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#646206 - 02/06/09 09:11 PM fix ur stock tachometer  
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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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Doober Offline
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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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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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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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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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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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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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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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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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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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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]  
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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!


Enjoy life, family first!
#881991 - 06/29/11 09:55 AM Re: fix ur stock tachometer [Re: 1 Slow SS]  
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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]  
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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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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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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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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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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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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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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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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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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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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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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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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)
#988613 - 11/22/13 08:03 PM Re: fix ur stock tachometer [Re: jigsaw]  
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hey guys. I think I could handle this, but could someone do a youtube vid and add it to the thread. I think that would be great as I am a visual learner.

#988664 - 11/23/13 12:02 PM Re: fix ur stock tachometer [Re: jigsaw]  
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Good point YOUTUBE can be used as a teaching tool. I have refered to it often for information on all sorts of repairs. Just be carefull which ones you take advise from. Typically the redneck way to do this and that are not reccomended.

#988676 - 11/23/13 06:13 PM Re: fix ur stock tachometer [Re: jigsaw]  
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Good point! I would agree with that statement.

#988726 - 11/24/13 05:33 PM Re: fix ur stock tachometer [Re: Ole Blue]  
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The only thing that would prevent a person from making a video at this point (IMO anyways) is the fact that most/all who have replied in this thread have already fixed their tachs and I doubt they would want to tear their dash apart once again just to make a video. Also, the work really isn't that hard and just going by the info provided here in this thread can be done by most people with basic do-it-yourself skills. I have done 3 of them now(both of my SS's and a fellow member's car), but doubt I will ever have to do one again since the chances of working on 1980's cars is becoming near extinct in 2013. My only advice would be to not let the fact that you have never done this before keep you from doing it. I doubt anyone who has posted in this thread has ever done this before.
At this point in the year, most of us have 4-6 months to get things done because of winter so just take your time, keep looking through the pics/posts/info provided here, and give it a try.

#993271 - 01/25/14 05:00 PM Re: fix ur stock tachometer [Re: jigsaw]  
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Could u replace the whole chip resistor

#993445 - 01/27/14 08:00 AM Re: fix ur stock tachometer [Re: jeremybrown]  
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jigsaw Offline
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Originally Posted By: jeremybrown
Could u replace the whole chip resistor

If u find it. this mod will work for any stock tach that has this chip not just for g bodies

#1029046 - 09/07/15 11:43 PM Re: fix ur stock tachometer [Re: jigsaw]  
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I did mine today soldered them in this afternoon worked perfect I don't know how to act.

Thanks,

Chuck

#1029136 - 09/08/15 11:52 PM Re: fix ur stock tachometer [Re: jigsaw]  
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mmc427ss Online content
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My tach reads three hundred rpm light at 3K. Don't know if the resistor fix will work. Guess it time to get the tach out and meter the resistor.
Bob

#1032344 - 11/23/15 01:46 AM Re: fix ur stock tachometer [Re: jigsaw]  
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Does Anybody have repaired tachs for sale ?

#1032427 - 11/25/15 02:22 AM Re: fix ur stock tachometer [Re: jigsaw]  
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Mike's Monte should have a good used one for sale, and also would have the 8K refaced one available.
http://www.mikesmontes.com/1986-1988-tachometer-Used_p_359.html

http://www.mikesmontes.com/8-Grand-Tachometer_p_734.html
Bob

#1042200 - 08/08/16 01:37 PM Re: fix ur stock tachometer [Re: jigsaw]  
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Bringing this thread back, I just repaired two tachometers that I had acquired over the last couple years that were bad. Tested in a car earlier to know they were bad and checked resistance recently, both were in the 120-140 range. I followed Ron's repair method, cutting the connections on one side and putting a 200k resistor on the other side. My soldering skills aren't the greatest but it didn't take too long to get right for a novice like me, tested them both out and viola. Resistance was 197-198 and the seem to read accurately and compared to my known good one. One more to possibly repair, seems half the tach's I come across lately are bad.


-86'SS 383 CCC QJet- BRF 2004r-8.5" 3.42
-87'LS 350 MAF/SD TPI- CRF 2004r-7.5" 3.42
-81'Grand LeMans Safari Wagon 350 CCC Q-Jet/CZF 2004-r/8.5" 3.73
-07'TBSS Stockish daily driver
[Linked Image]
#1042679 - 08/22/16 08:41 PM Re: fix ur stock tachometer [Re: jigsaw]  
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I just did this and havent had a chance to try it yet because im replacing all my cluster bulbs with LED's and carefully cleaning dust off the gauge faces and a couple other things but...

Should the needle on the tach just float somewhere other than on the 0 rpm peg after doing the resistor fix? I havent put power to the gauge yet but it used to float with ignition off at like 3000rpm. Unless it hasnt fully settles its floating at about 1000rpm now. Should it drop and read properly once i actually put power and the rpm signal to it?



85 SS - C&C T Tops, Black Cherry Paint, Summit Fast 5's & in need of drop spindles/springs!
L69 HO - Holley 650 4bbl, 053 intake, HEI, duals & no cats + lots of chrome!
200-4R - 1400 Stall (Eww!!), Shift Kit + Red Eagle plates/Kevlar bands & B&M Shifter
#1042742 - 08/24/16 01:34 PM Re: fix ur stock tachometer [Re: jigsaw]  
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The tach needles can move freely without any power to them if moving the tach around or touching the needle by hand, some move much more freely than others too I've found. The only real way to find out if it's working properly is to hook it up, bench testing measuring resistance should be an indicator as well.


