Bosch CD Box Failure Modes

Jeff Higgins · 06-02-2005, 05:28 AM

I have finally just gotten on top of a very frustrating and elusive problem that I was chasing since about February or so. Those of you that have helped me along through this by answering my various questions my remember how long I have been troubleshooting a very intermittent poor running condition. It turns out it was the CD box; they can have an intermittent failure. Common wisdom is they either work or they don't. With that (and the cost of replacement) in the back of my mind, I set about checking everything else. The happy result, after going through CMA a couple of times, rewiring everything in the engine compartment, and checking virtually everything else in detail, is it now runs better than it ever has. I put an MSD 6AL in it last weekend. Its poor running condition had deteriorated from at first a once a month "incident", to every couple of weeks, to maybe once a day, to not taking any throttle at all. Hopefully my experience can save some one else this frustration. Once again, lesson learned is the CD box can fail intermittently. In my case, at first, it could go weeks running just fine.

On a side note, I know these are hard to find, and that some one who knows their way around them can diagnose and fix them. I would hate to throw mine on some back shelf of the garage and take it out of circulation. Anyone want it? It's the old three pin version. Make me an offer; since it doesn't work it's real cheap...

evren · 06-02-2005, 05:53 AM

how about $50?

rick conrath · 06-02-2005, 07:38 AM

I know of a place that fixes them in Texas. The cores are worth more than you might think!
Rick
'78 930

pwd72s · 09:39 AM

I think the Pelican core charge is $100, but Evren's offer is fair...I picked up a core at Rothsport for $50...off an early car being hotrodded into a bigger bore engine with crank fired ignition. (edit) Yep, I just checked pelican's parts site: "Core charge of $100 will apply to rebuilt units. 911 Turbo CD-units will need to be sent into us for direct repair." But then the mystery...rebuilts are NLA??? Not from Pelican, anyway...I do know that rebuilding services exist.

Porschephanatic · 06-02-2005, 09:41 AM

Stay away from Perma-tunes! Mine only lasted about 2yrs each. Going to a MSD if I get enough positive feedback.

pwd72s · 06-02-2005, 09:53 AM

Brian, that seems to be the concensus here...that Permatunes are not the best way to go. There have been several hot debates here on Bosch stock vs. MSD, but those seem to be the 2 favored ignitions here, with followers in both camps. Not many Permatune fans here.

KNS · 06-02-2005, 10:12 AM

I have heard that some places are no longer rebuilding them. The rebuilder will send out the good unit and never get the bad core from the customer. The customer gets a working unit for $100 bucks and never sends in the bad unit. I hope these are isolated cases.

Jeff Higgins · 06-02-2005, 10:53 AM

O.k. Evran, it's all yours. Fifty bucks it is, plus shipping to "Istanbul". I'll probably have to get an export license to ship this high-tech piece of electronic equipment to an ITAR-regulated destination such as yours.

Seriously though, pm me an address and for fifty plus shipping you can have it.

rick conrath · 06-02-2005, 10:55 AM

I paid $186 for my rebuilt CD box.....the core was closer to $450 at the time, but I was only paying $186 for the exchange. The rebuilder (as I understood) supplied a number of parts houses that added their markup in the end. That's only fair. It's pretty hard to beat the reliability of the Bosch unit. My last CD box lasted for 23 years of "turbo" engine compartment heat and abuse.
Rick
'78 930

Early_S_Man · 06-02-2005, 01:48 PM

Guys,

I don't know where the 'myth' that CDI units either 'work' ot 'NOT' came from ... but it is pure fantasy!!! Of course, the Permatune folk add a bit of myth and hype to the discussion with their utterly false claim of being FAILSAFE!!!

Bosch CDI units are less complex than the radio that came with the cars ... and that brings some advantage, statistically, because of the reduced component count. The large number of point-to-point wiring connections between the discrete components does add a bit of an issue with bad solder joints, though!

Failure modes include open or intermittent transistors, diodes, SCR, and Zeners that are due to internal wire bonding failures. Shorted semiconductors are usually due to a thermal overload causing a permanent fault in the silicon structure.

Defective capacitors and resistors are also possible ... as is a shorted transformer winding, but those are usually secondary effects from another failure in an active [semiconductor] component.

