Do Tachometers need to match CD Boxes?
 

I recently bought a used "T" tach on the "parts for sale" classified, in a "throw money at it" attemt to fix my bouncy tach problem.

Anyway, the tach I bought was D.O.A. (but still worth the price paid); and I thought "hey i can send this one to NH speedo or Palo Alto and they can fix it; I can still have a useable tach in my car in the meantime."

When I sent the DOA tach to them, they called me and said they needed the number of my Bosch CD box in the car (which is 0227-200-001).

Which brings me at last to my question: Are there different #s of CD units and do certain ones only work with certain tachs? Or were they just checking for a Porsche CD box as opposed to a Volkswagen or something? The 2 tachs have the same markings on them and were manufactured within a few months of eachother, 9/71 and 6/72.

Really I'm just trying to make sure that I didn't pay a premium price for a guage reface, and my tach might have been fine.

Any ideas? I did a some searches, but didn't fare too well. Thanks

In a word, yes.

In more words, look here:

http://forums.pelicanparts.com/porsche-911-technical-forum/280714-tachometer-compatibility-chart.html

And post your data if you can, I want to compile a database of tachs and the applicable boxes and the years they were made.

How's your distributor? Points?

New points 'fixed' my tach on my 73.

Have you checked or monitored your system Voltage while driving? High system Voltage above 14.5 Volts ... can cause erratic or bouncing tach. Any fluctuation in headlamp brightness at night over the rev range?

Worn, pitted points can also cause bouncing tach!

The Bosch 0.227.200.001 3-pin CDI unit was used from the fall of 1968 until 1976/77 with a few circuit changes. The final 0.227.200.008 3-pin unit had another minor circuit change that would not affect tach compatibility, and really didn't justify a part number change, compared to previous changes. I documented all of the 3-pin units in the following thread:

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

The change from 33 Ohm pullup resistor to 120 Ohms occured at the end of 1972 production or the beginning of 1973 production. A tach dated-stamped 9/71 should have been a 1972 model tach, and you shouldn't have any compatibility issues with it.

Thank you, tcar and Warren. I have read many a "bouncy tach" thread. I am right now in the midst of changing my VR to the proper one. In fact, Warren responded to a thread of mine about this a long time ago:

http://forums.pelicanparts.com/porsche-911-technical-forum/187867-name-vr.html

So yes I understand that my tach may not be at fault, but I had a chance to buy a second one cheap, and I did. (Not so cheap anymore, after Palo Alto speedo, however.)

Just for kicks, I put the "refurbished" tach (6/72) in the car, and it was not bouncy. If anything, it was kind of slow-to-respond.

But for now, I'm gonna leave the "bouncy" one in there and see if I can de-bounce it with fresh points and a new VR.

Thanks