Dead tach....need tach filter?

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Re: Dead tach....need tach filter?

Post by g3chevy on Sun Aug 17, 2014 9:43 pm

Well, here are the results of my testing. The new circuit board that is made for 78-82 corvettes does not work and any of the working tachs that I tested it on even with adjusting the potentiometer thats built into for fine tuning. I also tested a working tach circuit board on a fuel gauge....this does not work either. That lead me to testing the resistance of the windings on each gauge. The tach measured about 600 ohms and the fuel gauge measured 200 ohms. So my thoughts are that a corvette tach has a different amount of windings within the gauge that requires different specs for the circuit board. My next project is purchasing a used 78 corvette tach and using the guts on my 75 Laguna tach with a new circuit board and go that route.
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Re: Dead tach....need tach filter?

Post by Hawk03 on Sun Aug 17, 2014 10:40 pm

That's a let down, thanks for doing the leg work.

This seems like a good fix for the 78 Camero tach but I am not sure how to transalte it into the chevelle tachs. I looked at the '73 one I have and it is competely different so I don't have anything to compare except the pictures.

http://www.howtoalmanac.com/kevin/projects/automotive/tachfix.htm
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Re: Dead tach....need tach filter?

Post by Mcarlo77 on Mon Aug 18, 2014 12:15 am

Not the news I was hoping to hear, either. Frustrating to know the Corvette guys have the much cheaper option of buying replacement circuit boards and doing their own repairs...and, our only option is to spend nearly $300 to have someone do it for us!

Your efforts are greatly appreciated, though. Little by little, it helps remove some of the mystery surrounding these tachs.
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Re: Dead tach....need tach filter?

Post by Oldschool454 on Mon Aug 18, 2014 1:34 pm

Mcarlo77 wrote:Not the news I was hoping to hear, either.  Frustrating to know the Corvette guys have the much cheaper option of buying replacement circuit boards and doing their own repairs...and, our only option is to spend nearly $300 to have someone do it for us!

Your efforts are greatly appreciated, though.  Little by little, it helps remove some of the mystery surrounding these tachs.  
What he said goes for me as well. I appreciate you putting the info on here from your results and all your research. We will figure out an affordable solution in time...
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Tach Repair Successful

Post by g3chevy on Mon Jan 26, 2015 11:14 pm

Well I was able to play around with a couple of my dead tachs over the weekend and ended up repairing one of them by replacing the IC chip that controls most of the functioning of this circuit board. The chips are less than a $1 each on k4supply.com and are relatively easy to replace if you have a solder removal tool that can be purchased at any radio shack store. Below are a couple of pics for reference. BTW, this is for 75-77 tach boards only. And there are 2 different versions of chips for these boards as the other one I have is chip number 25005061. If I recall the 73-74 tach circuit boards are quite different from the 75-77 since they ran a points type distributor without HEI. I would highly recommend purchasing a new tach filter when doing this repair on the 75-77 boards since that was a major culprit in the failure of these tachs to begin with. You can purchase them on ebay for about $30. Here's a link to one I found.
http://www.ebay.com/itm/75-thru-89-Corvette-Tachometer-Tach-Filter-NEW-Replaces-GM-14020025-/331353521382?pt=Motors_Car_Truck_Parts_Accessories&fits=Year%3A1978|Model%3ACorvette&hash=item4d26357ce6&vxp=mtr



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Re: Dead tach....need tach filter?

Post by Keith Seymore on Tue Jan 27, 2015 9:18 am

I fixed mine back in the mid 1990's by using the guts from a IROC Camaro tach.

I noticed the numbering and sweep were the same between the two:  7000 rpm across about 270 degrees.

I removed my tach face and screwed it to the IROC unit, wired it up, turned the key to "on" (so that the pin would zero itself) and pushed the hand on.  I recall I did have to clear some of the blue cluster substrate away so that the new board would package.

Been running it that way ever since.

K

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Re: Dead tach....need tach filter?

Post by Keith Seymore on Tue Jan 27, 2015 9:23 am

I procured two tach assemblies at that time;  I think I still have the second one around somewhere on a shelf (I can picture it sitting there in my mind's eye, assuming I didn't throw it away recently).  

I could post some pics if anyone is interested.

K
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Re: Dead tach....need tach filter?

Post by Hawk03 on Tue Jan 27, 2015 1:13 pm

Thanks for the update. Next is to find a solution for the '73 tach.
I was reading this thread where a guy modified his '69 Camaro tach with new guts. The only problem for us is that there doesn't seem to be many aftermarket 7K tachs with a 270 degree sweep.

http://www.camaros.net/forums/showthread.php?t=238337
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Re: Dead tach....need tach filter?

Post by g3chevy on Tue Jan 27, 2015 9:49 pm

Another fix would be using a 78-79 corvette tach circuit board and guts to put behind a tach face for our vehicles. The range is the same on the corvette tach and it uses the same size circuit board so you wouldn't have to modify the gauge housing. I've already tried using one of these new corvette circuit boards on one of my bad tachs and it did not work. I believe the guts have different windings than the corvette tach. I'm in the process of attaching my tach face on the corvette tach guts which should produce a fully functional gauge.


