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#646206 - 02/06/09 09:11 PM fix ur stock tachometer  
Joined: Sep 2007
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jigsaw Offline
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jigsaw  Offline
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Laredo TX
i tested a good unmolested stock tach and at 3,000 rpm it was off by 250-300 revolutions. at the same speed & doing the mod below tach was now off by 80-100 revolutions. at idle you cannot tell much of a difference, but u will start to notice it as the rpm increases. not 100% accurate but pretty close.


this how-to is to get the stock V8 tach to read back to factory specs. if ur needle is stuck in the down/up position or idles at a speed that u know is off then this mod should get it to read correctly. this should work on any other tachometer as long as the black & white resistor pack is found

tools needed for repair:
*100K ohm resistors (part #: 271-1131 from radioshack)
*soldering pen & solder
*solder vaccum or solder wick (braid)
*dyke pliers
*multimeter

what happens is that resistor #4 changes its value over time affecting the accuracy of the tach. its up to you to decide whether u decide to fix ur current tach or go aftermarket. the resistor is embedded in a resistor pack (ic chip) making it difficult to locate, but here is the schematic

[Linked Image]

resistor pack is the black and white chip and this is the pin arrangement from the top

[Linked Image]
actual board: (quartz is below the BOT lettering on the board)

[Linked Image]

as u can see there is a quartz which is parallel to resistor #4 that means u can check resistance on it without bothering with the chip. resistance should read about 200K ohms (.200 mega ohms) if you get another reading, that means the tach is not reading correctly and u must decide to fix it or replace it.

should u decide to fix it, first thing u need to do is find pin 10 and cut it with the dykes

[Linked Image]

now go back to the quartz and check the resistance on it. it should read infinite meaning there is no connection (open circuit) once u've done that. its time to solder two 100K ohm resistors in series to make 200K ohm (reason behind this the closest resistor is 220K ohms)

twist them together and apply solder, then cut the leads off
[Linked Image]

once u have that, this is where i chose to solder the resistor. one pin is going to go in the same contact (hole) as the quartz and the other pin is going to go on the adjacent contact which is covered by solder. u have to remove the solder using the wick or vaccum.

[Linked Image]

u can solder on the backside on the same or any other place as long as the 2 combined resistors are in parallel to the quartz. once ur done, take ur meter and measure the resistance on the quartz again it now should read very close to 200K


this is my finished product and it worked like a champ yahoo

[Linked Image]

some sources i found when i did some research

http://www.thirdgen.org/techboard/electronics/279765-tachometer-waaaaayyy-off.html
http://www.thirdgen.org/techboard/electronics/357893-90-92-tachometer-fix-4.html
http://www.fieros.de/en/articles/tach.html

Last edited by PB86SS/87LS; 11/08/17 08:45 PM.
#653030 - 02/25/09 01:10 PM Re: fix ur stock tachometer [Re: jigsaw]  
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Posts: 5
Doober Offline
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Doober  Offline
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Swartz Creek, MI
Great info, thanks for documenting this laugh

I tested mine 'on the bench' with a 12v power supply, the sound card in my computer, a function generator program, and an amplifier. I also found 220k-ohms gets the tach extremely close, if not dead-on (that is if the square wave function doesn't cause the tach to act other than what it should).
http://maliburacing.com/forum/viewtopic.php?f=27&t=58097&start=13


-Darren-
'78 Malibu (6.0/T56 project), '81 Malibu (305/3-gear Saginaw, possible LS project) - MalibuRacing/MalibuThunder.com (good Malibu/G-Body sites)
#672970 - 04/16/09 09:52 PM Re: fix ur stock tachometer [Re: Doober]  
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PLANT PROTECTION Offline
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LaPorte, IN
208.3K has me dead nuts on @ 3000RPM but roughly 50RPM off @ idle. Gauges are generally most accurate in the middle of their resolution so I plan on leaving it as is. I tried 220K and it still read too high in the real world Doober, I do, however, appreciate your scientific approach. For the record, my stock resistor was reading roughly 260K. Props jigsaw. thumbs

#776786 - 03/22/10 07:53 AM Re: fix ur stock tachometer [Re: PLANT PROTECTION]  
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camarokid Offline
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Lebanon IL
Why didnt you just put one 200 ohm resistor in line instead of two 100 ohm resistors? just curious

#794898 - 05/18/10 12:28 AM Re: fix ur stock tachometer [Re: camarokid]  
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DGM Offline
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DGM  Offline
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WinterSprings,Fl
i decided to give this a try last night - tach was reading 2000rpms at idle before and now around 750 about perfect and sweeps normal now - thanks.


