Porsche 924 Scheiße - understanding why people fit carbs...

[Zelandeth]

Keep it simple.

You need a few things for spark.

You need a solid power supply to the positive side of the coil.  Easy enough to check.

You need a switched ground on the ground side of the coil.  This is probably the most likely to be causing you headaches.  The ignition amp is what takes the tiny signal from the pickup in the distributor and turns it into the switching of the coil's ground.  

You need timing.  If the distributor is rigidly fixed, and the rotor is properly keyed onto its shaft, that's not likely to have changed dramatically.  

However...

The signals going into the ignition amplifier ALSO can impact the timing, as it also takes over from the old fashioned mechanical advance in the distributor.  

So I'd be concentrating on making sure the wiring to the ignition amp is sound (there are half a dozen wires, probably three or four actual signals) don't forget to verify the ground (or groundS), and seeing if you can get hold of a spare - not bad thing to have a spare of anyway.

The pickup itself is worth checking - Saab used both a hall effect or an optical chopper depending on the year - if these get full of crap (especially when the camshaft seal fails and the distributor fills up with oil) it can cause issues.

The fact that it sounds like the timing is screwy in the video makes me look even more suspiciously at either the ignition amp or it's pickup.

[Talbot]

12 hours ago, N Dentressangle said:

When it cut out, it did so suddenly.

For the next day, it got harder to start but would stay running ok and rev for up to 10min before eventually stumbling and dying. It wouldn't idle.

It will now barely start as you can see in that latest video.

I think the timing is OK, but part of the ignition system - either the electronic ignition in the distributor, or the ignition module I cleaned up earlier - is dying / dead. It's not the coil as it (a) tests OK and (b) has been swapped with a different one to no effect. Same goes for the HT leads, cap and rotor arm, and the plugs are brand new correctly gapped NGKs.

Are they genuine plugs and not some of the fake crap that seems to have infiltrated the market.  Both @juular and I have had experiences with fake spark plugs that break down under load so badly that an engine simply won't run.

If you take all the plugs out of the engine, ground them all and spin over the engine, do they all spark strongly?  If so, put them back in the engine, and use each lead as a trigger for a timing light.  When a plug breaks down under load, most timing lights fail to be triggered.

[N Dentressangle]

Yes, genuine plugs, all gapped before fitting.

They worked fine before, so they're not my prime suspects at the moment. I'm looking at:

- the ignition module

- the hall sensor in the dizzy

Testing either of these definitively is not so straightforward as far as I can see. I'm tempted to try an ignition module first as it seems more likely that its complexity and function would make a slower, intermittent failure more likely than the basic, pretty binary operation of the hall sensor, which tend to either work or not IME.

[Nibbler]

Listened to your video. I had a Capri Mk.2 that sounded just as bad as your 924 and was really hard to start. The cambelt had skipped two teeth. Just a thought but maybe worth checking out.

[N Dentressangle]

Yes, I had thought of that but it did run smoothly again after the initial cut out and the belt looks ok.

[juular]

My 240 (also Kjet and Bosch electronic ignition) has been doing the same instant death with no warning.  It's had new leads, cap, rotor and coil seems OK. 

I've got a pump hotwire switch to bypass the relay so I know it's not that. 

It has to be the ignition module or the hall sensor.

I think the hall sensor needs an oscilloscope probed in to test it properly. IIRC it's an 0-5v signal.

Worth mentioning when I removed my ignition module it was really damp, so might be worth checking the condition of yours.

[mercedade]

Recency bias and my total lack of actual knowledge says it's the rotor arm:

 

 

[Christine]

I haven't watched the video , but rotor arm ..is it a rev limiter one ?  My old Scirocco  died once , when giving it the beans .. flames and popping and banging ..to a standstill . I though the cambelt had jumped Turned out to be the spring loaded bit in the rotor arm had jammed , making the spark plugs  fire at the wrong time..  cheap fix!  

[N Dentressangle]

I've swapped out the rotor arm (boring solid one) for known good, so confident it's not that.

Currently trying to sort a used ignition module to swap in.

[N Dentressangle]

10 hours ago, juular said:

My 240 (also Kjet and Bosch electronic ignition) has been doing the same instant death with no warning.  It's had new leads, cap, rotor and coil seems OK. 

