V 8 Tach Mod.

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V 8 Tach Mod.

Postby 1100kaw » Wed Jan 02, 2013 6:12 pm

I did a little searching. Found some info here on board. Most of the links are dead to actual articles. One pic posted hard to see details in.
Does anyone have the Tach mod. info for early Vegas? details for the do it yourselfer kinda guy?
I am running LS, and all computer control engine stuff.
Can I modifiy Tach for V8, and then, do we know if the ECM will send a signal the old school tach can use? Or is that another issue? Maybe Programmer can set ECM to send a 4cly signal, no tach mod?
??????????????????????????????????????????????????????????????????????????????????????????????????????????????????????????????????????????????????????????????????
This seems to be getting expensive to try and keep original guages.
1st a $300 speedo adapter so compter can spin cable
now this tach?
havnt even started to do senders and stuff for the rest. Temp, oil, etc....
I'm almost ready to give up and do all aftermarket guages and build custom dash panel.
I just really wanted it to look stock inside....
AAARRGGG.
Thanks for your help
Darrell
72 Vega GT LS
69 Skylark Wagon BBC
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Re: V 8 Tach Mod.

Postby cjbiagi » Wed Jan 02, 2013 7:44 pm

Not sure on the tach but the other gauges shouldn't be as bad, just a matter of adapting the the stock senders to your engine. Maybe a few adapters here and there but otherwise I would think it would be pretty straight forward.
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Re: V 8 Tach Mod.

Postby Wizeguyrc » Wed Jan 02, 2013 8:01 pm

Here is a copy.
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Re: V 8 Tach Mod.

Postby spencerforhire » Wed Jan 02, 2013 8:56 pm

Wow; that's a whole lot clearer than the old photocopy of that tach mod that I've had for a lot of years. I just printed myself a new one. I wonder if instead of doubling the capacitor value to make a 4-cylinder tach read properly on an 8-cyl. we could cut the value in half to make it correct for a 2-cylinder signal(that's what the LS computer has). I wonder if Tim McCabe would be able to modify and calibrate the OEM tach to accept the LS computer's output? Might be worth sending him a PM.
A few years ago I was involved in an LS7 GM crate motor swap into a 1995 Camaro. The tach in the dash was looking for an 8-cylinder signal(from the previous LT1's "Optispark" ignition); I installed a ratio converter to quadruple the LS7's 2-cylinder signal so the tach would read right. Also had to do the opposite to the speedo signal because the pulses/mile was different as well. Tuning in the speedometer was lots of fun; this thing was putting 500+hp to the rear wheels. MANY "test runs" out onto the highway were necessary. :wink:
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Note- the very act of listing all of these has made me realize I have some kind of problem.....

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Re: V 8 Tach Mod.

Postby ROB » Wed Jan 02, 2013 9:02 pm

Anyone know if you could use just 1 capacitor, just with 100v or 25v instead of 50v? Also, does this trick work on Cosworth tacs?

Rob
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I'm not a purist, this is gonna have a V8.
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Re: V 8 Tach Mod.

Postby waybad » Wed Jan 02, 2013 9:31 pm

good to know... thanks
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Re: V 8 Tach Mod.

Postby spencerforhire » Wed Jan 02, 2013 10:28 pm

Rob- It's not the voltage rating that determines what a capacitor does but the capacitance which is measured in microfarads. The way I see it, adding the two 1mf. caps in series(makes it equivalent to one .5mf cap) with the existing one on the circuit board it halves the tach reading. I presume the existing cap is another .5mf one, therefor if it were replaced by a single 1mf. cap, the tach might read properly with a 2-cyl signal. Might require some experimentation; my electronic theory might be a tad rusty.
Last edited by spencerforhire on Wed Jan 02, 2013 10:30 pm, edited 2 times in total.
The "fleet"-
72 Vega HB Drag Car -383/'Glide/9"(9.35@146.19)(5.94@117.28 1/8th)
77 Vega Estate wagon- project(someday)will have TPI305/T-5, S-10 spindles/axles
76 Vega GT- 400/4spd/9" retired from active duty(rusty)
06 Silverado 2WD ex.cab daily
03 Silverado 2WD ex.cab (retired)
06 Haulin' 20ft enclosed car transporter
06 GMC Canyon Shop truck
07 Colorado project( 5.3 4L60e swap)
99 Saturn SL1- wife's car
01 Saturn SC2- son's project
07 Saturn Ion Redline project
and 4 more Saturn "parts cars"
Note- the very act of listing all of these has made me realize I have some kind of problem.....

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Re: V 8 Tach Mod.

Postby 1100kaw » Wed Jan 02, 2013 10:29 pm

Spenc
I bet dialing in that speedo was the best part. Thanks for the info from all so far.
I will try to contact Tim. I had no clue what the LS computer signal was. You say 2cyl? WOW. Surely the tach could be just as easily converted that direction as the other?
I will do some searching.
Thanks for the better pic WIse.
As far as the oil and water guages, the only pain on those will be creating extra ports to tap motor because all the originals have to stay in place to feed ECM info., and adapting everything from metric to standard. I think i've seen a water temp adapter for the LS, but the oil has to be picked up from filter housing. Gotta drill and tap a port there.
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Re: V 8 Tach Mod.

Postby TimMcCabe » Sat Feb 09, 2013 6:22 pm

Based on what I have been able to find, the LSx ECM actually outputs a 4 cyl tach signal (not a 2 cyl), but the voltage might be too low for an old school tach to trigger correctly.
You should be able to build a voltage pull up circuit using an opto coupler or pull up resister that would allow the stock 4 cyl tach to work.
I can modify the first gen Vega tach to read any number of cylinders, but it is alot more finicky then a '74 and newer (or Cosworth) tach when it comes to setting it up.

Cheers,
Tim
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Re: V 8 Tach Mod.

Postby 1100kaw » Sat Feb 09, 2013 8:27 pm

Tim
I Appreciate your reply.
I guess what i would like to know is, CAn you do it?
Is there a way to test your idea?
I am stuck on what to do.
I really want to run stock guages. Its quite expensive to do the speedo cable drive unit, and now we dont know what it will cost to do tach, if it can be done.
The other guages are easy.
If this tach cant be done for resonable cost, i need to cut bait and buy a complete set of aftermarket guages and start building a dash panel.
Thanks
Darrell
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Re: V 8 Tach Mod.

Postby 72 SS PNL » Sat Feb 09, 2013 8:43 pm

hey tim , do you have any idea on a GM eco tec tach set up , to work with the stock tach , or cossy tach ?
first and best car owned 72 GT(changed to SS) panel wagon

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Re: V 8 Tach Mod.

Postby TimMcCabe » Sat Feb 09, 2013 9:08 pm

HI Darrell,
You can send me your tach and I can test it to see how accurate it is to start with and I can add the pull up circuit right onto the tach.
Send me a PM and I'll give you all the details.

Cheers,
Tim
1972 TPI Vega T56 6 speed

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Re: V 8 Tach Mod.

Postby TimMcCabe » Sat Feb 09, 2013 9:11 pm

72 SS PNL

I'd recommend the Cosworth tach as it has a higher redline that is more in line with the ecotec. The factory tachs are notoriously way off in their calibration, so even though it is already set for 4 cyl, I'd suggest you have it calibrated to be sure it's accurate.

Cheers,
Tim
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