tach differences Options

[mightyohm]

Has anyone ever opened up a 914/4 tach and compared it to a 914/6 tach?
What is different inside? Are they totally different units or are just one or two components different?

Are there schematics anywhere for the 914/4 tachs?

I ask because I installed an MSD this weekend and I find that I am going to need to buy the PITA tach adapter to get my tach to work. It's not like I don't have enough HUGE RED BOXES on my firewall now. Sheesh.

I would like to investigate the possibility of modifying the factory tach instead of using the adapter. The 914-6 tach is made to work with a CD ignition, so if I knew what they changed between the two tachs it might help.

[Rusty]

Different unit inside.

The 914/6 unit requires an external ballast resistor (pn 901 602 601 00) to pick up the signal.

I know that PA Speedo can do the conversions, if you need them to.

-Rusty (IMG:style_emoticons/default/smoke.gif)

[mightyohm]

I have talked to them. Let's just say it's not an option (for me).

Thanks for the info Rusty.

PS. Do the MSD tach adapters generally work ok?

[Rusty]

I totally understand. I want to send all my gauges there and have some work done... but the price of admission over there is pretty high. Someday, I guess. <_<

On the plus side, their reputation is impeccable.

[maf914]

Rusty or Jeff,

Come on guys, spill the beans. What does North Hollywood charge to convert the tach for an MSD box?

Could you locate the MSD tach adapter under the dash, or does it have to be placed near the MSD box?

Mike

[ArtechnikA]

i have both MSD tach adapters; make me an offer, help me recover some shelf space.
basically, they're just loading coils that increase the current draw to a level the stock tach can see. they worked fine when i used them in the GTI, but i sh!tcanned the MSD and don't drive the GTI...

[mightyohm]

Does anyone have a working 914-4 tach they want to donate to science? Appearance is not an issue but no guarantees it will survive my testing.

I would like to take one apart and figure out how it ticks. I am an EE so I should at least have a chance of finding a way to modify the existing tach circuit to work.

[mightyohm]

Rusty or Jeff,

Come on guys, spill the beans.  What does North Hollywood charge to convert the tach for an MSD box?

Could you locate the MSD tach adapter under the dash, or does it have to be placed near the MSD box?

Mike

$180 at Palo Alto Speedo.

I am pretty sure the adapter could be mounted under the dash too.

[mightyohm]

basically, they're just loading coils that increase the current draw to a level the stock tach can see.

I have a pretty good idea of what the tach adapter does. It's a big inductor sitting on the points/tach wire. This introduces high voltage peaks onto the square wave produced by the points/CD box combo. I am pretty sure the tach wants to see voltage spikes of several tens of volts in either the positive or negative direction (not sure) to trigger properly. That's why they don't work when connected without the adapter. The +12V square wave doesn't have the right voltage peaking to trigger the tach.

[Rusty]

Come on guys, spill the beans. What does North Hollywood charge to convert the tach for an MSD box?

I dunno. My plan doesn't include that service. The estimate for the work I wanted done was approaching a grand. It'll happen... just not today.

-Rusty (IMG:style_emoticons/default/smoke.gif)

[mightyohm]

See the number I gave above. The reason it is so expensive (other than the fact that those guys get a premium for anything they touch) is that they replace the whole electronic guts of the tach with a new unit. I am hoping that I can just replace one or two components on the existing circuit but I don't know if it's possible until I open one up and see what's inside.

[maf914]

$180! (IMG:style_emoticons/default/ohmy.gif)

I guess when you throw in a couple of hours labor its understandable. At least that's how I rationalized the bill after letting the Nissan dealer repair a running problem in my pickup I had been unable to cure over the past year! (IMG:style_emoticons/default/laugh.gif)

Mike

[shoguneagle]

About fifteen years ago I had my 914 tach rebuilt and calibrated by PA Speedo; cost $100 back then. I think they changed the elements that you will need to look at when determining how the 914-4 works. I think I have a spare 914 tach in the garage I can give you. I will check if you need i; just let me know where to send it.


Steve Hurt

[cgnj]

Hi,

My car is a 76. I used a MSD 6 for over a year with no tach adapter. In fact, my tach was inop with the adapter. Looking for one cheap?

Carlos

76 2270 "Black Jack"
Black cause is
Jack cause it spent more time on the stands than the wheels

[Dave_Darling]

Does anyone have a working 914-4 tach they want to donate to science? Appearance is not an issue but no guarantees it will survive my testing.

You should'a asked at breakfast Saturday... Drop me a note or some such. It's even local. And partway apart already.

--DD

[mightyohm]

My car is a 76. I used a MSD 6 for over a year with no tach adapter. In fact, my tach was inop with the adapter. Looking for one cheap?

Where did you wire the tach to? And the adapter? Did you put it from the white wire to +12V?

I would like to try the tach adapter. I can't find them locally. Who wants to send me one? I would like to get it ASAP so I can try it.

Dave, I'm sorry I didn't make the breakfast, I had other priorities that morning (sleep was one of them). You know since you live 5 mins away we could probably meet up sometime that is not on a defined breakfast morning. (IMG:style_emoticons/default/wink.gif)


Jeff