Do I need to replace my CDI box?
 

Hi -
My car is a '74 911 w/a '77 2.7 engine and I've spent the last few nights doing a lot of ignition trouble shooting that started with this thread . Here's the current cliff notes version of where I am:

With the ignition key turned on, the center pin of the 3 pin terminal at the CDI box has power, but there is no power at the + terminal of the coil. From all my searching, it seems there should be power at the coil when the ignition is turned on - is that correct? Also, the CDI box does NOT whine or make any noise whatsoever - although to be honest, I never noticed it whining when it was working.

I have gone through the entire ignition system, traced every wire, cleaned and checked every connection and all the wires seem good to my untrained volt/ohm meter reading skills.

So, are these symptoms of a failed CDI box? What type of box do I have? Here's a pic of the (possible) offender. I've been assuming that it's the original Bosch unit. The only markings on it are a date on the back (5/5/76) and the numbers 3313 printed on a foil strip on the top.

All the trouble shooting began for me when the car wouldn't start after replacing the rotor, cap and plug wires. Could any of that have caused enough trauma to blow up a CDI box?

http://forums.pelicanparts.com/uploa...1176872280.jpg

Re: Do I need to replace my CDI box?
 

Hi,

That looks like a Permatune unit. The Bosch Units look like this:

http://forums.pelicanparts.com/uploa...1174242550.jpg

Permatune units do not whine and can fail unexepctedly. If you can, find someone with a known good unit and temporarily swap it out. Your answer will be obvious.

I looked at your other thread. It appears you are using a Bosch blue coil.

http://forums.pelicanparts.com/uploa...1176783687.jpg

Based on second hand info, this can cause the CDI Box (Bosch or Permatune) to fail.

<b>CDI systems don't have constant power applied to the coil ... they get very brief 350 - 460 Volt pulses from the CDI unit when sparks are needed!</b>

Bosch Blue coils work just fine with Permatune or Bosch CDI units.

Here is some reading material [a lot, actually] on CDIs:

http://forums.pelicanparts.com/showthread.php?s=&threadid=160537&highlight=bosch+ permatune

http://forums.pelicanparts.com/showthread.php?s=&threadid=226517&highlight=cdi+hi story

Have you done a resistance test between the Permatune case and engine block? The resistance should be less than 0.5 Ohms.

Have you done a 'Telegraph Key' test [remove lead at distributor and touch it to distributor case several times, repeatedly] with the distributor trigger lead and a test spark plug as load plugged directly into the coil with a spare spark plug lead. This eliminates the distributor as the trigger or load on the CDI system.

Hi Warren,

A few questions.

His box is blue. Could it be one of the really old, yet very good Permatune boxes? Is there a way he could tell?

I have always heard that the Blue coil would work but the turns ratio and impedance is wrong for a CDI system and this may create a load that is not good for the box. These same sources typically recommend you should use either the correct Bosch (black) or a MSD Hi Vibration coil.

Lastly, I am confused about the "telegraph key" test. I am assuming we are testing the coil to verify that it can make a spark without any help from the CDI unit (aka Kettering ignition). Are you saying that I would connect a spark plug to the high voltage connection (center lead) at the coil and ground the other end of the spark plug. I know one terminal on the coil is grounded but where does the voltage for the other side come from? Or is it something different?

Thanks so much for your help over the years.

Harry,

The purpose of the 'Telegraph Key' test is to eliminate possible distributor problems from the equation ... such as points, cap, or rotor. The CDI and coil are left wired normally, or tested on a bench with a new test harness as suggested in the Bosch test document ... the bench test confirms proper operation, or failure without the question of wiring harness or distributor problems.

That blue Permatune box does look like one of the old & reliable ones like the spare I have.

Harry and Warren -
Thanks for ID'ing my CD as an older Permatune unit. Warren's first statement tells me I may not be testing for power at the coil correctly? I have read many, many posts in my research thus far - including most of the key ones by Warren!