I've got a pump hotwire switch to bypass the relay so I know it's not that. 

It has to be the ignition module or the hall sensor.

I think the hall sensor needs an oscilloscope probed in to test it properly. IIRC it's an 0-5v signal.

Worth mentioning when I removed my ignition module it was really damp, so might be worth checking the condition of yours.

I'll hopefully have time to take the ignition module apart tomorrow morning and see if there's anything obvious wrong. I've ordered a good used one which should arrive on Thursday, so we'll see what that does.

Glad you agree on my two suspects. If it doesn't run properly for a few seconds on rocket fuel, it HAS to be ignition or timing, and we've already ruled out timing because it ran OK after the first cut-out.

[Zelandeth]

Thought it was just my phone being it's usual glitchy self earlier, but this is the same on my desktop.  Apparently Imgur is now blocked in the UK, so a bunch of those images aren't visible unless you're using a VPN.

It's the Photobucket debacle all over again.

[inconsistant]

Ah I wondered why I was seeing those instead if 924 bits and bobs!

[rusty_vw_man]

I can’t see the photos either, but if it’s the standard Bosch 3 pin hall sender there is a simple test.

remove king lead from dizzy, hold near an earth point, to create a spark gap .Obviously mine fingers/fuel etc  

Unplug three pin sender plug off side of dizzy, stick a wire in the middle pin (paper clip is ideal). Now when you ground this middle pin (with the ignition switched on!) the king lead should spark. 

If it sparks and you’re not getting spark now then suggests sender is kaput. No sparks then it’s something else. 

Should be 5v or more across the outer pins of the plug. 

[N Dentressangle]

8 minutes ago, rusty_vw_man said:

I can’t see the photos either, but if it’s the standard Bosch 3 pin hall sender there is a simple test.

remove king lead from dizzy, hold near an earth point, to create a spark gap .Obviously mine fingers/fuel etc  

Unplug three pin sender plug off side of dizzy, stick a wire in the middle pin (paper clip is ideal). Now when you ground this middle pin (with the ignition switched on!) the king lead should spark. 

If it sparks and you’re not getting spark now then suggests sender is kaput. No sparks then it’s something else. 

Should be 5v or more across the outer pins of the plug. 

That's really helpful, thank you - will try it later.

[N Dentressangle]

7 hours ago, Zelandeth said:

Thought it was just my phone being it's usual glitchy self earlier, but this is the same on my desktop.  Apparently Imgur is now blocked in the UK, so a bunch of those images aren't visible unless you're using a VPN.

It's the Photobucket debacle all over again.

What a total PITA

Hopefully they'll unblock it soon. I have no idea what other platform to use.

[juular]

18 minutes ago, N Dentressangle said:

I have no idea what other platform to use

WordPress is OK for image hosting. Dump photos in the media library and use the copy image link button to get a url to paste in here.

[N Dentressangle]

Everyone will just use a VPN soon. This is too tedious.

[N Dentressangle]

Took the lid off the ignition module and cleaned everything up. There seemed to be some dampness, but nothing obviously blown, so I wasn't expecting great things, and I wasn't disappointed - no change.

I did pull #1 plug again to check for spark. It was damp with fuel, but not overly so. Cleaned and tested:

I know there is a spark, but to me it looks weak and irregular. Symptoms I've seen before with failing electronic ignition. I followed this up with another good dose of brake cleaner into the inlet, and it still refused to start, so I can't believe fuel is the problem.

What do you reckon?

[N Dentressangle]

2 hours ago, rusty_vw_man said:

I can’t see the photos either, but if it’s the standard Bosch 3 pin hall sender there is a simple test.

remove king lead from dizzy, hold near an earth point, to create a spark gap .Obviously mine fingers/fuel etc  

Unplug three pin sender plug off side of dizzy, stick a wire in the middle pin (paper clip is ideal). Now when you ground this middle pin (with the ignition switched on!) the king lead should spark. 

If it sparks and you’re not getting spark now then suggests sender is kaput. No sparks then it’s something else. 

Should be 5v or more across the outer pins of the plug. 

Unfortunately it's a 2 pin sender. With ignition on, I'm getting 0.5v across the pins. Both pins have a path to earth.

