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Weak spark problem - likely cause?

 
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Porver9two4  



Joined: 23 Jan 2004
Posts: 103
Location: United Kingdom

PostPosted: Wed Oct 10, 2018 2:05 am    Post subject: Weak spark problem - likely cause? Reply with quote

Can anyone offer some wisdom on where to look for the likely cause of the latest ailment to maroon my 924......
I've finally got the fuel system properly sorted (having been into every single component of the fuel system) only to find myself stranded again with a 924 that wouldn't start...... This time it's ignition

I had a 'no start' episode the other day, investigation of which showed no spark. Changed the king lead, still no spark. Turned out the rotor arm in the distributor had burned out, with obvious arcing and burning from the central contact out. A replacement from my spares stash saw fat spark restored and I was on my way again - all fixed. Visual inspection showed no issues with the cap or anything else. Happy 924 days once more....

EXCEPT, yesterday I again had a 'no start' episode Luckily I still had a spark tester on board and this revealed a very weak spark - a bump start was enough for it to burst into life and once running it got me home. I parked it at home then tried to start it again - no dice, spark too weak. Came to it this morning.... big fat spark again and it burst into life immediately.

So clearly something is failing or breaking down as it is getting hot. What is the most likely cause of this? Anyone experienced these symptoms before? Is this a distributor cap issue? or a coil issue?
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924RACR  



Joined: 29 Jul 2001
Posts: 8118
Location: Royal Oak, MI, USA

PostPosted: Wed Oct 10, 2018 4:11 am    Post subject: Reply with quote

Well... if it will bump-start easily, seems as if the starter is drawing down the voltage enough that the ignition box won't work, right? Had a bit of that with my Turbo this summer. Can be impacted by bad grounds, of course...

Of course, when hot it takes more juice to crank, as well...
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'79 924 #77 ITB racecar
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Porver9two4  



Joined: 23 Jan 2004
Posts: 103
Location: United Kingdom

PostPosted: Wed Dec 05, 2018 10:40 pm    Post subject: Reply with quote

I'm still really struggling with this.....
I haven't found the root cause. I'm still getting an issue with the spark weakening as the car gets hot. I can drive it for an hour flat out and it runs and drives fine, start getting into heavy traffic though and it'll start to flutter a little as the spark weakens. If I then switch off, it won't restart as the spark is too weak, although leave it to cool right off (like overnight) and a great big fat spark is back.....
I have found I can improve the spark enough to get it to start by hot wiring direct off the battery +ve to the coil +ve and juicing the coil with 12v, which gives a decent spark, but I can't really use it like it is.....
So far I've replaced the plugs, leads, rotor arm and cap and checked voltage at the coil (which all measures correctly) and have taken to carrying a spare coil, in case it's that but the last time it wouldn't start and I jury rigged the spare coil in it didn't make a huge amount of difference.
It's the most frustrating sort of fault as obviously something is failing, but not yet failed and that makes it so hard to trace. Start it from cold and it's perfect.
Help! I am not sure where to hunt......
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KDJones2000  



Joined: 14 Sep 2010
Posts: 302
Location: Phoenix, AZ

PostPosted: Thu Dec 06, 2018 12:11 am    Post subject: Reply with quote

Well it could be in your ignition switch.

When you are cranking it is supposed to deliver 12V directly from the battery to the coil. When not, 12V goes through another path to the coil.

Trace your circuits and see if there is an issue.
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924RACR  



Joined: 29 Jul 2001
Posts: 8118
Location: Royal Oak, MI, USA

PostPosted: Thu Dec 06, 2018 1:36 am    Post subject: Reply with quote

I eventually broke down and swapped out my ignition box... not sure if your car uses the same part, mine (82 931 US-spec) is part # 191 905 351 A Ignition Switch Unit.

BUT

It MUST be installed on the factory heat sink, using heat conductive paste. As you would use to install a CPU on a motherboard. The original paste on my original ignition box had dried out and was no longer conducting properly.

I am actually still using an older, used ignition unit, though I got a new one from a VW supplier (it's a common part)... but I will note that the new replacement came with a smaller heat sink riveted on, which I don't know would be sufficient for the 924.

If you do indeed have this style of ignition unit (on the front left fender), you might try cleaning and reinstalling it to the heat sink with new silver conductive paste from a computer store, see if that helps...

Good luck!
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Vaughan Scott
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'79 924 #77 ITB racecar
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staticsan  



Joined: 19 Jan 2009
Posts: 448
Location: Sydney, Australia

PostPosted: Thu Dec 06, 2018 7:06 pm    Post subject: Reply with quote

Could you post a pic of your ignition unit? I've had issues with mine, but I am puzzled by your description of a distinct heatsink (yes, I know what heatsink compound is).

Wade.
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924RACR  



Joined: 29 Jul 2001
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Location: Royal Oak, MI, USA

PostPosted: Fri Dec 07, 2018 1:18 am    Post subject: Reply with quote

I don't have a photo, but here's it without the heat sink:
https://www.ebay.com/itm/151747513189?lpid=82&chn=ps&ul_noapp=true
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Vaughan Scott
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'79 924 #77 ITB racecar
'82 931 Plat. Silver
#25 Hidari Firefly P2 sports prototype
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staticsan  



Joined: 19 Jan 2009
Posts: 448
Location: Sydney, Australia

PostPosted: Fri Dec 07, 2018 3:13 pm    Post subject: Reply with quote

Oh! That's really different from mine - and I think I know why. Your's is a Turbo.
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safe  



Joined: 18 Mar 2017
Posts: 20
Location: Sweden

PostPosted: Sat Dec 08, 2018 4:36 am    Post subject: Reply with quote

924RACR wrote:
I don't have a photo, but here's it without the heat sink:
https://www.ebay.com/itm/151747513189?lpid=82&chn=ps&ul_noapp=true


That's an early one up to 79 I think. It sits on the fire wall close to the distributor.
Later, and turbos have a bigger unit located behind the left headlight on the wheel well.
Right?
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924RACR  



Joined: 29 Jul 2001
Posts: 8118
Location: Royal Oak, MI, USA

PostPosted: Sat Dec 08, 2018 8:59 am    Post subject: Reply with quote

safe wrote:
924RACR wrote:
I don't have a photo, but here's it without the heat sink:
https://www.ebay.com/itm/151747513189?lpid=82&chn=ps&ul_noapp=true


That's an early one up to 79 I think. It sits on the fire wall close to the distributor.
Later, and turbos have a bigger unit located behind the left headlight on the wheel well.
Right?


Actually what you describe is the opposite of what I've seen; the one pictured above (which I had heat sink issues with) is on my '82 931.
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Vaughan Scott
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