What goes wrong with OEM CDI units?

tiorio · 03-17-2003, 11:51 AM

So here's the deal, consensus seems to be that I'm gonna need a new/rebuilt CDI unit for my 89 930, and I'm wondering...

What is it that goes wrong with these units? What happens when they go to be rebuilt? I guess I'm wondering if I get out my soldering iron are there any 'user servicable parts inside'?

Anyone taken one of these apart or know what's replaced in a rebuild? Are there cold-solder joints that crack or boards that crack or what?

Lets say I crack it open and check it out, can I still have it rebuilt or am I gonna break something by opening it up?

Getting antsy with the 'little' tools,

Superman · 03-17-2003, 12:13 PM

I guess I wonder about the "I'm gonna need..." part. CD boxes either work or they don't. I am not aware of instances where they work poorly. They work, or they don't. Sort of like being pregnant. You either are or you're not. There are no "pre-pregnant" warning signs.

tiorio · 03-17-2003, 01:17 PM

Here's what's going on:

When I get her hot, she just shuts down...

Is there something else you think it might be?

Thanks!

Early_S_Man · 03-17-2003, 01:20 PM

There are several failure modes for the CDI-units, and two different models ... three-pin [point-triggered] and six/nine-pin [magnetic pickup coil triggered] ...

Two most common failures are:

1. a quiet box ... no normal whine or whistle while power is applied -- failure of the 2N3055 DC-to-DC inverter power transistor is usually the problem, though cases of broken solid point-to-point wiring are also possible due to to vibration and heat in the engine compartment ...

2. Fried/bad switching SCR ... due to fedback of high secondary Voltage into CDI-unit from a short in the coil.

Also possible are a multitude of broken wire or bad solder-joint failures within the case.

Lastly, trigger circuit problems of an intermittent, heat related nature ... are also possible.

The internal layout of the Bosch unit is very crude and difficult to troubleshoot ... due to the fact that several semiconductors are discrete components bolted to the magnesium case, which is used as a heat sink, and the circuit board is fastenened to the main capacitor and case-mounted semiconductors (and transformer) with point-to-point solid wires ... with must be unsoldered in order to get to the semiconductors [a power Zener Diode, main TO-3 case power switching transistor, and the SCR] ... in order to do any conventional troubleshooting with oscilloscope or multimeter you would have to fabricate an extension wiring harness of 15 - 20 wires to separate the circuit board from the chassis!!! Correct wiring based on photographs and labels would need to be 100% correct, of course!

Not an easy task ... exchange repair costs in the $100 to $200 range would seem to be worth it!

dd74 · 03-17-2003, 01:36 PM

Are there better CDI units than the stock version? With engine performance, how would one be able to tell a better unit from one that is just average? Is the engine crisper? More powerful, etc?

turbo6bar · 03-17-2003, 02:22 PM

I've read good things about the MW replacement CDI box, but I haven't seen dyno numbers to support the good word.

Jurgen

Early_S_Man · 03-17-2003, 02:27 PM

My comments on the failure modesand troubleshooting difficulty should NOT be interpreted as a condemnation of the reliability of the factory Bosch CDI-unit!!! They are built tough, and can withstand many years of severe vibration and heat. It is probably the most reliable CD ignition ever installed on a production car ... hence its' use by Mercedes and Porsche for so many years! The reliability of Ford Motorcraft and GM Delco electronic ignitions is a joke compared to Bosch over the production lifespan of the Bosch CDI system.

The Perma-Tune CDI replacement unit has had a spotty reputation, at best, and many consider the current quality control to be non-existant ... also many complain that intrermittently failing Perma-Tune units are being returned 'NTF' [No Trouble Found] by the Perma-Tune factory technicians ... much of the 'technical' info and claims at the Perma-Tune web site is pure 'marketing' BS! Perma-Tune units seem to suffer intermittent failures under electrical conditions which do not cause any problems for even some 30-year old Bosch CDI-units!
Not at all intended to be used on the Bosch CDI-unit ... the static bench tests for the Perma-Tune units detailed at their web site are only a guide, not a deterministic method of declaring a unit 'good!' A dynamic test with an ignition scope such as a Sun machine is the only real way of evaluating a CDI-unit ...