Over the years word has spread about checking for the whine of a Bosch CDI, but that isn't a comprehensive check! It verifies that the inverter oscillator is working ... but nothing else! A 'telegraph' key test at the distributor signal wire with a grounded test spark plug connected to the coil takes very little time, and tests the ENTIRE system ... up to the distributor, anyway. Here is what I humorously refer to as a complete test set for a 3-pin CDI unit ... same WWII surplus key [recently cleaned up after finding it in a dusty box in the garage] I used back in '65 to receive/send 90/65 wpm CW code!

evren · 06-02-2005, 10:49 PM

Thanks Jeff!... actually if it's ok could you send it to Loren in CA? then hopefully Paul can help me get it over here... I don't have Loren's address with me right now, but he's in area code (310), so that should be near LA i guess. If you let me know what the shipping will be I can send you paypal or a check.

Jeff Higgins · 06-03-2005, 05:17 AM

Actually Evren, I just got a note from Paul. I'll send it his way early next week. He said he would forward it on to a rebuilder for you. At first I thought your "Istanbul" location was just another humorous Pelican-esq tag line: I've seen some pretty incredible ones. Pretty neat that you are actually there. It must be very hard to find Porsche parts. Boy, and talk about coincidence - I just finished up doing some work on the Peace Eagle program, the 737 based AWACS for the Turkish Airforce. Anyway, Paul should have it by the end of next week or so.

Jeff Higgins · 06-03-2005, 05:25 AM

Warren, I'm living proof of just how missleading these myths can become. I sure chased this one for a long time. So if I had tested it, even with it being such an intermitant problem, would I have found anything anyway? Electronics have always been a mystery to me and I get pretty intimidated around them. Maybe in purely practical terms to a guy like me they either work or they don't. When that scary mystery box quits doing what it is supposed to, I'm certainly not qualified to figure out why. At least my experiences might help some one else diagnose a similar problem faster by helping to debunk that myth a bit.

Superman · 06-03-2005, 06:08 AM

Quote:

Originally posted by Jeff Higgins
Warren, I'm living proof of just how missleading these myths can become. I sure chased this one for a long time. So if I had tested it, even with it being such an intermitant problem, would I have found anything anyway? Electronics have always been a mystery to me and I get pretty intimidated around them. Maybe in purely practical terms to a guy like me they either work or they don't. When that scary mystery box quits doing what it is supposed to, I'm certainly not qualified to figure out why. At least my experiences might help some one else diagnose a similar problem faster by helping to debunk that myth a bit.

I don't even understand electrical theory well enough to ask Warren questions. So, for me, I think that the obervation Jeff makes above is yet another reason to carry a spare. In the event of trouble, plug the other one in. Plug them back and forth. Then you'll know.

And now that I've completely bored you all with this inane information, here's a factoid: When your engine is turning 6000 rpm, each spark plug is firing fifty times per second (the pistons are going up....and down....one hundred times per second). So, the CD box is making 300 flashes per second, altogether.

evren · 06-03-2005, 11:24 AM

.. jeff, thank you -- yes Paul is indeed my guardian angel, anybody who searches the pelican archives will see what he has done for me over the past x number of years. please let me know when / where to send you money -- you can reach me at evren @ turkven . com

randywebb · 06-03-2005, 02:24 PM

Electronics have always been a mystery to me...

- I'll bet there is a dummies guide... If not, there will be texts used for community college students - so you can surely learn at least a little bit.

If in doubt, start with V = IR, then cover capacitors and inductors. You can learn about active devices (transistors, etc.) later...

pwd72s · 03:58 PM

Guys, I'll confess. I'm confused here. Nothing unusual, because I'm easily confused. But between Jeff, myself, Warren, and Evren, who ships what where is becoming a mystery to me. Tell ya what...since the CD is going to be Evren's, let's all work through him, letting him be the CD project manager. I'm standing by to hear from Evren, willing and able to do any mailing asked...note Evren's email posted above. Mine: donkin @ proaxis.com
Hope this sounds okay to all. Paul

Jeff Higgins · 06-03-2005, 08:16 PM

Works for me, Paul. Evren, tell me who to ship it to and it's on its way. Either here, or a pm, or e-mail at jhigginsfamily@mac.com.

limble · 06-04-2005, 05:17 AM

Ditto on the myth. I was having intermittent starting problems. And the CDI whined more than a three-year nephew. I pulled the CDI out and replaced it with a standard coil and condenser. It always started.
I know you horsepower junkies will cringe because of the loss of horsepower, but rest assured, I'll replace it when the rebuild is done.