VDO gauges

[Baxter2012]

So trying to figure out tach adaptor selection
.seems I read some where that on the back of the tach there is a1 or a C and that designated a time frame. Anyone out there that can unlock the secret let me know. This has something to do with 8910 or 8920 MDS adaptor and mines a 1970

[zundfolge]

there is a thread on the bird about that but I forget the answer

[304065]

Cutoff is October, 1970. You should see a "C" after that.

Think about it. . . in 1969 they came out with BHKZ (known here as CDI). That box has four terminals

A= Achtung = 350V coil output, to terminal on coil (look at the markings on a later inductive coil, like one off a 924 or something with the terminals on the same side, you'll see an "A")
B= Batterie = +12V input
C= Contact = Points
D= Ground

The tach input is common to the points and the CDI box has a 33 ohm pull-up resistor in it that produces 420ma of current in the points circuit. Just enough to give you a discrete signal but not enough to erode the points like the earlier ignition.

Of course they didn't change the tach until October, 1970, so from the 1969 intro of CDI till that change, they used the "Intermediate unit." And the Tach said /1 on it which is the coil negative, even though it was connected to the "intermediate unit."

The MSD 8910 is basically a doorbell transformer inside a plastic case. One pole goes to switched +12v and the other to the wire to the tach. Its role is to charge up like an ignition coil when the points are closed, then when they open (1) the CDI triggers and (2) WHAM! A 200 volt EMF spike runs up the tach line, triggering the tach's input. Electrically as dirty as it gets but it sure worked in my race car for a while.

If it were me I would get one of Bob Ashlock's tach adapters, they are a fine product and people report success with them. I have one somewhere in the Fortified Barn, I forgot where it is.

Good luck!

[Jerry L]

+1 For Bob's TacAdapt.
The MSD kind of pissed me off, said with the VDO I would not need the adaptor, then when the tac would not work per their instructions and a few tests from them, they said I needed it, after making a very nice mounting bracket to tuck the MSD away. So no to the big klunky & ugly chunk of plastic.
Go with Bob's it is neat clean design small and works, as well as being a member here.

Best Regards