How do I mod an early tach for a V8 signal?

I've got an early car and will be running Ford EDIS which outputs a normal V8 tach signal. I searched an found a few references to info on Porsche Hybrids, but the links go nowhere useful. Anybody know what's up? I could take care of it externally with some diodes, but I'd rather take care of it inside the tach itself.

Edit: I've got this working. See my post farther down the page for more info.

Is this the commonly used 'voltage pull-up' circuit that you are looking for? (from LT1swap.com website).

That won't modify the frequency of the signal, just the voltage. What I'm looking for is something to modify the tach itself to display correctly with a traditional 8-cyl tach signal. I can get a 4-cyl tach signal by using a couple diodes on the coil packs, but I'd prefer to make the tach work with the signal I'll have instead.

You need a tachometer converter. One of these two might work.

http://www.dakotadigital.com/index.cfm/page/ptype=product/product_id=127/prd127.htm

http://www.ashlocktech.com/Home.php

If your original tachometer gets the signal from the original ECU, you probably need the Dakota. Haven't checked, but I believe all 944's do that.

If the original tachometer is conected to the negative terminal on the coil, then it's the last one. I need that one for my project ('76 924 straight six swap)

Again, I'm trying to mod the tach internally. I'm not paying $50-90 for a box that I can build for under $10. I would prefer to just change the way the tach displays to messing with the signal at all. What I'm looking for is information like what exists for the late tachs.

I've never opened a late 944 tach...any chance it uses the ever popular 555 IC as a pulse counter?

There is a ton of info around if it does. Starting here:

http://www.electronicecircuits.com/electronic-circuits/analog-frequency-meter-circuit

944convert wrote:I've never opened a late 944 tach...any chance it uses the ever popular 555 IC as a pulse counter?

That's a good question for somebody who has a late tach. I can't help you with that one.

All I know is I've seen specific instructions on which resistor to replace with what value in the late tach to make it read correctly for a V8 signal. I'm sure it's after any signal processing and before the movement. I'm asking if anybody here has that same sort of info for an early tach. I'm not looking for external solutions because I can make one of those on my own. I'm not looking for how to modify a late tach because I don't have one of those.

haroldk wrote:What I'm looking for is information like what exists for the late tachs.

oops...I misread this bit

If you are still interested i have a write-up on how to do this. I did it to mine.

v8carreragts wrote:If you are still interested i have a write-up on how to do this. I did it to mine.

Yes, please.

It's in WORD format so I can't post it here. Send me a PM with your e-mail.

I would like to know as well, but I have too low of a post count to be able to send PMs. If you e mail me at admiralkhole @ gmail. com I would really appreciate it! Thank you.

Alright, I've got this working. The relevant resister to replace is by the adjustment pot on the left side of the tach (as you're looking at the face). There's a 16k ohm resister inboard of the pot in between another resister and a diode. Pull the face off Make sure you note the position of the needles before you remove them and use a twisting motion to pull them off. You don't want to snap the shaft. Unsolder the 16k resister and replace it with a 931 ohm to 951 ohm resister. Replace the face and the needles.

Here's the part that made me thing I broke it: you'll need to turn the pot a good deal counter-clockwise before you attempt to calibrate it. I actually drilled a hole in the case where the pot is so I could adjust it while it's assembled since the ground is a lot easier to attach to when it's assembled.

Thanks to Peter for the info on this.

I just bought an a 924S with an LS3. It has the early dash and I want to connect it. I understand that the LS1 ouputs a 4 Cyl signal. Does the LS3 do the same? Also, where can I pick up the tach signal on the LS3 wiring? Any info on modifying the tach if the signal is 8 cyl, if necessary, would also be appreciated.