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Dead tach....need tach filter?

Post by Mcarlo77 on Wed Jan 28, 2015 12:01 am

I think you've maybe come up with the most viable solution. It looks like the three contact studs for Ground/12V/Coil are located in the same position as on a '75 board so that it could be plugged into the existing cluster housing without wire modification.

This might be the best solution for repairing my '73 style tach, too.  Buy a used, non-working '78/'79 Corvette tach for cheap in order to get the 3-peg coil and then spring for a new Corvette circuit board.  All told, probably cost a third of Tach Man's price of $300 (with shipping factored in)...and, you've got a readily replaceable board for a fraction of the cost if needed down the road.
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Re: Dead tach....need tach filter?

Post by Hawk03 on Wed Jan 28, 2015 9:17 am

g3chevy wrote:Another fix would be using a 78-79 corvette tach circuit board and guts to put behind a tach face for our vehicles. The range is the same on the corvette tach and it uses the same size circuit board so you wouldn't have to modify the gauge housing. I've already tried using one of these new corvette circuit boards on one of my bad tachs and it did not work. I believe the guts have different windings than the corvette tach. I'm in the process of attaching my tach face on the corvette tach guts which should produce a fully functional gauge.

Mcarlo77 wrote:I think you've maybe come up with the most viable solution.  It looks like the three contact studs for Ground/12V/Coil are located in the same position as on a '75 board so that it could be plugged into the existing cluster housing without wire modification.

This might be the best solution for repairing my '73 style tach, too.  Buy a used, non-working '78/'79 Corvette tach for cheap in order to get the 3-peg coil and then spring for a new Corvette circuit board.  All told, probably cost a third of Tach Man's price of $300 (with shipping factored in)...and, you've got a readily replaceable board for a fraction of the cost if needed down the road.

I think that is the best solution for the '73 tachs, good thinking! On eBay there are a few Corvette tachs going for around $60, plus the new tach board for $55 so for about $115 you can upgrade your tach and not have to worry about it frying again.
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Re: Dead tach....need tach filter?

Post by Hawk03 on Sun May 10, 2015 3:11 pm

What is the best way to separate the coil from the tach, drill it out? How about attaching the new coil to the tach?
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Re: Dead tach....need tach filter?

Post by Mcarlo77 on Sun May 10, 2015 7:32 pm

Go at those two black-headed pins that hold the coil enclosure to the tach face from behind. IIRC correctly, those pins were flared on the backside and I just removed enough of the flash to allow removal of the coil enclosure. My thinking was I'd reuse the pins (for cosmetic effect) and apply some JBWeld on the end of the pins (where I removed the flash) and around the perimeter of the enclosure to further secure it. Never got that far, however...
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Re: Dead tach....need tach filter?

Post by Keith Seymore on Mon May 11, 2015 7:29 am

Mcarlo77 wrote:Go at those two black-headed pins that hold the coil enclosure to the tach face from behind.  IIRC correctly, those pins were flared on the backside and I just removed enough of the flash to allow removal of the coil enclosure.  My thinking was I'd reuse the pins (for cosmetic effect) and apply some JBWeld on the end of the pins (where I removed the flash) and around the perimeter of the enclosure to further secure it.  Never got that far, however...

I used very tiny threaded fasteners (panhead screws and nuts) to re-attach the tach face.

K
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Re: Dead tach....need tach filter?

Post by Hawk03 on Mon May 11, 2015 1:43 pm

Thanks guys. I have an '82 Corvette tach on the way to switch the guts out of.
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Re: Dead tach....need tach filter?

Post by g3chevy on Mon May 11, 2015 6:42 pm

I dont think an 82 tach will be accurate since the number sweep is positioned differently from the 78-79 tach. I'm in the process of making one with the guts from a 78 tach. I'll post the results as soon as its done.
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Re: Dead tach....need tach filter?

Post by Hawk03 on Tue May 12, 2015 8:36 am

They're a little off but I'm not too worried about it being dead on, as long as it works and is somewhat close then I am fine with it.

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Re: Dead tach....need tach filter?

Post by g3chevy on Tue May 12, 2015 8:44 pm

Looking good! You maybe able to dial it in by using the adjustable potentiometer on the back of the circuit board. Keep us posted.
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Re: Dead tach....need tach filter?

Post by Hawk03 on Wed May 13, 2015 9:09 pm

I switched out the guts of the tachs, it all fit well. I won't know if it works until I'm near the car which won't be for a while. I drilled out the rivets and used small screws, washers and nuts to connect it to the face.


Back Side


The hardware used which I purchased at Menards.


The insides of the coils. Right is original and left is from the Corvette.
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Re: Dead tach....need tach filter?

Post by thatfnthing on Wed May 13, 2015 10:13 pm

Nice work!
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Re: Dead tach....need tach filter?

Post by g3chevy on Wed May 13, 2015 11:41 pm

Looks like you have a winner! I guess there's enough difference from the guts of the corvette tach that it works only with the corvette circuit board. I tried bolting on the corvette circuit board to the back of my original tach but it did not work. This setup you put together should work like new. Let us know the results when you can.
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Re: Dead tach....need tach filter?

