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Tach circuit board replacement

Posted: 12 Mar 2019 09:29
by anguso
Last year I installed a 123ignition distributor in my 510, and soon discovered that the tach didn't work with it.
A bunch of reading followed, and I was faced with either replacing the internals with those of a more recent tach, or getting a third party adaptor.
Since the car is tucked away for winter, and I had time on my hands, I decided to go down the DIY route. (Also, being in NYC, junkyards aren't an easy option in the winter).
I found a design by a guy in the Ferrari Dino community which looked cheap and would fit inside the tach housing, http://www.dinoplex.org/tachoconversion/index.html

I had the PCBs fabbed at https://oshpark.com and got a few spares just in case.

Including the rest of the components I got from https://www.mouser.com, the parts bill came to around $15.

I built the PCB, and rigged up a little test bench using a cheap amazon signal generator:
https://www.amazon.com/gp/product/B075V41DJM

Everything worked well - almost. I discovered that there was a short circuit on the PCB (which led to a good email exchange with the designer). I fixed that, then I was able to adjust the tach needle with the little trim potentiometer on the PCB.

Once I had it working, I noticed that once the tach was set correctly at 1000rpm, if I changed the signal output to higher rams, it would read 3200rpm instead of 3000, and 6500rpm instead of 6000.
I puzzled over that for a while, and had a feeling that the problem might be mechanical rather than electronic. Sure enough, on the back of the tach dial, there is a tiny little counterweight that rides in a little slotted arm hanging off the needle shaft. I measured the position of the weight so I could return it to its original position if needed, and then slid the weight inboard a millimeter or so. It worked, and now the tach reads accurately at all speeds.

I bolted the PCB onto one of the original boards from the back of the tach, reversed the order of the brass standoff screws so there was room for the PCB to fit, and screwed the case back together, only to have a mishap and smash the clear plastic cover for the front of the tach. Where the heck am I going to find another one of those..?


Now I wait for warmer weather so I can get to the car and reinstall the tach!

Re: Tach circuit board replacement

Posted: 12 Mar 2019 09:38
by anguso
Adding images is killing me - I click preview so I can make sure they make sense in the order I insert them, and then they become unavailable!
Here's the images that fill in the story:

Re: Tach circuit board replacement

Posted: 12 Mar 2019 10:40
by yenpit
Wow, GREAT write up! Might you build more & sell? I would never even attempt to do this kind of electrical work, but would buy if it would work on a CA18DET in a 510!

I wonder if the speedo "lens" is the same? If it is, I will have a spare speedo lens!

TJ
Denver CO

Re: Tach circuit board replacement

Posted: 13 Mar 2019 00:08
by Byron510
I’m thinking the same, would you make these available after tried and true testing?

Great work.

Keep us posted.

Byron

Re: Tach circuit board replacement

Posted: 13 Mar 2019 06:33
by greenthumb
very cool work. Not familiar with the 123 distributor. What was the reason the tach wouldn't read the 123 signal? Is it coil pack based, as opposed to a single coil?

Re: Tach circuit board replacement

Posted: 13 Mar 2019 16:15
by anguso
Before I go making promises I won’t be able to keep, let’s see how this one works once it’s reinstalled!

Regarding Yenpit’s suggestion (thank you!)- does anyone know if the speedo lens will work in the tach housing??

Re: Tach circuit board replacement

Posted: 13 Mar 2019 16:23
by anguso
Thinking back, it would’ve been massively easier to swap the guts out of a modern tach; Bosch makes some inexpensive stepper driven 250° sweep 8K rpm tachs.
I think the only reason I got so far down this rabbit hole is because I had a few too many nights to myself with my wife away on work trips...
However, the proof will be in the pudding! Fingers crossed!

Re: Tach circuit board replacement

Posted: 15 Mar 2019 19:35
by SteveEdmonton
anguso wrote:
13 Mar 2019 16:15
Regarding Yenpit’s suggestion (thank you!)- does anyone know if the speedo lens will work in the tach housing??
Yes, a speedo "lens" will work just fine on a factory tach. I overhauled my speedo and tach at the same time a couple of years ago and confirmed that the lenses are identical.