I have not done a resistence test btwn the Permatune unit and the case. I will do that. Quick (unedumacated person) question - my volt meter has an adjustment on the side of it that adjusts the Ohm readings. Do I need a resistor of a known Ohm rating to calibrate it?

I have done a version of the telegraph test I think, although not on my bench and not with new wire. After checking and getting no spark at the plugs (by unplugging a spark plug wire, inserting a new spare plug and holding it to an intake runner while someone cranked the engine) I unplugged the wire from the coil to the top of the distributor and positioned it close to the case while someone gave the engine a quick crank. No spark.

Maybe my next step is to try to set up everything w/new wire on my bench? Or revisit the telegraph test? I'm having a hard time concentrating as I've got a whining, sick 2 yr. old in the background!
I'm not sure it matters, but I have Pertronix, not points.

Thanks,
Tom

Tom here is the Permatune website link to troubleshooting your Permatune CDI: http://www.perma-tune.com/T-ignition-Porsche-911.html

Kurt - thanks for the link. Sometimes the most obvious resources are overlooked! I've printed out the article and will go through it today.

Waren - I've done a little bit of searching re:the 'Telegraph Key' test. I think there's a good explaination in the MSD instructions but I'd like to list the steps in case I'm still not understanding the procedure. Directly from the manual (w/a little paraphrasing and assuming the power to the CDI and wiring is good):

1. Ignition switch in off" position.
2. Replace coil wire from distributor cap with a spark plug wire w/a plug in the other end held to ground.
3. Disconnect white wire from CDI to Pertronix ignition at distributor.
4. Turn ignition to the "on" position (but do not crank engine).
5. Tap the white wire to ground. There should be spark at the plug.
6. If yes spark, ignition is working correctly.
7. If no spark - I'm not sure what to do. The MSD instructions say to replace the coil w/a known good one (which I don't have) and repeat. If there is spark w/the new coil, then the old coil was bad (duh). If the new coil still doesn't produce spark, and all the wiring is known to be good, then the ignition is bad.

I will do at least the first part of the test tonight to see if I get spark (I'm hopefull, but not very optimistic!)

Tom

Tom,

I would replace the PermaTune CD unit with a refurbished Bosch CD unit. They are almost always available here and they aren't expensive. I've had 2 PermaTune CD units fail in less then 3 years on my '74. Let me know if you need any fuel injection/airbox parts as I have 2 complete CIS setups to sell since I switched over to Webers. Good luck!

Tom,

I looked at your other thread and it seems that you may have multiple issues here.

I see several issues you need to verify as go or no go:

Bad Coil (shorted or open)
Bad (Early) Permatune unit
Bad rotor/Cap (fit or defect)
Bad Pertronix

In the other post you showed a damaged rotor. Did you get this fixed?

Did you do some bench testing of your coil (primary and secondary resistances?

Any news on your permatune tests?

What is the status of your pertronix unit?

for eacjh of these, there are posts on how to trouble shoot each part. Try a search and see if you can find the procedures. Posts by Warren (early_s_man) and lorenfb can be enlightening. We have other "wizards" that post on these topics as well.

Keep us posted.

More info
 

Harry - Thank you for taking the time to read through my other thread. I would agree that I have lots of loose ends and "possible" issues. Hopefully my info from tonight will cross some things off the list. Here's what I did tonight:

Telegraph test - as described in above posts, using the existing wiring. The result was no spark. I tried it w/a plug wire and new plug as well as with the coil wire itself. There was a noise - static/scratchy - comming from the Permatune CD box when I grounded the white distributor wire. Not sure what that means. No spark from this test would lead me to believe that the Pertronix unit is probably NOT at fault?

Coil test - I must preface my findings here by first saying that I'm not fully confident in my multimeter usage. Is it possible my findings are off by a factor of 10? Primary (between + and - terminals) = 10 Ohms. Secondary (between + and center terminals) = 108 Ohms. I'm not exactly sure what these readings should be, but relative to what I found in my searching, they seem high?

