VDO 10K tachometer conversion 6 Zyl to 4 Zyl, Diodes, resistors? What do I need to change? Options

[flxzcat]

Guys,

I have a beautiful 1974 (yes, I have had this for 50 years) 10K VDO tach installed in my 1974 914-4. I have 123Tune distributor and all works well except the tach reads @2/3 speed.

I have tried the Ashlock Tech. Tach-Adapt that is sold by 123Tune and it does not work with the 10K. When I hook up the 10k to the Tach-Adapt it is totally dead. I am using all the same tach wires as the original 914 tach uses.

Interestingly, the Tach-Adapt does work with the original 1974 914-4 tach. I can alter the speeds shown on the 914 tach with a flick of a micro switch inside the Tach-Adapt.

To further the confusion, when I hook up the 10K VDO tach without the Tach-Adapt, it works fine at 2/3 speed as you would expect. I am using the same wires on the harness as the 914 tach uses.

Now, I am electronically challenged, but I have soldering skills. I also have access to an electronics lab here at work. I do not have access to any technicians as we have shut down that part of our EV business. The lab has hundreds and hundreds of diodes, resistors, connectors, wire of all sizes, flavors and colors and of course all the analyzing equipment.

My question for the smart guys out there is: Is it a matter of changing a diode, resistor, capacitor or other and to what value or color stripes. If I had this info and knew where to remove and replace the parts, I could do this and not give $400 to Hollywood or other gage repairers.

Below is a schematic of a 73ish 911 tachometer which, one would think would be very similar to the 10K (with different lighting of course). Frankly, this is all I have to go on and it could be a foolish assumption. I have read that I need to change Resistor R8 and Capacitor C4 to make this 6 cyl tach work on a 4 cyl engine, but unfortunately values were not specified.

Is there an EE or other smart experienced person out there that can help me?

Thanks in advance,
Paul VDL

[Superhawk996]

Talk with the Tach-Adapt folks

Late model (74’ & up) 911 tachs were designed to work with CDI ignitions which have a much lower “drive” signal than a standard coil.

It very well may be that by driving the Tach-Adapt with the more powerful coil signal from the /4 that it is saturating the Tach-Adapt circuit (ie overdriving it) or there is an impedance mismatch that keeps the 10k tach from working.

I would have thought the Tach-Adapt might compensate this but I don’t have hands on experience with that adapter. Maybe the Tach Adapt is not compatible with the 10k tach for some reason (IMG:style_emoticons/default/confused24.gif)

That would be my 1st move before trying to modify the tach based off a questionable schematic for an early 73’ tach.

[Cfletch]

Curious if you pulled the little chip they tell you to pull on the unit to get the unit to work with lower signals (or higher?)- Have this unit for my car with MSD and with it installed, no tach, after pulling it, my tach works.

Good luck!