Another CDI box fail?
 

Gents,
Have pretty much searched all that has been posted on this subject.. cruised the Rennlist board as well.

Here is what I have going on.. 1988 Porsche 911 Turbo Slant. No spark. Verified no power to the CDI unit terminal 15. Found it by replacing the ignition delay shut off relay. Terminal 31 ground is good to the coil and the stud at the coil mounting plate. The green wire had some sporadic continuity between the sheild and center wire.. replaced. The CDI connector wires are in the correct order in the connector and connections are good. There is healthy voltage at terminal 15 at the CDI box when the key on and cranking. The signal from the distibutor is clean.

The tach has been ruled out. Sounds like a no brainer? Here is more info. The car had a Permatune.. early silver box with a matching long black coil. After restoring power at terminal 15, ect. the car started and ran well but with would stop then restart after a cool down, I heat cycled the car several times then finally it quit at idle and has not started since.

Out with the old Permatune and in with a Bosch reman CDI unit along with a short Bosch silver coil. Both from the dealer. No change no start , no squeal from the box... So all values re-tested and order another CDI (reman) from the dealer.. still no go. these boxes will not sing.. no high pitched squeal

Is there anything about the alternator that could be killing it? Years ago I once had a 911 SC that burned through two CDI units at terminal 31/1 due to a disconnected ground strap from the alternator to the top of the case.. this one is good..

Minimum what do I need at the CDI connector to hear the unit squeal when powered up? Simply terminal 15 and ground? As these are reman units I guess it is possible but unlikely that I have two defective reman units... make ya nuts!

Thanks in advance..

Sounds like it could still be a fault in the Turbo's run on timer circuit. Either that or the silver Bosch coil is dead and it is destroying every CDI you plug in.

Yes, all you need is 12V and GND to make a CDI whine.

Killer Bosch silver coils.... use something else. MSD, Partsklassic, many others.

This is a 6-pin Bosch CDI, correct? (Not an 8-pin? ...the 8-pin does not 'squeal' with power applied unless you have distributor rotor rotation.)

My thoughts:

The box only requires ground at terminal 31/1 and +12V on terminal 15 to operate the converter, but it is not always easy to hear them squeal... they operate around 3.5 - 4 Khz and sometimes the transformer just doesn't make a lot of noise.

A shorted coil will definitely 'take out' the CDI unit. It will cause the thyristor to fail (shorted) and then the converter will try to push power into the short and it won't 'squeal' like normal.

The silver coils are notorious for shorting and (more often) arcing internally, resulting in no spark out of the high-tension terminal.

Remove the 'A' lead to the coil. Connect an Ohmmeter from ground to the CDI "A" lead and see if you have a short or very low Ohms. If you do, the SCR/thrysitor has gone bye-bye.

It's 28+ years old, stuff fails. If it's not a DD, send it to Lorne, Ingo or Steve Wong to be tested. As to the coils the new ones have a high failure rate right out of the box. Turn around could be as little as a week depending on shipping time.

Even if you box tests good enuff to function, there are soldered connections that maybe close to going that can be fixed at this time. Not sure but they may also be able to test your coil as well.

Thanks all..

tested "A" lead wire disconnected from the coil (white wire) to ground engine and body... open.

Well, that's a positive sign that the CDI SCR is not shorted.

I think you already verified there is +12V at the terminal-15 pin on the CDI plug, correct?

A final quick test is to leave the coil A-lead disconnected, turn key on and see if you can hear the CDI squeal. It should! If not, then there is a problem within the CDI and it will need to be sent in.

I call it the "deliverance" box

it should squeal like a pig ..

Thanks, did that. No squeal, I have heard them before over the years.. where would you suggest I send it? Not much chance in convincing the dealer the silver coil they sold me has zapped two CDIs at this point. I do have a new PVL brand blue coil to go along with the next box.

update, plugged in the Permatune which I do not trust at this point with new PVL coil and she runs. The silver coil has fried two reman CDI boxes.