Personally, I have a spare Perma-Tune unit that was purchased in 1981, and it has been tested periodically ... and used for testing without any problems. It was not 'potted' in black epoxy the way current versions are. When I have test-installed the Perma-Tune on my car ... there was no perceptible difference in the way the engine started or ran, compared to the Bosch CDI-unit!

Superman · 03-17-2003, 02:55 PM

I have a Permatune unit and a Bosch unit that both work, and I have switched them recently. No difference. Warren is on the mark again. Methinks that Permatune products may not perform as well as you might think after hearing the hype. If the Bosch units can be repaired for reasonable cost, then I'd suggest considering that option. Bosch automotive electrical products are the envy of the world.

emcon5 · emcon5's Garage

Warren, any opinions on this?

Tom

subdewd · subdewd's Garage

Went through this nightmare w/83 sc years ago. Problem
came and went, Switched to permatune only to have it fail about a year later. To their credit they replaced the unit though it was out of warranty. While the perma was in the car I sent the bosche cd to be repaired. Unfortunatly I forgot who did the fix but I think it was an outfit in texas, I do recall they advertised
in excellence and did the job for less than $100. This was about 4
years ago. I've got the bosche in there now but the perma
stays in the trunk.

Doug Zielke · 03-18-2003, 06:26 AM

Warren.... That is the best synopsis of Bosch CDI info I have read since your last post on the topic a long while ago. Thank you. I printed it out for the "archives".

Jim.... Last time I was visiting JW, he had 2 or 3 dead Permatunes laying on the bench. Lots of info on the 'net about their lack of reliability. They are cheap, and you get what you pay for.
MSD seems to be a better value, although still not up to Bosch OE quality.

Superman · 03-18-2003, 06:49 AM

Doug, it was JW who reported replacing several MSD units, and also reported never having had to replace a Permatune he installed. He did say that while they can work with the stock Bosch coil in my car, their longevity is reduced unless you change to a 'real' coil. Yes, it seemed to me that PT had some failures, and that MSD seems reliable, but I went with John's advice this time. I'm not so sure that PT has a quality problem compared to MSD. At least the PT units are weatherproofed. We'll see. At this point, I have a spare system, so I'm not afraid of a failure.

Early_S_Man · 03-18-2003, 07:23 AM

Jim,

Did you buy one of the new Permatune coils as a spare? They are supposedly impedance matched to the output requirements of the Permatune units ... which logic says should also be close to the Bosch unit and coil ... since the Permatune is supposed to be an exact replacement for the Bosch units!

If you have the Permatune coil, I would be interested in the resistance measurements of the primary and secondary windings, for future reference?

island911 · 03-18-2003, 09:20 AM

Quote:

Originally posted by Doug Zielke
Warren.... That is the best synopsis of Bosch CDI info I have read since your last post on the topic a long while ago. Thank you. I printed it out for the "archives".

. . . what he said.

Thanks!

Superman · 03-18-2003, 09:59 AM

Warren, the Permatune guy I talked to suggested the fancy Permatune coil, but they're pretty proud of that thing (about $80 I think), so I am using a Bosch Blue. He seemed knowledgeable to me, but everything I know about electricity can be written on an ordinary post-it. He said that the Permatune unit does not work the same way as the Bosch CD. It's a different critter, he said, which is why it should not be used with the stock coil in my car. He did say that any ordinary coil will work with the Permatune unit, just not the "coil" thing in my car. He said that the reliability of the coil is important since a coil failure destroys the CD unit, ergo the suggestion to use the sturdy Permatune coil.

emcon5 · emcon5's Garage

The MW one supposedly has short circuit protection built in, so if the coil shorts or if you hork up the wiring the box will be safe.

I just wish someone would be the guinea pig and install one so we can know how well it works.

Tom