Retro fit Tach

[ezcruiser]

I'm curious as to whether a 240z, I think 240, Tach will fit into the blank in the 620 dash and look half-way stock and good?

 

This is the Stock 620 Tach via spiffinspudater:

 

This is the 240?z Tach:

[rat tail dragger]

hey looks like it sould work i wish i had one i would try it i have a dash already out let me no if it does

[yello620]

I did some measureing for this a couple of years ago. I i remember correctly the face of the 240 tach is a little bigger and and the RPM's would not be corrrect for the four cylinder. Getting it installed looked very doable, possibly even just switch the faces after removing the needle, But the problem of it reading for a 6 cylinder went unresolved and i dropped the idea.

[ezcruiser]

I'm not sure how either work but I can imagine it being something like ignition pulses from the coil, hopefully otherwise what I'm about to do is useless...

 

RPM = (Hz * (60sec/min))/(1/2(X))

 

Hz = pulses per sec

X = number of cylinders

 

ex @ 60Hz

4 cylinder: 2 pulses per rev @ 60Hz(3600 pulses/min): 3600/2= 1800rpm

6 cylinder: 3 pulses per rev @ 60Hz( " ): 3600/3= 1200rpm

That equals a 33.33~% difference.

It's late, I'll think more on it tomorrow...

Edited November 21, 2008 by ezcruiser

[Llittle_Llama]

you could do a 6 cyl swap....just a thought, lol

[SUN520]

I'm not sure how either work but I can imagine it being something like ignition pulses from the coil, hopefully otherwise what I'm about to do is useless...

 

RPM = (Hz * (60sec/min))/(1/2(X))

 

Hz = pulses per sec

X = number of cylinders

 

ex @ 60Hz

4 cylinder: 2 pulses per rev @ 60Hz(3600 pulses/min): 3600/2= 1800rpm

6 cylinder: 3 pulses per rev @ 60Hz( " ): 3600/3= 1200rpm

That equals a 33.33~% difference.

It's late, I'll think more on it tomorrow...

 

I like your thinking ... will be interesting to see how close you are ..

 

you could do a 6 cyl swap....just a thought, lol

 

NOw that would be the easy option ....:D and my choice !

[72 327]

I'm not sure how either work but I can imagine it being something like ignition pulses from the coil, hopefully otherwise what I'm about to do is useless...

 

RPM = (Hz * (60sec/min))/(1/2(X))

 

Hz = pulses per sec

X = number of cylinders

 

ex @ 60Hz

4 cylinder: 2 pulses per rev @ 60Hz(3600 pulses/min): 3600/2= 1800rpm

6 cylinder: 3 pulses per rev @ 60Hz( " ): 3600/3= 1200rpm

That equals a 33.33~% difference.

It's late, I'll think more on it tomorrow...

 

True Ratsun

[ezcruiser]

I have a battery charger that has a 60Hz Wave somewhere, all's I need to so is get some tachs lined up. I've got some spare potentiometers resistors and such. It may be to dang cold for a junk yard run... I'll see how many layers of clothes I can pile on and still move if I fall down and go for it! ("A Christmas Story" style):lol:

[ezcruiser]

you could do a 6 cyl swap....just a thought, lol

 

Lol! Why not a v12 and run two v-6 tachs in parallel?

[datsunaholic]

I have a battery charger that has a 60Hz Wave somewhere, all's I need to so is get some tachs lined up. I've got some spare potentiometers resistors and such.

 

 

I hope you're joking... That's not how tachs work. Pots and resistors would only make the pulse less strong, basically the tach wouldn't pick up the signal.

 

Oh, and a 60Hz wave would only read 120 RPM on a 4-cyl tach, IF it could read a waveform input. Tachs generally read ground pulse input, or in some cases are inductive (but still read pulse).

[ppeters914]

Why bother? IMO, the stock 620 tach looks more classic.

[BarryA]

tweak the circuitry in most any electronic tach to make it work with 4,6 or 8 cylinder engines. Downside is that you'll most likely have almost as much in it as an original 620 tach would cost.

 

For a different alternative, you might try looking for a Porsche 924 in a wrecking yard that still has the dash panel intact - they used a VDO white markings on black face tach with a "cone" shape lense that looks very much like the stock 620 instrument lenses - even has the black "button" in the center. Most older VDO gauges came in standard size cans (small ones are 2 1/8", large are 3 1/8") - big ones are an almost perfect fit in the tach hole, and since 924's were 4 cyl, there should be no adaptation issues..

[ezcruiser]

I hope you're joking... That's not how tachs work. Pots and resistors would only make the pulse less strong, basically the tach wouldn't pick up the signal.

 

Oh, and a 60Hz wave would only read 120 RPM on a 4-cyl tach, IF it could read a waveform input. Tachs generally read ground pulse input, or in some cases are inductive (but still read pulse).

 

Thanks for the feedback datsunaholic. I realize that putting a resistor in series before the tach would weaken the signal. Seems like if it did get signal from the coil that it would be converted to DC so it can power a meter within the tach's circuitry. So essentially, all I have to do is apply a resistive shunt to the circuit.

Isn't a pulse a wave? Even though it may read ground pulse, it still reads pulses which are waves which have frequencies that are measurable in Hz, which is modifiable? But if it is inductive then it might be magnetic?

