911S engine cuts out hot – bad CDI or voltage issue?
 

Hey guys, I need some help with my 1976 911S 2.7L.

The car shuts off after it gets warm—usually around 30 minutes of driving. This started around the same time I noticed the charging light came on. The tach has also started jumping around.

Here’s what I’ve done so far:
• Suspected bad grounding, so I replaced the ground strap from the transmission to the chassis.
• Swapped in a new ignition coil.
But the issue is still there.

I’m starting to think the alternator might be shot, and that I need to replace it to keep a stable current to the car. The injection system does rely on steady voltage to run properly, right?

Could it be the CDI box? The car starts easily and runs fine for 30 minutes, so I’d assume the CDI is OK—or am I wrong here?

Any ideas on what I should check next?

CDI's will break down from heat so maybe after 30 minutes of driving it will quit and that seems to happen quickly with little warning. When my CDI quit on me I could let it set for 20 minutes (even in the Florida sun) and engine would restart and get me back to the house.
Maybe Bob Ashlock who rebuids CDI's will see this thread, he's the go to guy and always helpful.

Hi and thanks for your reply, kltarga72.SmileWavy I hope Bob Ashlock or another specialist on these CDi´s will give me some advice.

I have had similar symptoms from a coil and from an ignition switch

The tach jump is a clue. This could mean the 12V power to the CDI is dropping out since the CDI has an internal 33 or 120 Ohm pull-up resistor to inyernal 12V that feeds current to the points. The tach is connected to that signal. OR, have you checked to be sure the points are gapped properly? (Around .016 inch). Sometimes their rubbing block wears down so much they are only opening a few thousands of an inch and completely close as they heat up.

My CDI box failed. After a 30-minute heat soak the motor would start to splatter and quit. Found a Porsche shop with a known good CDI for back to back test. Sent the old box out to a guy who fixes them. I believe he just touches all the solder joints and replaces whatever fiddly little gizmos on the circuit board that have failed. All in, a couple hundred dollars.

Hi gwmac, and thanks for sharing your info. I will check it out.

Hey Bob,

Really appreciate you chiming in – that’s super helpful!
I had no idea about the CDI’s internal pull-up resistor and how that could tie into the tach signal. Makes a lot of sense now that you mention it. I’ll check the 12V feed to the CDI and see if it’s dropping out when the problem happens.

I’ll also take a look at the points gap. Haven’t measured it in a while, so it could very well be worn down more than I thought. I’ll set it to about 0.016” like you said and see if that helps.

Thanks again for taking the time to explain – much appreciated!

By the way, if the CDI is losing 12V intermittently, would you expect the engine to just cut out instantly, or could it also cause the “runs fine for 30 minutes, then dies” type of symptom I’m seeing?

Hey smadsen,

Thanks for sharing your experience – that actually sounds a lot like what I’m seeing.
The “runs fine for 30 minutes, then starts to splutter and quit” description is pretty much spot on for my issue.

Good to know a bad CDI can behave that way after a heat soak. I might try to source a known good one for a swap test like you did – seems like a quick way to confirm. Also good tip about getting the old one repaired instead of buying new. A couple hundred dollars is way better than the price of a brand new unit!

I’m also going to try the things Bob suggested – checking the points gap and making sure the CDI is getting stable 12V – just to rule those out before I swap the box.

By the way, do you remember who you sent your CDI to for repair? Always good to have a trusted contact if it comes to that.

Thanks again for the insight – really helpful to hear from someone who’s been through the same thing.

What brand CDI box do you have? Not sure if it matters, but you may have a Perma-tune rather than a Bosch and I don’t know if those are repairable?

I would like to point out that I had a new silver coil fail straight out of the box so if you think your old black original coil is good I would reinstall it. The new one I bought would fail after about 30 minutes of drivin. I don’t want to see you chasing your tail with two parts causing same symptoms.

Hi Rutager West, I have the original Bosch so maybe it is repairable if that´s the one that is causing the problem.

Thanks for the tip, Chris. Yes, I have bought the silver coil. Have you heard about many other that has experienced problems with this one?

Re your alternator and CIS: If your 1976 car still has the CIS fuel injection, it does not depend on a steady voltage to work. The only electrical parts of the CIS in your car are used during cold start and warm up. Once the engine has warmed up, they do not have any effect.

It's still possible that a bad alternator could affect the CDI ignition. Check it with a voltmeter. If it produces about steady 14v, then go back to troubleshooting the CDI ignition. Do you have a voltmeter in your gauges? If not, get one that plugs into your cigarette lighter socket to monitor voltage while you are driving.

Answering your earlier question about the 'runs fine for 30 min' ... I was trying to start off with the basic stuff first, checking power integrity and points as you initially commented that the tach was jumping around. Does the tach actually behave 'fine' for the first 30 min and THEN when the problem begins does the tach jump come into play at the same time? Is that a kind of 'violent' jumping or just a 'dipping' as the engine starts missing? One of the most common CDI failures is the discharge capacitor shorting or opening after the unit heats up. Of course this kills the spark, and the tach is going to start changing as the rpms come down. I think a checkout of the CDI is in order and there are others on this forum that have bench test setups to do this, as I also do. Running hot on a bench test usually exposes this capacitor problem right away. And then of course you still have what appears to be an alternator issue to contend with. The suggestion by others to monitor the 12V is important because if your alternator is going berserk and putting out higher voltage or the dc voltage is carrying a large ac component (bad diode?) then the CDI unit will be dissipating more power, getting hotter than usual, etc., all aggravations of the problem.

Hi Bob,

Thanks for the explanation – that really helps. I will check everything. Did some testing yesterday.

To answer your question, the tach runs fine for about 30 min. After around 15–20 min, the engine starts cutting out for about a second at a time, and eventually it just shuts off completely after roughly 30 min from startup. Yesterday when I tested, the tach stayed mostly steady with just a couple of small jumps.

I also checked battery voltage:
• Idle: 12.4V
• 2000 RPM: still 12.4V
• Engine off: 12.6V

All measured at the battery posts. So this means that the alternator doesnt do the job. Could this relatively low voltage also be what’s causing the engine to die after 30 minutes?

Surprised that no one has suggested testing for spark when the engine cuts out (unless I have missed it). You need to make sure you have no spark before condemning the CDI. Might be fuel?
When the engine next cuts out check for a spark; easier said than done if you are on your own as you need someone to operate the starter while you are at the back checking a plug for a spark. Suggest you carry a spare plug rather than attempt to remove one and find a buddy to go with you.

Roy

Hi Roy, and thanks for the advice. I will surely try that, too.

Re your voltages: If you are only measuring 12.4V with the engine running, then your alternator is not charging the battery. Therefore, the battery should be getting weaker each time you start and drive it, until the battery dies completely.

But you said that the battery voltage was 12.6 (which it should be). You did not say that you had to recharge it with a charger. If you do not have to recharge the battery, that indicates the alternator is producing enough voltage to recharge the battery. Something does not make sense here.

Are you sure your voltage meter is accurate?

A simple test for a heat related failure of the CDI is to use an electronics coolant spray (https://a.co/d/2vb1at0) or ice to cool the unit. If it comes right back, it’s heat soak.