Yes- Finally got my 87SS!!
#795599 - 05/20/10 09:25 AM Re: fix ur stock tachometer [Re: DGM]  
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Rebel87SS Offline
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Rebel87SS  Offline
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NC
Will this fix a tach that pegs out as soon as you turn the key on? The needle sweeps clockwise around the tach and rests pointing straight down. When I shut the car off, it goes to zero.


Gwen - SMCC member
87 Aerocoupe burgundy/burgundy, stock and stripeless
85 El Camino black & silver/burgundy, work in progress
95 Impala SS black/grey - new project
#795845 - 05/21/10 02:32 AM Re: fix ur stock tachometer [Re: Rebel87SS]  
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DGM Offline
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DGM  Offline
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WinterSprings,Fl
mine was doing some funky things too and works great now


Yes- Finally got my 87SS!!
#863914 - 03/30/11 08:46 PM Re: fix ur stock tachometer [Re: DGM]  
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MeanBlue87 Offline
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Corpus Christi, Tx
Will this work even after the car's computer has been taken out?

#865311 - 04/06/11 01:18 AM Re: fix ur stock tachometer [Re: MeanBlue87]  
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Big3 Offline
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Big3  Offline
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Lancaster, Ohio
Originally Posted By: MeanBlue87
Will this work even after the car's computer has been taken out?



yes


87 MCSS Black w/Saddle
383 w/dart heads,Functional Goodmark hood.
700r-4, Gforce member,4.10s
1LE's w/Hotchkis uppers,2" drop & ZQ8's
#871639 - 05/07/11 01:32 AM Re: fix ur stock tachometer [Re: Big3]  
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Regal80.5 Offline
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Regal80.5  Offline
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Posts: 110
Crosby,Tx,USA
Hell freaking yeah!!!! The original poster deserves some sort of MCSS knghthood. I replaced my original tach with a used ebay tach twice and all three where messed up. One showed too high rpms and the other two pegged out and didn't do anything at all. Now all three work like they should. I recommened this post. burgside


Justin N.
Crosby,Tx-----Huffman,Tx
currently parked----->Burgundy/Black Primer 1988 Monte Carlo SS w/ Goodmark 2" Steel Cowl hood,Celebrity Euro Conversion Headlights
Driver------>1998 GMC Sierra 1500 X-cab Emerald Green
Newcomer----->1987 Black El Camino with 350/3spd w/overdrive work in progress
#871929 - 05/08/11 10:32 PM Re: fix ur stock tachometer [Re: Regal80.5]  
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Rich86SS Offline
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Rich86SS  Offline
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Lima, Ohio
Regal80.5...........let me know if you want to sell one.

#874242 - 05/20/11 01:32 AM Re: fix ur stock tachometer [Re: Rich86SS]  
Joined: Sep 2007
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1 Slow SS Offline
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1 Slow SS  Offline
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Small town USA
I have an extra tachometer if you need one. BTW that is a very good write up which will help several of us out. Thanks!


Be a car builder, not a googler!
#881991 - 06/29/11 09:55 AM Re: fix ur stock tachometer [Re: 1 Slow SS]  
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Aamer Offline
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Aamer  Offline
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San Francisco
My tach was showing 3000 rpms in idle and jumped to 6000 when i tapped the gas, i bought a 220k ohm resister from radio shack and now my tach works perfectly, just be careful when taking your gauges out, i broke two of my needles by barley touching it..im sure there all very brittle

#910267 - 12/29/11 01:01 AM Re: fix ur stock tachometer [Re: Aamer]  
Joined: Dec 2011
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ZMCSS88 Offline
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ZMCSS88  Offline
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Jacksonville, FL
Aamer, mine was doing the exact same thing. This was a easy fix. I just finished up with it. I cleaned it up a bit,popped it in and BAM! Works like new


1988 Monte Carlo SS
#912921 - 01/14/12 12:39 AM Re: fix ur stock tachometer [Re: ZMCSS88]  
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87TPI/SS Offline
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87TPI/SS  Offline
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Little Rock, AR
YES just fixed mine also what a great thing here I am so glad I joined this forumn....