Ground tests - I tested for ground all around the car:
engine case to chassis
Permatune CD box ground wire (unplugged) to chassis
engine case to Permatune mounting plate
ground strap at neg. battery terminal to chassis
Permatune CD box to engine case
Permatune CD box to CD box mounting plate
(note about the Permatune CD box - I could only get a reading from the screws holding the case together, not from the outside of the box itself)

All these readings were the same: 10 Ohms (which also leads me to believe I may be reading the mulitmeter incorrectly - but they were all consistent nonetheless)

As far a the cap and rotor, I have new ones, as well as new plug wires ready to go.

I'm not exactly sure where this leads me or where to go from here. As I am in a bit of a time crunch, I ordered some spare parts today (in a panic, which is never good). So now I have some compatability questions as many of the posts I've read offer differing opinions - imagine that! I really dislike using "swapping parts out" to trouble shoot. I can't afford it, and I'd really like to be able to understand the problem first - then be able to fix it. Unfortunately, I'm in a time crunch and am starting to get desperate.

1. Is the MSD High Vibration Blaster Coil compatible w/my Permatune CD box?
2. Are the OEM style Beru plug wires compatible w/an MSD CD?

Finally, this is an impossible question I suppose because I don't know exactly what's wrong yet, but do parts like the coil and CD box just all of a sudden decide not to work? I'd obviously like to avoid toasting new parts. Hopefully, some of the info above will help in the diagnosis and maybe point in the direction of the possible cause.

Thanks for all the help,
Tom

Tom,

I will let other experts speak to compatibility.

YEs, these parts can suddenly fail with little warning.

Ohm meter 101....

Set your ohm meter to the desired range and short the two leads together. Adjust the zero knob until the scale reads zero. If you cannot zero the meter, then your meter battery is weak and needs to be replaced.

For Coil tesitng, check out this thread: http://forums.pelicanparts.com/showthread.php?s=&threadid=299530&highlight=coil+o hms

For checking continuity, be sure you are making good contacts with your test leads.

You will get the problem solved.

I have an old functioning blue Permatune laying around.

It's original and i took it off at a 1999 rebuild to be used as a back-up to a new silver Permatune.

Silver one lasted about 25k mi as i was driving through Area 51.


keeping a cheap used CD box & coil around is like a spare tire. Even having a $20 Kettering coil, $5 resistor, and points is worth the time imo.

Harry -
Thanks much for the Ohm meter education. Yesterday, I bought some resistors w/a known value (10 Ohms) and calibrated it that way for my measurements last night. The readings still didn't seem to make sense. I will retest tonight after zeroing by shorting the leads together.

Thanks,
Tom

The diagnosis of this problem should take at most 5 minutes:

1. Get ANY known good ignition coil.
2. Make sure that the center pin of the CD box has a good +12 volts.
3. Make sure that the CD box case has a good ground.
4. Connect one terminal of the known good coil to the CD box output.
5. Ground the other coil lead.
6. Momentarily ground the pickup lead to CD box which will produce
a spark each time. Make sure nothing else (points, special pickups)
are NOT connected.

It's that simple!

Check here for additional info & specs (coil & etc.):

www.systemsc.com/diagnostic.htm

Loren - I've spent way more than 5 min. on this! But I now have a better understanding of what to test and how to test it. Spare parts should arrive today - one of which is a new coil. Just so I'm clear - if I follow your steps outlined above, as I did last night, after installing a new coil and I still get NO spark - then that indicates that the CDI box is the culprit? Correct? (assuming all the wiring is good).
Thanks,
Tom

"if I follow your steps outlined above, as I did last night, after installing a new coil and I still get NO spark - then that indicates that the CDI box is the culprit? Correct?"

That's correct!!!!!!