[24vdiamond]

Any part numbers on the ignition module? I've a few various new old stock ones hanging about 

[mercedade]

On 30/09/2025 at 10:30, N Dentressangle said:

I've swapped out the rotor arm (boring solid one) for known good, so confident it's not that.

Currently trying to sort a used ignition module to swap in.

Hard to tell from the latest video, but I'd agree that spark looks bollocked.

Are you confident that the rotor arm is engaged properly with the dizzy? I've had one that had managed to round out slightly so that it would only spin erratically.

[rusty_vw_man]

5 hours ago, N Dentressangle said:

Unfortunately it's a 2 pin sender. With ignition on, I'm getting 0.5v across the pins. Both pins have a path to earth.

They are easier in some ways!

If you disconnect the plug, and stick a multimeter in resistance mode across the two pins out of the dizzzy and turn the engine over by hand - you should see the pulses of on/off.

[rusty_vw_man]

Does the spark look more regular and stronger if you use the king lead? Would help to narrow down if it’s dizzy related or an issue before the spark gets there….. 

[N Dentressangle]

2 hours ago, rusty_vw_man said:

They are easier in some ways!

If you disconnect the plug, and stick a multimeter in resistance mode across the two pins out of the dizzzy and turn the engine over by hand - you should see the pulses of on/off.

Great - done that. We have no resistance between the two pins at any point, nor any voltage from them for that matter.

Conclusive?

[N Dentressangle]

2 hours ago, rusty_vw_man said:

Does the spark look more regular and stronger if you use the king lead? Would help to narrow down if it’s dizzy related or an issue before the spark gets there….. 

Looks just as weak

[Alan G]

10 hours ago, N Dentressangle said:

What a total PITA

Hopefully they'll unblock it soon. I have no idea what other platform to use.

Approximately zero chance of that. The cause is their unwillingness to comply with UK Online Safety Act. 

[Christine]

You said it has new ht leads ... Those caps that are on the end of the ht leads that connect to the plug ...they don't look new ?  I had a Cavalier that misfired, chased it for ages ...   An agricultural engineer  i know had a look .. On the cavalier   the metal   caps had a resistor ? in them . He pulled the caps off  , put the ht leads direct to the plugs ..and no more misfire ! !  

[N Dentressangle]

2 minutes ago, Christine said:

You said it has new ht leads ... Those caps that are on the end of the ht leads that connect to the plug ...they don't look new ?  I had a Cavalier that misfired, chased it for ages ...   An agricultural engineer  i know had a look .. On the cavalier   the metal   caps had a resistor ? in them . He pulled the caps off  , put the ht leads direct to the plugs ..and no more misfire ! !  

Those are the old leads I put back on to rule them out. It's not the leads.

[rusty_vw_man]

2 hours ago, N Dentressangle said:

Great - done that. We have no resistance between the two pins at any point, nor any voltage from them for that matter.

Conclusive?

It’s just a switch, so it should basically be open or shut. Depending on the set up it either causes the coils field to collapse and generate a spark when it opens or shuts. 

The confusing bit here is if there is no opening/closing effect then there shouldn’t be any spark, weak or otherwise, as the coil shouldn’t be generating a spark. 

if you want to examine it further it’s a bit of a faff but pull the dizzy off entirely (make a note of how it was in) and then you can spin it by hand and see if it sparks at the right point from each plug in turn.

Stupid question, but are you 100% sure all the leads or on in the right order? You seem to have a weak spark and at least something combustible, and it’s does seem to cough a bit. I had the same once because I’d swapped two leads over so it was 180 degrees out so it was basically fighting itself and wouldn’t go. 

If you think the spark from the king lead is weak then it suggests something is wrong outside the dizzy. If the coil is making sparks then the trigger side is working (at least to some extent) so it suggests the coil voltage is low (dodgy feed or earth) or possibly that the coil is weak (which if you can find the specs you can check the resistance of).

If it won’t start on easy start or similar it does suggest electrical (either weak or timing)  assuming you are giving it enough! 

[rusty_vw_man]

Here’s another idea - is the dizzy clamp fixed tight? Could it have moved? Was running, shifts a bit and is an arse to start, shifts a bit more and won’t go at all??

Could go back to basics, find tdc on no1 and check the dizzy marks line up?