924Board.org

[fiskolja]

Hi all,

Searching for some answers and hope you can provide.
Been having an ongoing problem with missfires for a while, the problem seems to come and go intermittently. but the car always start, sometimes only on 3 cylinders tho.

I connected a timing light and I seem to get no spark over a few seconds, doesn't matter which cylinder i connect to, same issue all over.
So I measured the resistance at ignition coil and are the readings enough to say that the problem is fixed with a new coil?

Primary - 1.6/1.7
Secondary - 12k

I tried to measure the resistance of the ballast but i couldn't get a reading, same with ignition cables. Rotor is within spec. Is there something else I can check before lunging for a new coil? If not, does it matter what coil i get?

Thanks in advance!
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924 NA 1977

[fiskolja]

Here's a small clip with timing light connected to cyl 1 to visualize the issue
https://i.imgur.com/P5Bi0Tb.mp4
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924 NA 1977

[Raize]

The ignition cables MUST give a reading of a few KOhms.

If they are reading open circuit they may still work but very badly. I've replaced so many failed ignition cables now. The resistors in the plug end of the cables tends to crack.

Can you also measure the voltage from coil positive to ground with the ignition on as this will indirectly tell us if the ballast is OK. Should be 9V or so.

The coil resistances are a bit high so you might have a 12V coil fitted instead of the proper 9V coil as well. Maybe you can find a part number on it to look up...

[fiskolja]

Thanks for taking the time!

Voltage
terminal 1 to ground - 0.48V
terminal 15 to ground - 7.5V

So that looks to be in spec as service manual states >5V on positive

The coil is indeed 12V (bosch 0221119027), tho I thought that was standard, atleast on ROW cars?

Regarding the cables I think I've messed up a bit since I got Bosch WR5DC and resistor cables (although the resistor cables dont have any resistant anymore)
I have new plugs (B7ES) on the way and I guess I'll order new OEM cables aswell

Still learning and lost in the jungle
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924 NA 1977

[copeina]

FWIW - I spent days fussing with my coil and trying to figure out a no-spark / intermittant-spark situation. For a change of scenery I started on other little jobs under the hood and found a bad ground on the ignition box. Fixed that and haven't had a problem since. Sneaky little gremlin!

Good luck with your project. Lots to learn as you go on these cars.
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1978 924 NA Project

[Raize]

The 7.5V is OK, ballast resistor doing its job. This would go up to 9V with the alternator spinning.

The way it's meant to work (for all 924) is they use a 9V coil, which gets a 12V feed from the starter motor as a "boost" for starting but at other times is only fed the nominal 9V (via the ballast resistor). In my opinion this system is unnecessary and I have left it disconnected with no problems.

Since you've got a 12V coil, what you could try is bypassing the ballast resistor to give the coil the full 12 volts that it's specced for. Or replace it with the 9V coil and keep the ballast resistor. Either option should be fine.

https://www.ngk.com.au/crossref.pdf I think your plugs are OK.

Resistor cables are OE, but if they are showing zero resistance it means the resistors cracked so you definitely need to replace them. They still sort of work because the spark jumps across the crack but it wastes a lot of energy doing that.

[fiskolja]

Thanks guys! The path forward is a bit clearer now
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924 NA 1977