Not uncommon. The silver-can coils are notorious for failing. You asked the question, so I will tell you: Our host offers a CDI Repair/Rebuild service under part number "AT-CDI Repair". I contract with Pelican to do them. I know, I know, this is 'self-promoting', but a recent related poster chastised some of us for not promoting the service offered by our host. There are also others on this forum that repair these units, as suggested several posts up and they are credible experts too.

Seems like that could be remedied with a more robust SCR that can take the surge current. Curious if the Bosch power supply limits its output if heavily loaded? I assume it would. That's another protective feature. Fred

As others said a wire to one of the two GND terminals of the box and one to T15 and the box needs to make noise with the coil disconnected. If the pitch is already way too high it's a shorted SCR. If there is no noise the box died due to over voltage and has most likely a burned PCB amongst other issues.

If it whines with ~3.6 kHz and you then (carefully) connect the coil you'll hear momentary dip in frequency and the tone needs to come right back to the exact tune with a half second. This is because the internal capacitor gets charged up taxing the DC/DC converter.

If the tone remains higher the internal capacitor is shorted. If the tone comes back to the exact note you can trigger the box once: hook an AA battery with its positive pole to box GND and then with the negative terminal touch the trigger input. If the note then changes to a high pitch and remains there the SCR is dead.

Ingo

I would have assumed that if the capacitor shorted the tone would decrease because the load increased, IE lower frequency. Just finished looking at a Warren Hall schematic for an early 3 pin. I can spot a couple of places where cheap protection could be easily added without disrupting the circuit function. Before I did that, I'd sooner rip the guts out and replace it with one of mine:) Although unless you are a female teenager, you probably can't hear the whine from my unit. Fred

When the main capacitor shorts it shorts the D.C/DC converter through the coil to GND and thereby the frequency goes up to about 4.5kHz.

Thank-you, That's interesting. The opposite of the type I make. I see why the power supply could be short lived if the SCR or the capacitor shorts. Fred

Doesn't surprise me (see my first reply to your above).

A number of people on here maintain that there is nothing wrong with the original Bosch CDI design. I beg to differ.

In fact, there are at least half a dozen flaws that would not pass a review in our or any other present day electronics workplace. No short circuit protection on the output is just one of them.

Considering all the common failure scenarios was the first thing we did when we designed CDI+. As such it is protected against pretty much every eventuality.

I haven't looked at a 6 pin circuit, but another fault of the 3 pin is no dedicated low impedance method of energy recovery after the initial discharge. This leaves a significant amount of energy in the capacitor (charged in reverse) that won't dissipate as spark energy but simply heats the coil. I can see two paths for energy recovery back to the discharge capacitor, but both are high impedance. One path is through the secondary winding of the transformer and the other is through the trigger circuit which is definitely not the right place in the circuit for this to happen. At high rpm, the coil is probably still ringing with useless energy transfer within itself, the capacitor and the power supply when the SCR fires again. That can't be good for the coil depending on the phasing when the SCR fires, and could make for a weak sparks (randomly). It also means the power supply has to be that much larger for high rpm and the spark duration will be short. That's an armchair review without testing one. Without scoping one, I am guessing, but I have tested many CDIs and have a feel for how they work. I have an 8 pin and I'm not impressed with it either. It's a power hog with no voltage control. Fred

Update, the car is running reliably now. What an adventure. This car had four things going against it. Age, salt belt corrosion, a really bad aftermarket alarm install back in the day..holes in the low side AC hose from a screw through the floor on the passenger side, spliced and poor connections tapped into the cars harness in several locations, and a rodent who made a home everywhere..
The final issue after getting coil sorted correctly and the CDI powered up reliably was the ignition switch. The detent for terminal 15 after releasing the key during engine start was poor. After some time heat wold build up due to high resistance and the car would shut down. Its hard to determine and isolate a bad switch if you have not felt what a good switch feels like. The detent for terminal 15 on the original switch felt very vague. The new switch has a positive feel.
Thanks for all the help all, this old dog was reminded of a few things forgotten and learned a few new things. I used to be able to hear 3.5 to 4 khz, not any more :).