Post by Mcarlo77 on Thu May 14, 2015 1:01 am

Can't wait to hear if it works...but, don't see why it shouldn't. You motivated me to buy a cheap used Corvette gauge just for the coil winding. I'll buy a new board and hopefully will have a newly refurbed tach as well.
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Re: Dead tach....need tach filter?

Post by 77mali on Thu May 14, 2015 8:30 am

+1 for the guts to change the guts & updating!

Hope it all works out.
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Re: Dead tach....need tach filter?

Post by Hawk03 on Thu May 14, 2015 8:57 am

Thanks guys, its super easy. I was worried I'd break something but it went as good as it could. I took the needle off with a regular fork which gave me some leverage and had openings to side the needle through. I drilled the rivets from back side and they popped out the front. I had to cut around the white face I put on and used a washer to cover the cuts of the face. The tach with a new board cost me about $90 on ebay.

It should work and if the board gets fried it will only cost another $50-60 for a new board instead the $300 for the tachman to fix.
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Re: Dead tach....need tach filter?

Post by Hawk03 on Mon Jun 22, 2015 9:00 am

I tried the tach and it didn't go as I had planned (never does it seems). I either need a new tach board, which was supposed to be a new one to begin with, hook up a filter or try a new wire to connect the tach to. When I turn the key the needle goes to zero and stays there.

Found this on another forum, since I now have a Corvette tach (except face) these are some helpful things to look for when if there are problems.

"TACHOMETER NOT WORKING ON YOUR 1975 TO 1982 CORVETTE ?.HERE ARE SOME HELPFULL INSTRUCTIONS ON HOW TO CHECK AND FIX YOUR TACH PROBLEMS

Be aware that there is an aftermarket tachometer circuit board made Overseas for 1975-82 Corvette.These product do not work, They short out when installed, so now it comes with washer to cure the problem, they are not calibrated, so they don't read correctly not even close,and no instructions or 1 year warranty. Bottom line is they don't work. They don't tell you that their circuit boards are not calibrated in their ads, but people think they are, do it yourself and save. The poor person who purchases these boards think he must have other problems with the tach, not knowing he could not possibly make these boards work correctly, if at all.


..
1. The tachometer circuit board is the most likely problem why the tach is not working, or working correctly, but not always. On 1975-76 Corvette tach the printed circuit board usually show a burned spot, actually melts the thin copper wire and the problem is obvious. 1977 to 1982 circuit boards don't , and you can't tell by looking at it.
2. Checks to do to see if the tachometer is working for 1975 to 1982 Corvettes, or if 12 volts or ground are missing, or the signal from the distributor is missing. The tach needle usually stays about 1000 to 1500 RPMs ,when the vehicle is off, this is normal .Turn the key to the on position engine not running, the needle should go to zero. If the needle goes to zero this indicates the tach is working and 12 volts and ground are present. If the needle does not move,either the circuit board is bad or 12 volts or ground are missing to the tach. Remove the tach from the dash and apply 12 volts and ground, tach needle should go to zero if working. On 1978 to 1982 Tachs, the Speedo and Tach housing's main wire connection causes problems and should be checked. Now , start the engine, if the tach does not move off zero, the signal from the distributor is missing, the likely problem is the tach filter, follow the wire from the distributor cap, you can't miss it, about the size of your thumb. Use a wire and bypass the tach filter all together.(Remove , unclip both ends completely) If the tach works now ,that your problem.The Tach filter also, if not working correctly makes the tach work erratically, jumps around, etc. bypass it if you think it's bad. On a lot of 1978 to 82 Corvette the problem is the tach is reading to high, 1000 to 2000 RPM's or more to high, Circuit board is bad, you need a new board. Also on 1978-82, sometimes the needle pegs past 7000 RPMs and stays there. It stay there because it can't get back to zero because it's bottoming out .You need to move it back to zero by hand, take the lens off and move it back. If the needle pegs to the bottom again either on the zero side or 7000 side, then the board is bad.
3.All these instructions assume we are dealing with a factory tach and wiring etc, that has not been messed with or replaced, etc. NOTE, if the needle is removed for any reason, you can NOT just put it back on at any place and expect it to read right.You must rezero the tach needle. To rezero the needle 12 volts and ground must be applied to the tach, then the needle must be set on so it reads zero. Also REZEROING is NOT CALIBRATION . Also, switching circuit boards, 1978-79 only, 1980-82 only. 1980 to 82 tachs have a hole for the choke light, 1978-79 don't have this hole.1978-79 tachs Calibrate differently from 1980 to 82. Another problem in warmer climates, the needle has a dampening effect, there is oil inside the meter head, if this dry's up, the needle swings freely and causes it not to settle down , it fluctuate a lot .A new circuit board will not fix this problem.

4. . *** Note, the original G.M., Delco circuit board sold over the counter were not calibrated, so buying a new old stock (N.O.S. )board will not make your tach work.
"
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