Dead tach....need tach filter?

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

Post by g3chevy on Sat Aug 09, 2014 2:21 pm

Here are some pics to help determine which posts are neg/pos/tach. Also there is a pic of a typical bad spot on these circuit boards. If you can find a good board level repair shop they can probably fix your circuit board for under $100. If you could decipher the markings on the two IC chips on this board you could rebuild the whole board without too much trouble if you have basic soldering skills. My board appears to have a "hot spot" next to one of the diodes which I may try to replace and test. My concern is when diodes go bad it usually takes out other components with it. Maybe I'll get lucky.


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

Post by Oldschool454 on Sun Aug 10, 2014 2:13 am

I have this older 73 style board, but it's the same tach and has the same post connections for +, -, and send unit as the pics he posted. I spent an hour today checking my grounds and even ran a separate ground from the firewall to the negative post and still nothing. Needle still stays at zero when ign turns on and car is running. I checked the circuit board but nothing looks fried. Not that it couldn't be anyways...
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Re: Dead tach....need tach filter?

Post by Oldschool454 on Sun Aug 10, 2014 2:17 am

Also...the bottom of the circuit board in your pic says "top" but if you're looking back of the tach right side up like in your pic it should be the bottom. probably just marked wrong or something? Not that it really matters anyway. Just noticed
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Re: Dead tach....need tach filter?

Post by 77mali on Sun Aug 10, 2014 7:14 am

g3chevy wrote:Here are some pics to help determine which posts are neg/pos/tach. Also there is a pic of a typical bad spot on these circuit boards. If you can find a good board level repair shop they can probably fix your circuit board for under $100. If you could decipher the markings on the two IC chips on this board you could rebuild the whole board without too much trouble if you have basic soldering skills. My board appears to have a "hot spot" next to one of the diodes which I may try to replace and test. My concern is when diodes go bad it usually takes out other components with it. Maybe I'll get lucky.



Thanks for that! +1

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

Post by Mcarlo77 on Sun Aug 10, 2014 12:39 pm

So...how exactly do you bench test to see if the needle jumps without frying the circuits?  Do you connect 12V to the + terminal, Ground to - terminal and then just touch another 12V to the Tach terminal?

Another question I'm curious about...when Gary the Tach Man reconditions/upgrades a tach, does he simply cut off that 4th attaching peg on the small "can" on the back of '73/'74 tachs and install a '75 style board?  Obviously, you can't drill a fourth hole in the circuit board to allow it to pass thru as there are circuits in the way. Or, is there something inside that "can" that is not compatible with HEI and it is changed out as well?
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Re: Dead tach....need tach filter?

Post by 77mali on Sun Aug 10, 2014 12:55 pm

According to his website Steve he installs a "Solid state circuit board" with 3 post coil.

"THE PICTURE ON THE LEFT IS GM'S FIRST ATTEMPT TO CREATE AN AIR COIL MOVEMENT FOR IT'S TACHOMETERS. THE WHITE CIRCUIT IN THE FIRST PIC HAD OUT DATED ACTIVE ELECTRONIC COMPONENTS THAT WERE COVERED IN EPOXY AND COULD NOT BE REPAIRED. NOTICE THE 4 PEG COIL FIRST USED IN THE FIRST PIC AS WELL. THE SECOND PICTURE SHOWS THE REMOVAL OF THE 4 PEG COIL AND REPLACED WITH THE MODERN 3 PEG COIL. ALSO NOTICE THE MODERN SOLID STATE TACH BOARD INSTALLED IN THE SECOND PIC. THE BOARD HAS AN ADJUSTABLE POT SO R.P.M.'S CAN BE ADJUSTED. G.M. NEVER OFFERED THIS ADJUSTMENT ON ANY OF IT'S BOARDS. THE THIRD PIC IS THE FINISHED PRODUCT, AN ACCURATE, WORKING TACHOMETER. 2 YEAR WARRANTY, ACCURACY AND RELIABILITY FROM THE TACHMAN."