#913024 - 01/14/12 03:31 PM Re: fix ur stock tachometer [Re: camarokid]  
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jigsaw Offline
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jigsaw  Offline
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Laredo TX
Originally Posted By: camarokid
Why didnt you just put one 200 ohm resistor in line instead of two 100 ohm resistors? just curious

because they dont sell 200K; at least in radioshack they dont. the closest one is 220K but it was too high for me. thats why u need 2 100K and wire them in series to come up with 200K keep in mind that they do have tolerances so u will want one with -/+ 5% tolerance (gold band) whenever choosing one

#913187 - 01/15/12 05:44 AM Re: fix ur stock tachometer [Re: jigsaw]  
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Turbo1dr Offline
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NC
This is a awesome post. I have several gauge clusters that may need this mod done. Thanks for the post!


You can buy adjustable potentiometer's that are very small. That way, you can fine tune your tach with a turn of a small screw.



1987 Monte Carlo SS Aerocoupe - Owned from 1990-1995
PROJECT: Aerocoupe - Started April 24th 2010...not yet finished. Planning to begin working on it mid Summer of 2014.
#913417 - 01/17/12 02:33 AM Re: fix ur stock tachometer [Re: Turbo1dr]  
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87TPI/SS Offline
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Little Rock, AR
well I have a problem this fix worked some what it read a lot smoother at low rpm's but after 3k its all jumpy again and all over the place. any Ideas? BTW when I had the tach out the green board around that "chip" was very dark and discolored.

#927888 - 04/14/12 02:08 AM Re: fix ur stock tachometer [Re: 87TPI/SS]  
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rons87mcss Offline
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PA
87TPI/SS, are you still using the tach filter on the firewall? Just curious since that is known to cause erratic tach readings when they fail. I've never had one fail, so just throwing it out there. I've read that you can just eliminate it.

Has everyone actually confirmed their tach somehow after installing XXXK resistor? I'm guessing my 86 SS is reading higher than it should at upper rpms(the rpm range that concerns me most).
I may give this a try when I swap the speedo. TIA

#940695 - 07/31/12 02:42 AM Re: fix ur stock tachometer [Re: rons87mcss]  
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k9mcss Offline
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Midwest, IA
I see there have been comments on breaking the needles- I haven't dug into mine yet, but the needle shouldn't need to be removed should it? TIA!


86 MCSS. Will have 406 SBC, built 700R4. Hopefully done soon!!!! wink
#967217 - 04/01/13 09:03 PM Re: fix ur stock tachometer [Re: jigsaw]  
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bbbrussell20 Offline
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Oxford AL
Okay when you give it gas like passing why dies it go past the last number on the tach?

#967548 - 04/04/13 12:32 AM Re: fix ur stock tachometer [Re: bbbrussell20]  
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rons87mcss Offline
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Just in case I ever end up doing this, I went on ebay and bought "RESISTOR CARBON FILM 1/2 WATT 200K OHM 5% 100 PCS." for a total of $3.20/shipped.
If you don't need them instantly, this is probably the way to get them and dirt cheap. I should have about 98 or 99 laying around now that I'll probably never use smile

#968638 - 04/14/13 01:33 AM Re: fix ur stock tachometer [Re: rons87mcss]  
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rons87mcss Offline
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Crappy/cold/rainy weekend so I decided to fix mine as well. Instead of just cutting one, I removed both 'pns' from #4 and #10 and flipped board over and soldered one 200K (read @ 196K w/meter) on the circuit board opposite of where #4 and #10 used to be.

[Linked Image]


Works great now. I had a feeling that I wasn't really spinning my stock bottom end motor to 6500-7000 before wink

Last edited by PB86SS/87LS; 11/08/17 08:48 PM.
#971772 - 05/15/13 10:45 PM Re: fix ur stock tachometer [Re: k9mcss]  
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rons87mcss Offline
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Originally Posted By: k9mcss
I see there have been comments on breaking the needles- I haven't dug into mine yet, but the needle shouldn't need to be removed should it? TIA!


No. The needle does not have to be removed at all. The circuit board seperates from the gauge face without removing the needle.

#980787 - 08/19/13 10:58 PM Re: fix ur stock tachometer [Re: jigsaw]  
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gza06955 Offline
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gza06955  Offline
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Springvale, Maine
Hate to dig an old thread up, but if anyone has a repaired tach for an 87 ss, I will gladly pay, and give up my core tach if needed. I have nothing but bad luck with soldering, and would like to retain the look of the original dash tach.

Please PM me


Thanks!
Travis


84 SS metallic blue 350/350th
87 SS T-tops 383 stroker t5 5 speed (sold)
85 SS hardtop needs motor (sold)
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