There are several different VDO tachometers out there. This thread is an attempt to classify the characteristics of each. Can anyone help by furnishing the VDO numbers from the back of any spare tachs they may have lying around?
1. 1965-1968 SWB Tachs. These may have VDO number 333.230/041/002, according to VDO, "this is a old version Tachometer with 24 Volt Square Wave signal from Ignition KL.1."
KL is German for "klemme" or contact, and #1 is the points contact, so this is a 24-volt square wave signal that comes off the coil. In another thread, Pelican 914GT tested it and found that the coil output was way more than 24 volts, but the spec came from VDO.
Tach Thread These also say "12V 14000 Imp." which I think is the impedance value to make them work? I think all 914 tachs fit into this category as well.
2. 1969-1970 LWB Tachs. These have VDO numbers 333.230/041/007 to 333.230/041/010. According to VDO, these are driven by an "11 volt square wave." Now, we know that up until around 1971 the tachs were driven from an "intermediate unit" which was a silver can with Bosch part number 0 227 990 001. We know that this contains an inductor that is probably used to simulate the original coil to drive the low-impedance tach. Some of these say "12V 24000 Imp."
3. 1971-1973 Tachs. These appear the same externally but have been altered internally to be driven off the high-impedance circuit between the points and the Bosch CDI box. In the early 3-pin CDI boxes, there is a 33 ohm pullup resistor in the trigger circuit that would allow only around 425 mA to flow in the circuit. The tach must be sensitive to 11 volts and 425mA, which is why the later CDI won't drive the earlier tach off this circuit-- the earlier tach needs higher voltage.
4. 1974-1977 Tachs. I know these were driven off the CDI box, but I don't know when the CDI box changed from an "001" to an "008" or when the cutover from a 33 ohm pullup resistor to a 120 ohm occurred. With 120 ohms the input current is only 116mA so the tach must have changed somewhere along the line to match. If you send your tach to NH speedo to be recalibrated they want to know what CDI box you have, so this must affect the input value to the tach.
5. 1978-1983 Tachs. These are driven from the "TD" tachometer drive output from the six-pin CDI box because the points were eliminated in favor of a magnetic reluctor. According to the following thread, the SC tach signal is the same as the later Motronic:
SC Thread
6. 1984-1989 Tachs. These are driven from the Motronic box. Chris Bennett posted the Motronic signal here:
Motronic Thread
7. 1990-1998 Tachs. 333.240/006/001. The 964 and 993 tachs are also driven from the Motronic box, no?
If ANYONE has any information on the signal requirements of any of these tachometers, it would be very helpful to post it here. Also, can anyone post the numbers from the back of their tach? A helpful format would be:
230/041/002 12V 14000 IMP 6/72-- that way we can figure out what tachs are out there, when they were made and when the cutover to a different impedance was.
Thanks!