-86'SS 383 CCC QJet- BRF 2004r-8.5" 3.42
-87'LS 350 MAF/SD TPI- CRF 2004r-7.5" 3.42
-81'Grand LeMans Safari Wagon 350 CCC Q-Jet/CZF 2004-r/8.5" 3.73
-07'TBSS Stockish daily driver
[Linked Image]
#1049823 - 04/11/17 01:16 PM Re: fix ur stock tachometer [Re: jigsaw]  
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Finally did this fix and am very happy! Being that Radio Shacks are becoming more and more rare and the closest one to me is an hour away, I found my resistor(s) on eBay. They are +/- 1% as opposed to the 5% from Radio Shack, are 200k a piece, and mine actually came in a shade over 200k ohms at 201k. Works beautifully. Thanks to jigsaw for this write-up!


'87 Monte Carlo SS, 400 SBC, 800 Q-Jet, Edelbrock RPM Q-Jet intake, Howards BM Rattler cam, Flowtech headers, 200-4R w/shift kit, 4.10 posi, 2.5" exhaust, DeltaFlow 40s
'73 Mercury Montego, 460 BBF, AirGap Intake, 800 Q-Jet, Voodoo cam, C6, 9" 3.25 posi, 3" exh, Flowmaster 40s
[Linked Image]
#1051121 - 05/26/17 09:20 PM Re: fix ur stock tachometer [Re: jigsaw]  
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I just did this fix and I have to say Thank You! to the OP. Without your research this wouldn't have been possible.
I got the 2 resistors off of amazon. I tried radio shack but they were selling shelves, display cases and cash registers =[

This was the easiest fix I've done for the car. I am awful at soldering but this is very large through hole type soldering and it's cake.

First you do the worlds easiest solder of the 2 resisters together.

Then you just press the iron on the 2 spots he indicates on the board and let the solder melt and you push your resister ends through. Then put a dab of solder on each one.

If you can get the tach out of the dash you have done the hard part of the job.


Justin
1986 Black SS
Upgrades: 350 L05 - Hooker Longtubes - FlowMaster True Dual 2.5" - Electric Cutouts - Electric Fans - Transcooler - Proforged Steering Kit - Astro Shaft - AR62 OUTLAW II Wheels - Energy Suspension Bushings - Bitflipper Chip - 87 ECM - Moog Springs and Ball Joints - Bilstein Shocks - Bucket Seats
#1053147 - 08/31/17 06:25 PM Re: fix ur stock tachometer [Re: jigsaw]  
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Great thread and writeup, I just wish I had done mine before the Photobucket Ransom

Now there's no pictures or guidance?

Would anyone happen to have saved the originals?

I'm trying to do the repair on a board out of a '83 Grand Prix, and I'm struggling to find my orientation

What's Pin 1? I don't want to cut wrong on a chip I can't get anymore

[Linked Image]

[Linked Image]


'86 Monte Carlo SS, 122K
'83 Grand Prix LJ, 56K
#1053156 - 09/01/17 11:36 AM Re: fix ur stock tachometer [Re: jigsaw]  
Joined: May 2016
Posts: 539
Witness86SS Offline
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Witness86SS  Offline
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Joined: May 2016
Posts: 539
Hatfield, PA
That looks very different than what I remember from the Monte. The chip looks the same so cutting the pin should be easy. Figuring out where to solder in the resistor might be more difficult to translate even with the pics


Justin
1986 Black SS
Upgrades: 350 L05 - Hooker Longtubes - FlowMaster True Dual 2.5" - Electric Cutouts - Electric Fans - Transcooler - Proforged Steering Kit - Astro Shaft - AR62 OUTLAW II Wheels - Energy Suspension Bushings - Bitflipper Chip - 87 ECM - Moog Springs and Ball Joints - Bilstein Shocks - Bucket Seats
#1053243 - 09/06/17 04:36 PM Re: fix ur stock tachometer [Re: jigsaw]  
Joined: Dec 1999
Posts: 12,359
PB86SS/87LS Online beer
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20+ Year
PB86SS/87LS  Online Beer
Administrator
20+ Year

Member

Joined: Dec 1999
Posts: 12,359
New Berlin, WI
I printed out the thread awhile back before Photobucket turned into what it is. I'm also planning to repair a few tachs in the near future, I'll try to replicate those pictures and post them in this thread at some point.


-86'SS 383 CCC QJet- BRF 2004r-8.5" 3.42
-87'LS 350 MAF/SD TPI- CRF 2004r-7.5" 3.42
-81'Grand LeMans Safari Wagon 350 CCC Q-Jet/CZF 2004-r/8.5" 3.73
-07'TBSS Stockish daily driver
[Linked Image]
#1054307 - 11/08/17 08:50 PM Re: fix ur stock tachometer [Re: jigsaw]  
Joined: Dec 1999
Posts: 12,359
PB86SS/87LS Online beer
Administrator
20+ Year
PB86SS/87LS  Online Beer
Administrator
20+ Year

Member

Joined: Dec 1999
Posts: 12,359
New Berlin, WI
I just updated this thread by using an imbed fix online for Mozilla, had to download a more recent one, and stole the photobucket pictures (ha, take that photobucket!).


-86'SS 383 CCC QJet- BRF 2004r-8.5" 3.42
-87'LS 350 MAF/SD TPI- CRF 2004r-7.5" 3.42
-81'Grand LeMans Safari Wagon 350 CCC Q-Jet/CZF 2004-r/8.5" 3.73
-07'TBSS Stockish daily driver
[Linked Image]
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