As far as a bench test my thoughts previously were to test what is believed to be a dead tach, in order to see if it is receiving the proper voltage/power for the needle to at least react and move from "zero". I don't think you can do a proper operational test without some expensive equipment.
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Re: Dead tach....need tach filter?

Post by Mcarlo77 on Sun Aug 10, 2014 2:52 pm

I remember reading that about the 4-peg coil, but I was confused as to whether the replacement of the 4-peg coil was only necessary to accomodate the newer 3-peg style board since the circuitry didn't allow for the 4th hole.  So, was just wondering if he was doing the whole replacement thing as a precautionary measure since the coil is old and failure was just a matter of time.

I have an older 4-peg tach with excellent face...but, no circuit board on back.  Also have a '75 tach with ugly face and a circuit board.  Maybe I'll drill out the two rivets holding that coil A$$'y to the face and swap the '75 works onto the older tach.  Just can't justify nearly $300 (by the time you pay shipping back and forth to) to repair what would in effect be a spare for me.
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Re: Dead tach....need tach filter?

Post by g3chevy on Sun Aug 10, 2014 3:44 pm

Here are some more pics of tachs and circuit boards.

These are 3 different circuit boards all from HEI cars. The one in the middle is a circuit board I pulled from a 78 Z28 and bolted to the back of my 75 tach face. Worked perfectly with a few mods. The first was grinding out the gauge housing to accommodate the wider circuit board. The second was pulling out the original contacts from the housing that connect to the printed circuit on the back of the housing. Then I ran my own wires to a 12v source, ground and one to the HEI tach connection.


Here is a closeup.


Here is a closeup of all three circuit boards. The first one is from a 79 V-8 Monza I believe, the second is the Z-28 and the last one is from a 75 Monte.


This poses a new set of questions. Here is a fuel gauge and tach side by side. They appear to have the same windings inside the gauge and the same posts on the back. I wondering if you could have a fuel gauge face re-done as a tach and put a tach circuit board on it???
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Re: Dead tach....need tach filter?

Post by Mcarlo77 on Sun Aug 10, 2014 8:03 pm

Quote: "This poses a new set of questions. Here is a fuel gauge and tach side by side. They appear to have the same windings inside the gauge and the same posts on the back. I wondering if you could have a fuel gauge face re-done as a tach and put a tach circuit"

That is a good question...and, one I had to explore.  So, to remove some of the mystery as to what's inside,  I removed the casing on the backs of a fuel gauge and tach to expose whatever was in there.  As expected, there is a single coil.  Here are pics of a '75 Fuel Gauge (Unleaded Style) and '75 Tach coil side by side...with the Fuel Gauge coil on the left/Tach on the right.  I'm no expert on coils, but I think I remember that the amount of resistance is based on number of windings?  They are very similar...but, do look a little different in that regard with the Tach coil looking more bulbous; as though it has more windings.  Don't know if that would be relevant to the function of these gauges.  Maybe someone in the know can expound on this.  Also, it was duly noted there is a single wire coming off the coil to each of the three pegs.  







Back to my previous question...is the 4th peg on the early Tachs just another contact peg only necessary for that gen of circuit boards and could be eliminated when upgrading to the newer boards without changing out the coil?
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Re: Dead tach....need tach filter?

Post by Hawk03 on Sun Aug 10, 2014 8:41 pm

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

Post by g3chevy on Sun Aug 10, 2014 11:15 pm


The answer is yes, but you have to make modifications to your gauge cluster to make it work. You can reference my pics and information in an earlier post that show a 78 Z-28 circuit board on a 75 Monte tach. Hope this helps.
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Re: Dead tach....need tach filter?

