/engine etc of a negative earthed car whilst (stupidly) touching the metalwork/contact of the positive terminal will cause a short, which can give you a nasty jump and even a burn. Advertised on motor mechanic courses here with photos of burns to wrists of guys who contacted with a metal wristwatch band.

Disconnect the negative and touch the body/engine at the same time it doesn't matter.

Other than the above safe working procedure Fred, you are spot on, a circuit is a circuit and you break it whichever side of the power source you disconnect.

Al

P.S. Is "like 5min" another way of saying five minutes, or does he mean 4 minutes and 59 seconds, which is similar but not the same? So therefore could be described as like. ;-)

Yep. I reckon the word "only" must have crept in there by mistake. Otherwise you'd have a hell of a job replacing the battery ;o).

Cheers,

Colin.

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Besides burns, you can fry the electronic boxes in the car...

Yep. :o)

Cheers!

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He's saying: At least 5 minutes. I usually make it 15...

Cheers!

Well I tried disconnecting the bat for around 5 minutes. No change. Thanks for the pin-out that helps. I'll check pin 5 (assuming I can find my scope :-) as soon as it stops raining.

Thanks again to all for your help, John Hooven

Sorry, I should have been more specific. I agree, you should disconnect the battery. But why *only* the negative (ground) cable? You could do both.

I think you meant that you should always disconnect the negative *first* for safety reasons.

-Fred W

Actually, I knew where he was going, but it was the "only" the negative which I wanted him to clarify.

Also, just touching those with your body even with a wristwatch on will not cause one bit of a problem. The resistance of your body / skin is high enough that a 12v source will not even cause you a tingle. This is one of the reasons they stick with the inefficiencies of low voltage in auto and marine applications.

But, swing a wrench around and contact some grounded metal while disconnecting the positive lead (with the negative one still connected) and watch the sparks fly.

Yes, "like 5 min" is 'merican slang for "approximately 5 minutes or so, old chap", but like, I didn't say that man... It was some other dude. ;-)

-Fred W