CDI Part Numbers and Resistance

[Scott Clarke]

I have a 3 pin CDI box, Bosch # 0227 200 001, that is of the 30 ohm resistance variety. I understand that there is another box with 120 ohms of resistance. Does it have a different part number? The box I have causes the tach to read too high by about a factor of two. I have a Permatune box that has about 100 ohms of resistance, which works just fine with the tach. I'd like to get an original Bosch box, and it would be handy to know if the part numbers are different for the different applications. My understanding is that the switch from one type to the other occurred early in the '70 MY.

Thanks!

[304065]

I have a 3 pin CDI box, Bosch # 0227 200 001, that is of the 30 ohm resistance variety. I understand that there is another box with 120 ohms of resistance. Does it have a different part number? The box I have causes the tach to read too high by about a factor of two. I have a Permatune box that has about 100 ohms of resistance, which works just fine with the tach. I'd like to get an original Bosch box, and it would be handy to know if the part numbers are different for the different applications. My understanding is that the switch from one type to the other occurred early in the '70 MY.

Thanks!

There is no specific change over point from a 33 ohm pull-up resistor to a 100 ohm.

Expanding on Ingo's great advice, the pull-up resistor in the CD box is there to prevent a dead short to ground when the points are closed. When they close, the CDI trigger circuit is grounded, and current flows in proportion to the size of the resistor. E.g. for a 33 ohm resistor, 420mA of current: for a 100 ohm resistor, only 140 mA. It makes sense, right? The higher the resistance value, the smaller amount of current that flows when the points are closed.

The later VDO tachs from 10/70 onward are designed to run off an 11 volt square wave-- if you look at the tach circuit diagram, they are basically a monostable multivibrator or "one-shot"-- without posting the schematic, what basically happens is this:

You have two transistors, one of which is on and the other one of which is off. When an input signal arrives from the tach, it turns the transistor that is ON to OFF and starts turning the other one ON. How fast the other one turns ON is determined by the value of a capacitor that charges-- it takes an interval of time until the capacitor is full. While the capacitor is charging, the tach's meter needle is deflected. The more frequently the pulses arrive (higher RPM) the higher the needle "bounces."

So the problem is that the current flow from the 33ohm box is overwhelming the input circuit of your tach, giving you a pulse width that is double that of normal. Why don't you try a small resistor in the /1 circuit, in the black/purple wire, to limit the current from the CDI trigger circuit? Try 20 ohms.

[Scott Clarke]

John-
Thanks for your reply. I'll try what you recommend and report back.

[304065]

No problem

Turns out I posted the cutoff date years ago . . .9/73

http://forums.pelicanparts.com/porsc...ml#post2608829

Anyway, what you want is not more power in tacho drive circuit (as in, adding a 5 watt resistor beween the +12v lead and the black/purple to the points) but rather just between the black/purple and /1.

[Scott Clarke]

Well, I got some resistors and tried adding them to the circuit from the CDI box to the tach. I first tried a 22 ohm resistor, connecting it between the "1" terminal on the back of the tach and the black and purple wire. I got similar erratic behavior to that I had experienced previously with no resistor. I then removed the 22 ohm resistor and replaced it with a 68 ohm. This was no better, and in fact seemed worse. In any event, no go. Ideas?

[304065]

bad electrolytic capacitor in the tach

send to nh speedometer for a new board

tell them 001 box

[Scott Clarke]

The tach works perfectly with the Permatune.

[JKH]

Scott is it a 74-75 blue permatune , I have found two of these that appear to be factory ? when bosch couldnt supply, I was told. The two I have are 100 also. Jeff

[Scott Clarke]

Jeff-
It is blue, but I don't know the date. Is there a date stamp on it somewhere? Interesting that some might have been OEM. Replacement parts, I guess, given the dates you mention.