Hello again all.

Time for an update and unfortunately still looking for some more input. I ended up dropping the engine and checking the cis over properly. I replaced the airbox and the vacuum hose at rear of engine as both looked a little worse for wear. I was reasonably sure I didn't have any airleaks and indeed didn't find any. But no harm. Put the engine back in and no change to my original issue. Pushed the car into corner of garage and covered with sheet in disgust!

Had some time and pulled the sheet off yesterday for a fresh eyes look.

The issue is ignition related. I am getting a weak intermittent spark on no1 cylinder. I pulled all the plugs earthed them and turned engine over.

I'm getting 3-4 sparks on no1 and then nothing for a few cycles. I'm also getting occasional multiple sparks.

This is with new permatune cdi and black coil. Same behaviour with old permatune cdi and Bosch blue coil.

I've scoped the disi pulse signal at cdi plug and it looks good.

I've measured voltage at coil positive and coil resistance. These do look odd.

Resistance between 1 and 15 is 1.7 and 3.7ohms black / blue coil.

Resistance between center and 15 is 9.7 and 8.7kOhms black / blue coil. Same reading between center and 1.

Voltage at 15 when cranking less than 1 volt AC!

Shouldn't there be 400-500 volt pulses at 15 on the coil? With such a small voltage how am I getting any spark?

Just to confirm refitted plugs and get the same heavy backfire poor run up and v poor running as before. Timing light confirms random delivered timing and intermittent presence of spark. Same behaviour on both coils and both cdi units.

What the hell is going on???

So I think main questions are:

Is the coil resistance correct?

What should the output from the cdi look like at terminal15 on the coil?

Can I run the cdi with coil disconnected and check white wore output without doing any (more) harm?

Solve some of the mysteries......
 

James (?),

Sorry to hear about your nagging engine problems. This is what I would do if I were in your shoes. Test and confirm that both the PermTune CDI and ignition coil are good and working. Locate someone local with a '78~'83 SC with CIS and request to have your parts tested in your friend's car. Not until you could show and demonstrate the these critical ignition components are working, you are guessing (?). Make the trouble shooting simple and convenient. They might be good but you don't know it as a fact. Test and confirm. Then proceed on your investigation.

Were the seven (7) fuel injectors delivering sufficient fuel during cranking in your earlier tests? Keep us posted.

Tony

Hi Tony. Unfortunately I don't know anyone I can swap components with.

A new (another) premature and coil is around $700.

The spark operation is definitely not correct.

Every aspect of the fuel system has tested out good.

Got a working coil in your collection you could check resistance values on? The references I've found agree with my values for the two coils I have other than confirming that having the same resistance between center and either side of the coil is correct.

So I removed the distributor, coil and CDi unit and bench tested the system with a homemade loom (apart from green disi wire which was still connected to disi) and spare 12v battery.

Nice big fat white spark at plug in time with DISI pulse signal. Scoped the white coil wire with coil disconnected and get the correct 400V almost square wave output.

The disi pulse signal is ok as scoped when in the car, the CDI gets constant power, the earth line resistance is less than 0.5ohms, the white wire the same.

When put back in the car the delivered spark is weak orange and not present 20% of the time.

This is with plug connected to the coil output.

When I then connect the rotor, cap and all the plugs I get the same behavior.

So it all works out of the car but not in the car. Apart from a loom issue (which I have allready checked for resistance and continuity) what could cause this? Floating earth? Alternator spikes? Spiteful German imp?

So, short answer - it was the (new) Permatune CDI unit.

I replaced it with a classic retrofit unit and spark timing and delivery is all good. Engine starts on the button hot and cold. Tach sweep at key-on very cool....

Does not explain how the old and the new unit had apparently exactly the same failure mode.

Does not explain why the system worked on the bench.

Does not explain if something in the car instigated the failure of both units.

Glad you got this sorted out.

Components test........
 

James,

That’s the reason I suggested to have the CDI and coil tested on somebody’s car. In spite of you being very systematic in troubleshooting, sometimes doing it the old fashion way was the right thing to do. Good to hear the problem is fixed. Good job.

Tony

Isn't the new permatune unit returnable under warranty ?

It is and has been. Doesn't compensate the confusion or time wasted though.....