Tach not working after restore

[davemccorkle]

Got my car back after a 3 year restore and have been working through various things. I am having issues with my Tach not working now. My car is a 72 but has many mods. The original motor was a 2.2 and the Tach worked. I now have a 2.7 RS, 3-pin CDI, and the Tach does not work. All the lights work and I have check the wiring but not sure what to look at next.

I traced the b/purple wire to the engine bay and it appears to be fine.

Can i connect a test wire directly from the distributor to the Tach?

Do i have the wrong Tach now that I changes the engine?

[Wilywilly]

I'm thinking you need your tach converted to a cdi configuration. N Hollywood does them

[davemccorkle]

Cool! I was going to call them Monday!

[ossiblue]

You said your car had many mods, so it's not clear what had been changed but, originally, your car had a CDI ignition so you shouldn't have to mess with the tach unless the PO had modified the ignition. If original, the wire from the tach originates at the distributor and shares a common connection with the wire from terminal C of the CDI. Without a diagram for a 72, I can't tell you exactly where it goes from the distributor but on a 71, it goes to a speed switch on the engine electrical panel and then to the tach. On a 73, it runs directly to the tach.

[davemccorkle]

My Tach is a VDo 230/41/10
CDI Bosch 0 227 200 001 12 volt

Not sure if these two are compatible or not.

[ossiblue]

Originally Posted by davemccorkle

My Tach is a VDo 230/41/10
CDI Bosch 0 227 200 001 12 volt

Not sure if these two are compatible or not.

That CDI unit was used up to model year 76, so it should be compatible. However, tachs went through several mods and though your tach number appears to be compatible, there may be some subtle differences. Can you find the date stamp on the back of the tach?

Look at the back of your tach for the plastic electrical connector. What is the marking above the connection for the black/purple wire from the ignition? It should read "C" or "1."

Check out this link:
Tachometer Compatibility Chart

[davemccorkle]

It read "C" and the date stamp show 2/72

[ossiblue]

Originally Posted by davemccorkle

It read "C" and the date stamp show 2/72

Two questions I forgot to ask: You stated in your original post that the tach worked with your 2.2 which originally had CDI ignition. What was never clear was, did your 2.2 set up have CDI or had it been removed? If it did have CDI, did you replace the CDI unit when you installed the 2.7 engine?

My understanding would be it should work with your CDI, connected to the distributor along with the black wire from terminal C, however you may want to check the date code on the CDI . Apparently, those manufactured before 9/71 have a different resistance value and these need to be matched with the tach. Possibly you can do some testing of the resistences mentioned in the thread from the posted link, or contact one of the members who contributed to that thread--Grady might be a good start.

From the posted link:

(posted my member 304065)
"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."


(From Warren Hall)
"The 33-to-120 Ohm changeover occured around the time of the '72/'73 model changeover. My CDI is an early '73 model, date-coded '227' and the 120 Ohm resistor is original. The date code is stamped on the same side as the 'Vorsicht!' sticker near the lower edge ... with very tiny 2 - 3 mm high numerals.

You can easily tell which pullup resistor you have by measuring from terminal 'B' to terminal 'C' with a multimeter set to measure low Ohms [200 Ohm scale] ..."

[davemccorkle]

Got my Tach back from North Hollywood and it works great now! Woo Hoo! It just needed to be converted! Thanks WilyWilly

[tobluforu]

How much did it cost to get converted?
Thanks

[joeblow]

Just to second this, NHS is the way to go. I had the same issue and they fixed it up for me. I also had mine completely rebuilt too for around $300 from memory. Now I have to redo the rest of the gauges as it looks so good in comparison!