Post by g3chevy on Sun Aug 10, 2014 11:21 pm

Mcarlo77 wrote:Quote: "This poses a new set of questions. Here is a fuel gauge and tach side by side. They appear to have the same windings inside the gauge and the same posts on the back. I wondering if you could have a fuel gauge face re-done as a tach and put a tach circuit"

That is a good question...and, one I had to explore.  So, to remove some of the mystery as to what's inside,  I removed the casing on the backs of a fuel gauge and tach to expose whatever was in there.  As expected, there is a single coil.  Here are pics of a '75 Fuel Gauge (Unleaded Style) and '75 Tach coil side by side...with the Fuel Gauge coil on the left/Tach on the right.  I'm no expert on coils, but I think I remember that the amount of resistance is based on number of windings?  They are very similar...but, do look a little different in that regard with the Tach coil looking more bulbous; as though it has more windings.  Don't know if that would be relevant to the function of these gauges.  Maybe someone in the know can expound on this.  Also, it was duly noted there is a single wire coming off the coil to each of the three pegs.  







Back to my previous question...is the 4th peg on the early Tachs just another contact peg  only necessary for that gen of circuit boards and could be eliminated when upgrading to the newer boards without changing out the coil?

Great pics and info! I would think the amount of windings would have an effect on resistance. I also noticed that some of the contact posts are copper and others are steel. This also would have some effect on resistance I would assume. I will test out the fuel gauge with a tach circuit theory next weekend and let you know what I find.
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Re: Dead tach....need tach filter?

Post by Oldschool454 on Mon Aug 11, 2014 8:36 am

Thank you So much for all the info! This is exactly the idea I was searching for. I'm much more willing to spend 100 bucks maybe on a tach from a 78 camaro and modify it to work. How is the needle response using that camaro circuit board? I know the originals sensitivity was a bit sluggish and didn't know if it might be more accurate since it's a little newer, though not much.
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Re: Dead tach....need tach filter?

Post by Hawk03 on Mon Aug 11, 2014 9:19 am

g3chevy wrote:



The more I compare the Corvette board to g3chevy's board the more they look interchangeable. It might be wishful thinking on my part and I wish my car was running so I could test it.



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

Post by Oldschool454 on Mon Aug 11, 2014 9:53 am

The more I compare the Corvette board to g3chevy's board the more they look interchangeable. It might be wishful thinking on my part and I wish my car was running so I could test it.

Just by looking at the sweep of the 2 different tach faces it looks like it would be different. If you look at the monte/laguna tach face, directly to the left of the pointer in the "9 o' clock" position is about 1250rpms. Whereas the corvette tach is about 500-700rpms. Unless maybe you use the monte/laguna coil and windings with justy the corvette circuit board? Though I don't think it would work in my case because I have the 4 post coil and the corvette uses the newer 3 post.
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Re: Dead tach....need tach filter?

Post by thatfnthing on Mon Aug 11, 2014 10:29 am

Oldschool454 wrote:The more I compare the Corvette board to g3chevy's board the more they look interchangeable. It might be wishful thinking on my part

Probably not wishful thinking. It probably is the same. From a factory standpoint, it makes sense for them to re-use the same parts on different models wherever they can, because it saves them money. And when you're talking millions of vehicles, it's a lot of money. You only redesign the stuff you have to. If it wasn't for that we wouldn't even have this board -- our cars are all very similar under the skin. There's every reason to expect other GMs of the era share some parts as well.
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Re: Dead tach....need tach filter?

Post by g3chevy on Mon Aug 11, 2014 10:22 pm

I'm fairly confident that this corvette circuit board will work as a replacement. To test this theory I just purchased a new circuit board on ebay. Here is the link to the one I purchased. http://www.ebay.com/itm/130877044046?ssPageName=STRK:MEWNX:IT&_trksid=p3984.m1497.l2649
Most of these boards have an adjustable potentiometer on the board so you can fine tune your tachometer. Below are some photos for comparison.

Side by side comparison of a 75 Laguna Tach and a 78 Corvette Tach


Side by side comparison of a 75 Laguna Tach Circuit Board and a 78 Corvette Tach Circuit Board
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Re: Dead tach....need tach filter?

Post by Hawk03 on Tue Aug 12, 2014 9:27 am

g3chevy wrote:I'm fairly confident that this corvette circuit board will work as a replacement. To test this theory I just purchased a new circuit board on ebay. Here is the link to the one I purchased. http://www.ebay.com/itm/130877044046?ssPageName=STRK:MEWNX:IT&_trksid=p3984.m1497.l2649
Most of these boards have an adjustable potentiometer on the board so you can fine tune your tachometer. Below are some photos for comparison.

