1981 911 SC Shutdown
 

Hi All,
After some advice/help.
I have a 1981 911 SC (ROW)
After a breakdown & flatbed truck home my 911SC refused to start.
Long story short, Nil Spark, replaced Coil (MSD), Distributor cap, Leads, Spark plugs, Checked Green wire, new Fuel filter & Accumulator etc but problem came down to CDI.

Following the below thread...

https://forums.pelicanparts.com/porsche-911-technical-forum/203100-6-pin-sc-turbo-cdi-unit-repair-documentation.html

I repaired the CDI with a new C8 capacitor, SCR.

The car started & ran perfectly for approx 30-40min then shutdown, no cough, hesitation or splutter, just an abrupt stop. Car started straight away, ran for 20-30 sec then same shutdown. Subsequent starts perfect, but with same shutdown after 20-30sec.

Allowed car to cool down for 20-30min then car started & ran again for 20 min then same abrupt shutdown.

Has anyone had this defect?

I have noticed the CDI does get extremely hot, but i'm not sure if this is a normal condition?

It seems to be a hard signal shutdown, either from CDI (spark)?? or fuel shutdown?

I have read further articles with regards to WUR, CDI "Lock up", oxygen sensor etc.

Just curious to hear from people what their experience is with this type of "hard" shutdown & their thoughts/repairs, or any articles/forums they could point out.

Thanks.
911SC 1981 ROW

What brand/model of SCR / thyristor did you use for the repair? Wondering if the specs for the new one are somehow different and causing a problem.

I’ve not had the problem you describe but i did fry my 79 SC CDI when boosting a flat battery and inadvertently reversing the polarity on the jumper cables, whups.

New zener diode set it right and no problems since, although i’ve only run it for a few hours since i’m still finishing up a front end rebuild. I’ve bought replacements for all the other components so i can rebuild when/if needed, even if components become NLA.

First let me chastise you for replacing stuff without finding the problem first. This has complicated your troubleshooting. Even if you used new parts, suspect them until you have proven they are good.

Now, swap back those components to see if any of them fix the sudden stop. Double check the green wire and wiggle it around. I had a green wire that was broken internally (right where it makes the bend to the black plastic plug into the distributor) causing intermittent no-start and misfiring.

After the car stops and will not immediately restart, spray starting fluid into the intake to make sure it is a spark problem, not a fuel problem (e.g., the fuel pump working intermittently).

I experienced something similar to what you described with my 83 SC (USA) about 15 months ago. Towed it to my local Porsche aircooled specialist. The CDI died. He replaced the CDI and ignition coil. You can check out my posts from that episode here: https://forums.pelicanparts.com/12040215-post87.html

Hey Brian,
Thanks for your input. I bench tested the old SCR, & it was serviceable. I re-installed it. (changed it out originally because I had the CDI apart & from reading can also be the cause) Capacitor C8 was the main problem originally, as it was blown.
Car is now normal operation.
The new SCR I installed was part number 50RIA80 so possibly not enough voltage. I'll source a 50RIA120.
Thanks again for your input.

Hey PeterKZ,

Thanks for your input. Much appreciated.
My Troubleshooting was not complicated at all. As I mentioned it was a long story made short for the forum. The original fault/defect was the car hesitated/spluttered/stopped & wouldn't re-start, Nil spark. The original fault changed after the CDI repair, hence the forum question.

Each part was replaced & tested, then original re-installed & I moved onto the next step, (I now have a good shelf of spares) until I determined it was caused by the CDI, even tho it was whinging/buzzing. The C8 capacitor was blown, and whilst I had the CDI apart I replaced the SCR/THY due to it can also cause the defect. But......the new SCR looked like it was the incorrect voltage/spec. Correct SCR/THY re-installed and the defect has now disappeared. Car is back to normal.
Thanks again for your input & time to reply.

Hey Silberpfeil,

Thanks for your reply.
Managed to fix the problem with the SCR replace in the CDI.
Glad to see you got your 911SC going.

Not a good methodology, random parts replacement of electronic components, especially when one doesn't know the key electronic parameters required, e.g. of the SCR & the output capacitor.

At this point, carry a spare CDI in the vehicle!

EDIT: never mind, sounds like you’re back up and running. If problem comes back, post here and we can compare CDI temps if convenient.


Actually that thyristor should be fine, i would hold off sourcing the 120, tho higher voltage spec won’t hurt.

You mention your CDI runs quite hot, mine warms up but not hot exactly. Wondering if something in the CDI is overheating.

Do you by chance have an infrared thermometer you could check temp after running for a few minutes? I could do the same as a sort of controlled experiment and compare temps. Sounds like the cutout happens pretty consistently after 20-30 min… is something overheating.

Also, Pete has a good point about the green wire, the copper strands can fatigue and break internally, so it’s worth checking or perhaps replacing just to be sure. But that is less likely to happen with regular timing.

Only one device in the CDI that can generate enough heat, and that's a bad power transistor.

Dave, are you quoting Lorenfb there?

To be fair to OP, it seems like he was following the parts list in his first post link to (the late, and highly respected) Warren/Early S Man’s thread, which has part numbers that align to the required component specs, definitely worth a read if you have not done so already:

http://forums.pelicanparts.com/uploa...1727929221.jpg

He used a thyristor that meets spec (and is the main controller of current in the circuit (800V / 50A) vs transistor 1 (250V / 40A).

So i feel like he’s done a pretty decent job of problem solving on this one - especially for a first time poster! Who knows, it might be him helping us figure out a thorny problem not too far down the road…

This is not the only key SCR parameters needed, e.g. have the turn-off time and current been forgotten? It's not as simple as some have indicated for reliable functionality!

Could you perhaps remember the turn off time, current and other relevant parameters for those of us that may have forgotten them?

For example this one has 110 microsecond turn-off time, 50A current (anode to cathode forward bias), and 100 milliamp gate current:

https://www.vishay.com/docs/93711/vs-50ria.pdf

Would it be suitable for this CDI application, or would you recommend a specific alternative component instead?

The community would certainly welcome constructive advice on component selection for this repair, if you can offer such.

These high current SCRs are not good choices for a CDI application, i.e. poor turn-off characteristics.

Ok, so what part number / manufacturer are you recommending instead?

Seriously, help us out here.

BTW dazandkim911SC, we’d love to see a pic of your 81 SC, it’s sort of customary for new posters to show the community their ride.

Cheers dazandkim911SC!

Congrats on getting your CDI sorted out. Very impressive that you're able to get into the CDI and test/diagnose which circuit component was the culprit.

Looking back, perhaps that could have been done for the CDI in my 83 SC, but not sure if the tech at the indie shop had the know-how.

Enjoy your SC!

Yes, that's key, and is typically not found on websites or on YouTube!