Bosch CD upgrade compatibility?
 

The 6-pin Bosch CDI on my '81 SC swap seems to have gone south. It was putting out a weak tach signal, and now it's not putting out any at all. The number on the box is 0227.300.004. The nice fellows at NH Speedo who've been trying to get me a tach that works suggested that the "004" boxes were unreliable.

Question is, is there another version of the stock CD box that will work that I might be able to find used, or should I just get a Perma-tune like everybody else seems to do?

Thanks for any advice.

HMM mine did the same thing. I don't know if there are more reliable boxes. I installed a Crane optical trigger, Hi6 and coil. No more worries.

Actually, this is one case where I don't care about a performance gain. I just want something reliable.

My change was due to it being more reliable. It worked and it was just sitting in a box on the shelf.

I know many people switch to an MSD setup when the Bosch unit craps out. There are several places that can rebuild the stock unit.

When I looked into this, I came to the conclusion that a rebuilt stock unit was the way to go. YMMV, but if there is an upside to using something else, I'm not seeing it.

btw, I picked up the rebuilt stock unit only as a backup . . . my 23 y/o OE CDI is still working fine. That seems pretty reliable, to me.

My 25 year old Bosch still going strong. I'd get a rebuild in your situation.

Jon,

The Bosch 0.227.300.004 CDI box was the ONLY version ever used on SCs from commencement of production in 1977 'til the end in '83, and was used on all 3.3 Turbo's from fall of 1977 thru the end of production in the summer of 1989! The SC's didn't get their reputation for reliability by having an 'unreliable' ignition system!!!

The only fault I can find with the '004' CDI unit so far is that Bosch deleted an over-Voltage Zener diode that had been used in all of the 3-pin CDI units ... and the '004' may have a higher likelihood of being damaged when a Voltage regulator fails and spikes the system Voltage to above 20 Volts!

I think whoever spoke to you at NH Speedo is trying to feed you a line of BS! It sounds like a similar 'tale' that many folks on this board tell when they bow to the MSD 'legend' ... when, in reality, they don't have a clue how to even begin troubleshooting an ignition system, and don't know how to do basic maintenance on automotive electrical systems!

The tach compatibility issue is one that all of the VDO repair guys should have worked out decades ago ... and making lame excuses now about the '004' CDI sounds a bit 'fishy' to me! I think the real problem is the number of revisions to the VDO tach circuit and keeping up with those changes ... there aren't that many technicians around any more that can troubleshoot discrete component digital circuits ... neither IBM nor the USAF teach component-level troubleshooting any longer, and I am certain there are many other similar policies in industry ... so where are the techs going to get the training?

I have a couple of ideas for a simple tach signal converter/adapter to drive the early tachs for the point-triggered 3-pin CDI systems from the '004' CDI 'TD' signal ... and may get a chance to test them next week. I should be able to post results in a couple of weeks.

We are all waiting, Warren. Thanks!

"The only fault I can find with the '004' CDI unit so far is that Bosch deleted an over-Voltage Zener diode that had been used in all of the 3-pin CDI units .."

Actually the 6 pin Bosch CDI's design doesn't need a zener across the +12 volt
input because the signal circuits are protected by a 270 ohm resistor and a 6.8V
zener. The 3 pin CDI was problematic with the zener across the input because
when the later updated internal regulator alternator was used and it overcharged
it would kill the 3 pin CDI because the zener was only 22 volts.

The Bosch 6 pin CDI tach output only provides a ground signal. You can simulate
this signal by taking a coarse file which is grounded with a jumper and then dragging
another jumper connected to the tach wire (black & purple) across the file. The
tach needle should move. The value shown on the tach depends on how fast you
drag the jumper across the file.

The Bosch CDIs are very reliable. It's very rare that one fails. If you need one,
Pelican can provide a rebuilt unit.

Loren,

The only problem with your argument against the 6-pin CDI units needing an over-Voltage protection Zener ... is that it doesn't hold water for the '82 and later models using the failure-prone Valeo and Paris-Rhone alternators that you have posted extensively about!!! The 6-pin CDI units do fail when the integrated-regulator alternators fail and put out 20+ Volts. But then ... you already knew that! I suspect that the 1 Watt rating for ZD2 is a problem when cars are driven extensively after the Voltage Regulator fails and maxes out system Voltage!

