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

by 1973 454 MONTE Fri Aug 01, 2014 2:52 pm

ive never used a filter on my tacs. the one in my monte now works great.

by Oldschool454 Fri Aug 01, 2014 3:07 pm

I believe the problem arises if you're using an OE tach (not aftermarket) from a pre 1975 vehicle, on a car that's made past 1975. I've heard it has something to do with them switching to HEI in 74 or 75 and that the tachs before then read different pulses that aren't as strong as HEI. So it throws it off or is too strong for the tach. Being that my tach and dash are from a 73 el camino and I'm putting it in a 77 malibu I could see how that might cause a problem if it was made for a different style (non HEI) system...maybe...lol

by 1973 454 MONTE Fri Aug 01, 2014 3:28 pm

i have a HEI dizzy in my monte. thats the first thing i would change,because i knew nothing about points.

by 1973 454 MONTE Fri Aug 01, 2014 3:33 pm

40 seconds in is my tach running.
https://www.youtube.com/watch?v=NGyZCSSzJDU

by thatfnthing Fri Aug 01, 2014 3:37 pm

Oldschool454 wrote:I've heard it has something to do with them switching to HEI in 74 or 75 and that the tachs before then read different pulses that aren't as strong as HEI. So it throws it off or is too strong for the tach.

The points system used a resistance wire to lower the voltage (as a full 12V would cause them to burn out too quickly), whereas the HEI uses the full 12V (and indeed should not be run using the old resistance wire), so it makes sense that the old tach would be expecting lower voltage. The full 12V then would probably play hell with it -- don't know if it would just give a bad reading or burn out the tach altogether. Probably both.

by Oldschool454 Fri Aug 01, 2014 3:55 pm

Well I can tell you for sure mine worked fine for a day and seemed pretty accurate reading. no needle bouncing or anything. Then just died. Just have stock HEI. Stock coil. No msd box or anything...been researching and talking to people nonstop tryin to figure this out. Driving me nuts not knowing!!! Aprreciate all the input though. I'm learning. lol

by Oldschool454 Fri Aug 01, 2014 5:59 pm

Has anyone here used the tach man to ugrade their tach? just curious what exactly he changes. I would guess just the circuit board that bolts to the back of the tach. I'm wondering if I could use the guts from a car with a similar tach maybe...

by Mcarlo77 Fri Aug 01, 2014 10:04 pm

I had left a couple messages for him to return my calls last March and never heard back from him. Third try I get a woman...who I presume was his wife. She's the one who quoted me the price of $250 to replace the circuit board.

Regarding filters...I can tell you I removed and kept the filter from a '75 Laguna parts car that I'll be putting on mine when the time comes. Can only presume that GM had a good reason to install them.

by g3chevy / Mr Pontiac Sat Aug 02, 2014 12:00 am

There were significant differences between the 73-74 tachs compared to the 75-77 tachs with the circuit boards and face. The newer 75 and up tachs had more modern ic chips to handle the hei ignition as well as a different type face and numbers that were oriented outside of the increment marks. I've attached a couple of pics for reference. Hope this helps.
Dead tach....need tach filter? - Page 2 75_tac10

Dead tach....need tach filter? - Page 2 75_tac10

75 Tach Face
Dead tach....need tach filter? - Page 2 75_tac11

75 Tach Circuit Board
Dead tach....need tach filter? - Page 2 73-74_10

73,74 Tach Circuit Board

by Oldschool454 Sat Aug 02, 2014 7:57 am

Thank you guys, this has been helping me learn a lot! So it appears I have 2 tachs that are both 73/74. And I'm assuming then my brand new one that I put in that stopped working is now fried or something then because I used it with HEI when it had the older style circuit board...fack...Well I was gonna try to make a trip to junk yard today to get tach filter and try it. But I have feeling it won't do any good now.

by dynchel Sat Aug 02, 2014 9:22 am

This may sound bad but since your car has a stock car vib, why not just get a big auto meter monster tach and call it a day?

by Mcarlo77 Sat Aug 02, 2014 9:24 am

Interesting info...first time anyone has actually explained the differences other than the style of the face. So...I've got a '73 style tach with a mint face plate but, no circuit board and a '75 tach with a corroded face but circuit board. Is it correct to assume a person could swap the circuit board from the '75 (HEI) to the '73 tach and it would work with either the HEI or points?

by 77mali Sat Aug 02, 2014 9:54 am

The Tachman uses a newer "3 prong board" from what I understand, although, unless you've bought one- you can't find a pic of it online & he covers it up in the picture on his website. I'm assuming he's trying to keep that info exclusive as no one else seems to be doing this conversion. I am planning to use him once the time comes if mine gives me grief. Worth the $$ IMHO. He's a small shop so I think he's prob overwhelmed with business sometimes. (No excuse for bad customer service though!)

On another note- if you look at Chris' video, when he gets in to turn the key the Tach needle reads what I presume to be curb idle speed. The needle goes down to zero when the key is turned and back up when the car is on again. If anyone is looking for these used online- that's needle position off the zero is a good sign. If the needle reads zero off the car it's probably got issues.

by Oldschool454 Sat Aug 02, 2014 10:31 am

dynchel wrote:This may sound bad but since your car has a stock car vib, why not just get a big auto meter monster tach and call it a day?

