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Discussion Starter #1
Hi all,

wanted to ask a few questions before spending time on this and I know tach questions are old :p

Does anyone know what typically fries on the tachometer circuit board? I pulled gauge out of the dash and wired it directly to the battery and the dist *with and without the filter* and alas all it does is turn all the way to the six o clock position and nothing else. So I took a cheap meter just in case didn't want to blow my fluke up and hooked it up to the dist output on the dist. put it on 500v and was getting a 11/12 changing back and forth reading but not in a true pattern. so then i put the filter on and got nothing on the other side. both with it bolted to a chasis ground and not. so here are a few questions.

Does the Tach filter need to be bolted to a chasis ground?

How do you test the tach filter?

Does anyone know what goes bad typically on the tach board?

Anyone have a factory tach for sale or know where I can find one?

Thanks a million
I D
 

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i have had the filter on my 86 fail and the tach wouldnt work ,it just did stupid s***.there is a dark spot on my tach circuit board but it has been that way for years with no effect on the tach operation. try replaceing the filter before you condemn the tach. i have never run it without it attached to the firewall so i dont know for sure if it needs to be grounded.mine died about a month after the module crapped out.not sure if related or not but odd timeing. i have a spare 6 cyl 86/88 tach without a needle . you could use it to test but if your tach is moveing it is probably ok.the speedo shop says they can calibrate the v6 ones to read correct on a v8
 

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gbody tachs are simalar to f body tachs(same guts) there is a great article on thirdgen (camaro/firebird site)about repairing/recalibrating them, basicly what fails is the big white resister chip(its got two or three resisters built into the same chip), i run a third gen f body tach in my fourth gen elky(whole guage setup really)i repaired mine and works great(im not even running a filter)
 

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Discussion Starter #4
gbody tachs are simalar to f body tachs(same guts) there is a great article on thirdgen (camaro/firebird site)about repairing/recalibrating them, basicly what fails is the big white resister chip(its got two or three resisters built into the same chip), i run a third gen f body tach in my fourth gen elky(whole guage setup really)i repaired mine and works great(im not even running a filter)
looking at that i too have found that the resistor value for all the others is ok BUT pin 4 and 10 looks burnt at pin 10. it's so small of a burn i used a small magnifying glass to see it plus when metered i have no value nor continuity. so I think I'm on to something.

http://www.howtoalmanac.com/kevin/projects/automotive/tachfix.htm
 

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Discussion Starter #6
looks like i need something in the 180k range i'm thinking a 120k resister and follow that with a 100k pot. or oh 2 100k pots in series and that will give me more range to tweak. i have a silly scope so just need a freq generator.... quickly batman to the 555's in the closet!
 

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Discussion Starter #8
LOL randy.

Anyway progress on the tach was put to an abrupt end when I used an old copper clad board. Should have seen it coming that the solder no longer would flow to it easily. anyway. Got to go back and redo the signal generator but according to the silly scope this might just work. "IF" it does work I promise before I install it I will bench all the parts post schematics and make a video or post pics. which ever seems to be the best bet.
 

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Discussion Starter #9
blah that wasn't the only reason the work had to end. my other chevy had some serious issues that needed instant attention. troubleshooting found 3 broken wires coming off the gas pedal it's a 2003 silverado 1500 with electric gas pedal. thats off topic i know. will be working on that since it's my primary vehicle.
 

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in the absence of a signal generator you can use an aftermarket tach/ or timing light with a tach on it to calibrate the pots,(thats what i did) (ie: hook up the tach and the timing light and adjust the pots "Live" till the two match.) i know people running two tachs with no problem but the timing light got mine close enough(im not racing, so tach is just window dressing/nice to have) GL :beer:
 
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