Did you get that 120rpm reading from plugging up a 620 tach to a 12v 60Hz source? What kind of source was it? If it wasn't a power source I'm curious to see your math. I want to know more!!

 

ppeters914 - The reason I'm doing this is to see if I can. I've seen the companies that do it for 50-60-70 bucks, I should be able to do it for next to nothing. The reason I don't have a stock tach is because I haven't been able to find a stock tach that is in my price range. Maybe if I was doing a resto.

 

If I can get it to work and it fits good, nissan v6 tachs are all over the junk yards here. And if it is accurate and reliable I'm going to spread the wealth. Also, I'm planning on not spending much more than 5-10 bucks, I hope that this is a reasonable project that will keep me working on my datsun.

I'm going to check into that Porsche 924 tach too! Will probably have more luck on ebay than in the junk yards here. Thanks BarryA

[Llittle_Llama]

yeah, i would be interested in a cheap tach for mine! thats a great reason to do it, help fellow ratsuners

[SUN520]

I will put my hands up for one ... but i may be able to use my old MK 1 GT Cortina Tacho that is sitting somewhere in the shed ...:D

[ezcruiser]

Thanks guys, I'm not bailing out. Maybe I will just hold off posting about it, get it completed and working good, then let the cat out of the bag.

 

SUN520- Off subject but, do you have a ?

[Llittle_Llama]

:(

[SUN520]

.

 

SUN520- Off subject but, do you have a ?

 

 

Had a couple of them actually .. my favorite car ever ... only model of car to win Bathurst 3 years in a row .. :D

I even had an Anglia once (the car behind the Cortina in the pic)

 

Shed a tear when i sold my 2 door a couple of months ago :(, i still have the rear end (trunk) of my old 4 door GT , it was set up as my BBQ , but since we moved house i havent got around to setting it back up ..

 

I think i will be using the webber i have from it on the L18 in the 520 .. :D

 

Cheers Phil

[Bleach]

Just see if you can find a tach from some other 4-cyl Datsun in the 70's.

[datsunaholic]

Thanks for the feedback datsunaholic. I realize that putting a resistor in series before the tach would weaken the signal. Seems like if it did get signal from the coil that it would be converted to DC so it can power a meter within the tach's circuitry. So essentially, all I have to do is apply a resistive shunt to the circuit.

 

All electronic meter movements (the part of a gauge that makes the needle move) are internally an ammeter. They measure current. The rest of the gauge electronics, be it a tach, or a voltmeter, or a pressure gauge, simply convert the input signal into a DC current for the meter to read. The meter movement itself does not take a lot of current, but it varies from gauge to gauge. Because of the design, putting a resistor inline with the meter movement might not have the desired effect- in essence, the tach MIGHT read correctly at say 1500 RPMs but be off 10% or so at 4000RPMs. Commercial aftermarket tachs have switches that run the signal though more transistors to change the input- I had one that had settings for 4, 5, 6, and 8 cyls.

 

 

 

 

Isn't a pulse a wave? Even though it may read ground pulse, it still reads pulses which are waves which have frequencies that are measurable in Hz, which is modifiable? But if it is inductive then it might be magnetic?

 

 

True. But if you put the distributor trigger on a o-scope you'd find it's mostly a sawtooth, with a pretty big "spike" when the points open (or when the EI unit sends a fire signal). But it's not fluctuating from Zero to 12V, it's fluctuating from "grounded" to "Open Circuit". The capacitor on a points system ensures that when the points are open it doesn't read zero (grounded). The "open" state could be much higher than 12V.

 

The inductive tachs take the trigger line from the distributor, run it through the tach, and then back to the coil (-) post. "Normal" tachs are hooked up in parallel, though the operation is the same in that the trigger line runs through a pickup coil that "counts" pulses. The electronics convert the pulses into a current that's run through the meter movement.

 

 

 

 

 

Did you get that 120rpm reading from plugging up a 620 tach to a 12v 60Hz source? What kind of source was it? If it wasn't a power source I'm curious to see your math. I want to know more!!

 

That was me posting when way too tired and watching TV at the same time. TV makes ones brain mush. I forgot to convert seconds to minutes.

 

It's 1800 RPM on a 4-cyl tach.

[Bleach]

how many rpms does your Gerbil run at?

 

[ezcruiser]

HAHA, I thought that was a twinky at first glance! Random... Damn random! I'm not backing out on this project yet, I know "most" of my options as far as other donors and such, but I would really like to see this come together. I don't really have anything better to do and hey, I may even learn something!

Edited November 24, 2008 by ezcruiser

[ppeters914]

 

ppeters914 - The reason I'm doing this is to see if I can. I've seen the companies that do it for 50-60-70 bucks, I should be able to do it for next to nothing. The reason I don't have a stock tach is because I haven't been able to find a stock tach that is in my price range.

 

Aaaahhhhh.....makes sense now. I thought you were replacing the stock one. Thanks.

[ezcruiser]

Hehe, I'm making progress, thanks to rubberman riding with me! Good to get some datsun motivation with all the crap going on!

The 280 tach is way too big for the 620 dash btw...

 

I currently have a 510 wagon tach that I picked up for 11 bucks, that should fit perfectly!:P:p:p

[RacnJsn95]

Too bad to hear the Z tack is too big... I have a couple of them... I was looking at the tachs in a 720, I've found quite a few of them... They're quite a bit smaller than the hole, but I guess you could rig something up?