Use an old VW coil or the blue Bosch coil which Pelican sells or can get
as a replacement (blue - VW is for testing), i.e. NOT the silver Bosch.

It's a miracle! I saw spark tonight! My Permatune CDI box was bad. I set up the same test as last night, but with all new wiring - just to be sure. W/the old Bosch coil and Permatune box - no spark. So I swapped coils w/a new MSD that arrived today - still no spark. So I wired in a new MSD 6AL ignition that also arrived today and along w/the MSD coil - SPARK!! I did not test the new MSD ignition w/the old Bosch coil as I didn't want to press my luck.

I called Pelican who recommended a used parts place looking for a used/rebuilt Bosch CDI box and found the rebuilt units run over $500 - although in hindsight, the plug and play capability may have been worth the extra $$. Oh well. I don't care because I've got SPARK!

I still have a lot of work ahead mounting and cleaning up the wiring etc... A few questions:

1. The tach: I'm not exactly sure how to hook this up. Do I have to go digging through the wiring bundles to find the black and purple wire? Does it run through the 14 pin connector?
2. Can someone verify that Beru plug wires will work w/the MSD ignition. They're brand new. I've read conflicting info on this.
3. I plan to set my spark plug gap to a conservative? .035" Seems like I read some posts here that mentioned bigger gaps w/the MSD system. Is there really any performance difference between .025" and .05"? Or "cause any harm" difference? Does this matter?
4. I'm not quite ready to fully strip out all the old connections yet that will go unused. Can I just tape over the ends of the connectors and leave them be for now?
5. I need to figure a way to mount the huge MSD box - though I've found some helpful posts on that.

Thanks for the help,
Tom

"My Permatune CDI box was bad. "

These early units rarely fail but usually when:

1. Bad troubleshooting procedures used;
a. grounding the coil wire
b. applying +12 volts to the coil

2. Reverse jumping the battery.

3. Overvoltaging the battery when charging, i.e. crank-starting charger.

4. A bad voltage regulator (usually internal type) overcharging the battery.

Sometimes it's less expensive to NOT be a DIY type!

Thanks for the support! It's absolutely possible that I am the root cause of the problems I've been having. But it went bad before I began any troubleshooting. I have never had to jump my car or charge the battery. It's quite possible the voltage regulator is bad - how would I test for that? : - ) Never mind - I did some searching and learned about alternators and voltage regulators etc...

Tom

Tom
The tach wire does not go through the 14 pin connector. Follow one of the white wires from the CD unit (there are only 3 wires going to the CD unit - 1 red and 2 white) to a white plactic connector near the CD unit. The black/purple tach wire should come off of this connector and run forward from there straight to the tach.

I have Beru wires on my car with MSD - no issues.

I have a complete write up on MSD installation for a M.Y. - but it's not near me right now. I can send it to you later today if you having issues.

Thanks for the info on the tach wire and also the Beru wires. I've got a few write-ups on installing an MSD system and some photos etc... all from the archives here. The wiring appears straightforward - the MSD directions are pretty good too. Mounting the beast of a box may be my greatest challange. I've got an external voltage regulator and rear window relay to deal with. That's my goal this afternoon - to get the mounting figured out.

Thanks,
Tom

Up and running! I installed everything last night and I seem to be good to go. The wiring was not too bad, I found the tach wire and it works. After re-adjusting my valve adjustment from over the winter, a new rotor and cap, new plug wires and the new MSD CDI box, its seems to be running really smoothly. (hopefully I didn't just jinx myself for my 5 hr. drive tomorrow). I've got some tidying up to do w/the wiring and elect panel in the engine compartement, but as it is now, it works and will get me through - even if it's not the prettiest. Also, I checked the battery voltage while running and it seems good at 14.5V. Hopefully that's the last electrical issue that I'll have to deal with for a while.

I'd love to know what caused my issues - whether it was something I inadvertently did or if something just wore out and quit. I plan to do some more investigating next week when I have more time.

Thannks for all the help,
Tom