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? Or was it after 71?) 911 tachs were designed to work with CDI ignitions which have a much lower “drive” signal than a standard coil. The early 911’s and 914/6 used to have a secondary “ballast” or signal amplifier to bring the CDI tach signal up to that of 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!

[flxzcat]

Superhawk,

10K tach date stamped 74.

No ballast that I can see, and ballast needs to be dis-connected anyway according to Tach-Adapt

Like I said, the original 914 tach worked without the Tach-Adapt.

The original 914 tach worked with the Tach-Adapt and RPM could be modified.

The 10K tach only worked without the Tach-Adapt but at the wrong RPM.


Cfletch,

I did try pulling the chip thingy but that didn't help.

[Superhawk996]

Yeah I get that and agree it’s a weird issue. That’s why I’d suggest talking with Tach adapt - there seems to be some sort of problem or incompatibility between the tach adapt and the 10k.

Their site makes some vague reference to their adapter may not work with all tachs but the 10k Porsche tach might be a known issue they can offer a solution to.

Lacking an answer from them, you’re in for an uphill battle setting up a test rig to drive the 10k tach with a frequency generator, and then coming up with an appropriate circuit mod to change the tach calibration from a six cylinder to a 4.

I have successfully set up my own tach bench test rig a few years ago (and did find a dead /6 tach I have). However - I’d be talking out my (IMG:style_emoticons/default/bootyshake.gif) if I tried to tell you I have the knowledge or experience to tell you want components need to be modified in the 10k tach to convert the /6 tach to a /4 tach though I’m sure it’s done regularly by NHS or even John Bell on this site. Sometimes it’s just better to pay for someone’s else’s expertise that has done it 100 times before.


https://www.youtube.com/watch?v=EO3XCJQ9tW4?si=npXEPwBlJWxkBtDX


I have an abbreviated video i can share if you want to set up a simpler test rig using just an Arduino and not need the frequency generator.

[flxzcat]

Their site makes some vague reference to their adapter may not work with all tachs

Yah, I saw that when I ordered the part. I had a feeling things would end up that way (IMG:style_emoticons/default/hissyfit.gif)

[Superhawk996]

Their site makes some vague reference to their adapter may not work with all tachs

Yah, I saw that when I ordered the part. I had a feeling things would end up that way (IMG:style_emoticons/default/hissyfit.gif)

Can’t hurt to ask though - my understanding is Bob Ashlock originally developed tach-adapt specifically to address the issues Porsche tachs had and the NLA ballast that was needed for early Porsche CDI boxes.

Maybe they have a quick fix or can point you in a helpful direction (IMG:style_emoticons/default/confused24.gif)

[Superhawk996]

If I get some spare time I’ll put that tach schematic into LTSpice and see if I can come to an understanding of how it works and if your suspected modifications would do the /6 to /4 conversion.

[sixnotfour]

John Bell,,
He does 10k conversions ,,and are nice...Bonus your 74, 10 k ,,is probably more important to someone else..I had 2, 1970, 10k Tachs ,, sold one for 3k.

I bought this from John for a 3.2 transplant.
https://forums.pelicanparts.com/porsche-911...rera-3-2-a.html

[JmuRiz]

You can always just grab a /4 tach right? I had my '73 silver button /4 tach sent and my engine matching 911 lack button tach sent to John Bell, he swapped the buttons, cleaned and tested them both. So I have a '73 /4 tach with a black button on it ready to go, I think the '74 changed over to black button needle right?

Let me know if you're interested....I think I also have a tach-adapt that I was going to use before I found John, so let me look for that as well (if you're interested).

[mepstein]

Another two thumbs up for John Bell.

[mb911]

Easy button John Bell

[flxzcat]

John Bell, tempting but, the problem is my excellent appearing 914-4 tach is a bouncer, so needs more than a face lift. New diodes I guess. Anyway, the car has a 7200 RPM redline now. I will send a note to John asking if he knows anything about or can perform the modification I need on the 10K VDO.

The 10K has been with me a long time though unused since my old timey racing days. Memories, can't part with it.

Superhawk, If you can load the schematic into LTSpice software and get results, that would be fabulous.

JmuRiz, Don't bother looking for your Tach-Adapt, I have a new one and it's not working with this the 10k.

[mepstein]

John has done a lot of modified gauges for me. Green triple gauges, green electronic speedo- moving the reset from the face to the bottom, repairing gauges, etc. He's been my go-to for a couple dozen gauges so far. A pleasure to work with, very reasonably priced and quick. I used to wait a year for the big names to just open the box and give me an estimate. John had done gauges for me in days or sometimes weeks if it's custom.

[flxzcat]

I used to wait a year for the big names to just open the box and give me an estimate. John had done gauges for me in days or sometimes weeks if it's custom.

That is something I was worried about although Hollywood quoted a 2-day turnaround for my particular problem.

Here is a pic of gauge in dash. Fun fact, I made the steering wheel almost 50 years ago!

[mb911]

John did this tach for me