This isn't actually on my car. It's on my dad's car. Which is also a 76 Laguna. He has all of his factory comforts still. No roll cage, original dash, heat, a/c, all that stuff...So he wants to keep the original dash look.

by Oldschool454 Sat Aug 02, 2014 10:37 am

77mali wrote:The Tachman uses a newer "3 prong board" from what I understand, although, unless you've bought one- you can't find a pic of it online & he covers it up in the picture on his website. I'm assuming he's trying to keep that info exclusive as no one else seems to be doing this conversion. I am planning to use him once the time comes if mine gives me grief. Worth the $$ IMHO. He's a small shop so I think he's prob overwhelmed with business sometimes. (No excuse for bad customer service though!)

On another note- if you look at Chris' video, when he gets in to turn the key the Tach needle reads what I presume to be curb idle speed. The needle goes down to zero when the key is turned and back up when the car is on again. If anyone is looking for these used online- that's needle position off the zero is a good sign. If the needle reads zero off the car it's probably got issues.

I'd be curious to find out either where he gets his, or if it's off of another car even. May have to dig some more online for info. If I hadn't just spent 450.00 on the dash I may have thought about having it done by him. And in Chris' video, how his tach was acting is exactly how mine was acting when it was working. I'm gonna dig some more and I'll post anything if I find out more.

by Mcarlo77 Sat Aug 02, 2014 10:40 am

I interpret that 3-prong conversion as nothing more than putting a '75 style board in place of the 4-prong tach boards on the '73/'74 versions...which, again begs the question of whether the internals of the tach are the same?

I would tend to agree with you, Paul, on the assessment of the position of the needle with no power to the tach. The NOS one I found last winter is sitting at around 2000 rpm.

by Mcarlo77 Sat Aug 02, 2014 10:44 am

Regarding the question of where Gary the Tach Man gets his boards, I'm sure he has a source who makes the printed circuits for him. I was in the business of supplying circuit board fabricators with supplies and you could find a smaller shop to do small runs. Of course, it's more expensive than if they were mass produced...so, I do understand the cost Gary requires to do this work.

by g3chevy / Mr Pontiac Sat Aug 02, 2014 10:46 am

I attempted to change out circuit boards in the past from a 75 tach to a 73 tach but then realized because the 73,74 tachs have the 4th post on the back and there is no 4th hole on the 75 tach circuit board then it was obvious I couldn't do the swap. There may be some board level repair shops still around that you can ship your circuit board to and have it repaired. I've used companies like this in the past to repair other electronics with good results. I also have contacted the Tach Man and found his customer service demeanor less than helpful. I'm sure he does a good job, but if you did have an issue after the sale it may not work in your favor is the vibe I get. Good luck.

by dynchel Sat Aug 02, 2014 11:07 am

www.redlinegaugeworks.com

This is who the magazines always pimp for this kind of work.

by 77mali Sat Aug 02, 2014 2:01 pm

dynchel wrote:www.redlinegaugeworks.com

This is who the magazines always pimp for this kind of work.

Good info! Although- I saw Jay Leno's mug on the bottom of the page ...
That almost always means a second mortgage lol!

by Oldschool454 Sat Aug 02, 2014 3:07 pm

g3chevy wrote:I attempted to change out circuit boards in the past from a 75 tach to a 73 tach but then realized because the 73,74 tachs have the 4th post on the back and there is no 4th hole on the 75 tach circuit board then it was obvious I couldn't do the swap. There may be some board level repair shops still around that you can ship your circuit board to and have it repaired. I've used companies like this in the past to repair other electronics with good results. I also have contacted the Tach Man and found his customer service demeanor less than helpful. I'm sure he does a good job, but if you did have an issue after the sale it may not work in your favor is the vibe I get. Good luck.

Do the other prongs line up though? They look like they would but I don't have one for comparison. I need to do the opposite. Go from 73 tach circuit board to a 75 or 76. I wonder if you could do that swap and if it would work ok?

by g3chevy / Mr Pontiac Sat Aug 02, 2014 3:12 pm

I haven't tried the reverse swap but the prongs do line up exactly the same on both versions.

by Hawk03 Sun Aug 03, 2014 10:14 pm

Everything I've read is that adding a filter will allow the pre-HEI distributers to work with the points systme tachs. Will any filter work and is the solution this easy? I've found this one for a Corvette so I would think it would work.

http://www.ecklerscorvette.com/corvette-tachometer-filter-original-barrel-style-1975-1989.html

by Mcarlo77 Sun Aug 03, 2014 10:30 pm

Looks pretty close to the one I removed from a '75 Laguna.

Dead tach....need tach filter? - Page 2 Tachga10

by Oldschool454 Mon Aug 04, 2014 7:50 am

Hawk03 wrote:Everything I've read is that adding a filter will allow the pre-HEI distributers to work with the points systme tachs. Will any filter work and is the solution this easy? I've found this one for a Corvette so I would think it would work.

http://www.ecklerscorvette.com/corvette-tachometer-filter-original-barrel-style-1975-1989.html

I thought it was the opposite. The filter allows the tach to read the hei waves. That's why they use then on a lot of gm cars past 74?

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