Your analogy between a wire dragged across a file is not a good one -- except for point-triggered 3-pin CDIs, as a well-documented problem with emitter-coupled Schmidt triggers is the fact that the output doesn't drop to ground because of the emitter resistor and the saturated transistor Voltage drop, plus, in this case ... the additional forward drop across D8 on its' way to the TD terminal! Nominally ... the TD signal never drops below +1.9 Volts when T3 is conducting! That is what is causing the earlier tachs designed for point-triggered ignitions to not respond properly to the 'TD' signal!

I'm an engineer and that still put me to sleep.

That's what is so great about this BBS.

I like to read just before bed time....

"I suspect that the 1 Watt rating for ZD2 is a problem when cars are driven extensively after the Voltage Regulator fails and maxes out system Voltage!"

No!

All the 6 pin Bosch CDIs which I've seen over the last 15+ years have yet to fail
as the 3 pins do because of an overcharging alt. Draw a schemtic, and do a
circuit analysis. You'll see that Bosch did a WELL protected design on the 6 pin.

Here's a brief the analysis:

1 watt zener max current = .16 amps (160 ma)

3 watt 270 resistor max current = 100 ma

Therefore, Vin (12 V input) max = 6.8V + 270 x (.10) = 33 volts.
That's much more than an overcharging alt will go.

The high voltage oscillator is also protected by a zener.

"the TD signal never drops below +1.9 Volts when T3 is conducting!"

So!

The 911SC tach has a pull-up resistor and only needs a ground or
a signal NEAR ground which the CDI provides. It DOESN'T matter
that the tach signal goes to ground. Use a pulse generator with an
SC tach and see what happens. That's what should be implied
(a pull to ground) from the anode of a diode which is the TD output.
A pull to ground is a better Tach signal. So if the CDI only provides
a pull to 1.9V, then a pull to ground will work, i.e. the tach needs
a "negative edge" signal and is NOT dependent on a min. positve
level (the requirement is a MAX postive level).

Bottomline: You need to draw a schematic of the 6 pin CDI!
It's a very simple, designed well, and thus very reliable.

Loren,

I think you missed the whole point of this thread and the tach problem ... nobody ever mentioned an SC tach! So that isn't the problem!

It is an engine transplant, and the early tach was what wasn't working with the TD signal ... that is why it was sent to North Hollywood Speedometer! Then they came forth with an excuse, citing the 'unreliable' BS about the '004' CDIs.

"The 6-pin Bosch CDI on my '81 SC swap seems to have gone south. It was putting out a weak tach signal, and now it's not putting out any at all. " - Rallyjon -

"The only fault I can find with the '004' CDI unit so far is that Bosch deleted an over-Voltage Zener diode that had been used in all of the 3-pin CDI units ... and the '004' may have a higher likelihood of being damaged when a Voltage regulator fails and spikes the system Voltage to above 20 Volts!" - Warren -

??????

So... this all means that if I can find a manually controlled voltage sensor and have the sensor pos flipped in place of the internal reg pos terminal I'd have a back-up internal reg? I guess the idiot lite is not used?

So Field pos on our "type P/pos" alt-- then to control pos-- then the neg control terminal to ground?

Who sells these manual voltage sensors, huh ?

Just to give this some closure, here's how things ended up:

NH Speedo apparently doesn't know their tach signals from, um, some part of their anatomy. Their new, special, modern internals, expecially for making early tachs work with 004 CDIs were looking for a conventional signal, not the "pull to ground" or whatever the 004 puts out. Not sure what they did for the two weeks we sent it back to be "fixed", since it's apparent they don't have a clue.

Anyway, Bert (my mechanic) ended up not using the tach output from the CDI at all. He took the signal from the distributor pickup and used that as the input to an op-amp, which then provides a tach signal that will drive the NH Speedo modified tach. Works fine.