Side by side comparison of a 75 Laguna Tach and a 78 Corvette Tach


Side by side comparison of a 75 Laguna Tach Circuit Board and a 78 Corvette Tach Circuit Board

Excellent! I'm excited to see the results.
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Re: Dead tach....need tach filter?

Post by Oldschool454 on Fri Aug 15, 2014 7:09 pm

g3chevy wrote:Here are some more pics of tachs and circuit boards.

These are 3 different circuit boards all from HEI cars. The one in the middle is a circuit board I pulled from a 78 Z28 and bolted to the back of my 75 tach face. Worked perfectly with a few mods. The first was grinding out the gauge housing to accommodate the wider circuit board. The second was pulling out the original contacts from the housing that connect to the printed circuit on the back of the housing. Then I ran my own wires to a 12v source, ground and one to the HEI tach connection.


Here is a closeup.


Here is a closeup of all three circuit boards. The first one is from a 79 V-8 Monza I believe, the second is the Z-28 and the last one is from a 75 Monte.


This poses a new set of questions. Here is a fuel gauge and tach side by side. They appear to have the same windings inside the gauge and the same posts on the back. I wondering if you could have a fuel gauge face re-done as a tach and put a tach circuit board on it???
If I wanted to use a board from a 78 camaro like you, and I have a "4 post" tack, instead of 3 like yours, would I be able to just remove that bottom 4th post? Or would that cause problems?
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Re: Dead tach....need tach filter?

Post by Mcarlo77 on Fri Aug 15, 2014 7:33 pm

Quote: "If I wanted to use a board from a 78 camaro like you, and I have a "4 post" tack, instead of 3 like yours, would I be able to just remove that bottom 4th post? Or would that cause problems?"[/quote]

That's the question I've been trying to find out!  Can't find any posts on other sites regarding this.  I don't think anybody but those who repair tachs knows.  Anyone?

BTW...I managed to break off the indicator needle spindle on the coil of my '75 spare, so now I'm doubly screwed trying to fix that one without finding a replacement coil in addition to possibly a circuit board. Wonder if a coil off a Corvette tach is the same? More questions...
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Re: Dead tach....need tach filter?

Post by Oldschool454 on Sat Aug 16, 2014 7:54 am

TOO MANY UNANSWERED QUESTIONS. Gaaaaah!!!! Never thought this would be so involved. lol
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Re: Dead tach....need tach filter?

Post by Hawk03 on Sat Aug 16, 2014 9:28 am

I think removing the 4th post would work. As long as the board is good & receiving a signal then the coil in the tach should be able to read & register it. Does the inside of the 73/74 tachs look the same as 75 & up?
Does the refurb places do something to the tachs or do they just remove the 4th post & install a new board?
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Re: Dead tach....need tach filter?

Post by Oldschool454 on Sat Aug 16, 2014 11:16 am

"Does the inside of the 73/74 tachs look the same as 75 & up? " ...Do you mean the windings inside? (coil or whatever)
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Re: Dead tach....need tach filter?

Post by g3chevy on Sat Aug 16, 2014 1:27 pm

Hawk03 wrote:I think removing the 4th post would work. As long as the board is good & receiving a signal then the coil in the tach should be able to read  & register it. Does the inside of the 73/74 tachs look the same as 75 & up?
Does the refurb places do something to the tachs or do they just remove the 4th post & install a new board?

Here is an option for keeping the 4th post on an older tach. Below are pictures of a Willcox tach circuit board I just purchased. It has the optional 4th hole built into the board. I will test this board today on my bad 75 tach and a fuel gauge to see how it works. I'll post the results hopefully later this weekend.


New tach circuit board front


New tach circuit board back
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Re: Dead tach....need tach filter?

Post by Oldschool454 on Sat Aug 16, 2014 6:56 pm

That would be great!!!! Keepin my fingers crossed.
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