Dead tach....need tach filter?

by Oldschool454 Thu Jul 31, 2014 9:35 pm

I just completed my dash conversion from idiot light style dash to ss dash with readable gauges and tach. Yesterday the tach worked fine as soon as I ran the signal wire to the HEI distributor. Today I went out, started it, and it worked for a minute then just died. Now when I turn it on it just drops to zero. Like it's receiving power but no signal. Would a tach filter possibly bring it back to life? Anyone ever heard of this happening?

by Oldschool454 Thu Jul 31, 2014 9:36 pm

I took the dash back out and rechecked alllll the wires, made sure it had a good connection, made sure nothing was fried. Can't find anything suspicious looking...

by 77mali Thu Jul 31, 2014 9:44 pm

Sounds like you may need "The Tach Man". Give Gary a call he does conversions to a more modern unit on our Tach's if you're interested. Maybe he can give some professional advice as well. He's a really nice guy. See the bottom of this page.

http://www.tachman.com/gm.htm

by Oldschool454 Thu Jul 31, 2014 10:15 pm

Thanks. I just emailed him and game my # too. Hope he can be of help. I can't afford to do much right now so hopefully I can just get this one working with a simple fix. It was working fine the other day and I didn't touch anything. Keeping my fingers crossed. It's supposed to be a surprise for my dad's bday so it'll be a little disappointing if I can't get tach to work.

by Mcarlo77 Thu Jul 31, 2014 10:18 pm

I was quoted $250 this past Spring to refurb a tach by Gary the Tach Man. Had done some reading up on how to bench test these and it's virtually impossible without having the right electronic equipment. Also, from reading threads on the subject on other car forums, seems there is a difference of opinion as to whether a tach filter is really necessary. If you do talk to Gary, maybe ask him and report back to us.

by Oldschool454 Thu Jul 31, 2014 10:20 pm

I did ask about if a missing filter might cause this kind of problem. I will let you know for sure.

by 77mali Thu Jul 31, 2014 10:20 pm

No problem. I can't say for sure but if the tach doesn't need to be "restored", I seem to remember he does that conversion for about $100(?).

by Oldschool454 Thu Jul 31, 2014 10:21 pm

Yeah the tach is in brand new shape. Circuit board and all. And like I said was working great yesterday.

by Oldschool454 Fri Aug 01, 2014 7:38 am

This was his response....."patrick:
the tach has died, almost certain, i can convert the tach to modern electronics for 275.00 with a 3 hear warranty". Didn't say anything about the tach filter question... :/ Wish I had that much to spend on a tach

by 77mali Fri Aug 01, 2014 8:13 am

Wow I guess I was way wrong on the price. That's too bad about the tach.

by Mcarlo77 Fri Aug 01, 2014 9:20 am

Here's a another question I was never able to get a definitive answer to: Were the electronics on tachs built for pre-HEI cars ('73/'74) compatible with the HEI systems used on '75-'77?

by Oldschool454 Fri Aug 01, 2014 9:43 am

I sent a nice, not too long, but good sized email to him too asking what might cause my symptoms and asked about the mystery of the tach filter possibly causing it to not work. And I had heard a few people say how great and helpful he is, so I guess I'm just a little disappointed. Basically what I saw in his response was," you're tach's dead. give me money and I'll fix it." With no response whatsoever to any of my questions. :/

by Oldschool454 Fri Aug 01, 2014 11:33 am

Just found this on a different site. I think I'm gonna try to get tach filter 1st before jumping up and spending 300 bucks on new tach circuit board........"Most ORIGINAL early aftermarket and most ALL factory tachs will NOT work with an HEI distributor (newer versions on these old tachs do work in most instances). The wave signals sent out by the 2 units are different and most analog tachs cannot read the HEI waves. You typically need to install a tach filter in series between the tach wire and the tach port on the HEI distributor. They show up on ebay pretty regularly. We do not make a product to do that, no. MSD and Accel do, but I understand they are a bit pricey. I have no personal experience, just what guys have related to me. The GM cars of the mid 70's on up thru the Monte Carlo SS and later Camaros all used a filter when equipped with a factory tach, so they are good cars to look in at yards. Those are the ones to try and get as they plug onto the distributor and have the male receptacle that the correct HEI tach lead plugs into. Now, one last thing. There are report (unconfirmed by me) that NOT ALL older tachs will give you this problem. There have been reports of some instances of some tachs working without using a filter, but I have not been able to get them to work ever. Both of the tachs in my old 71 Camaro with HEI and my old 70 SS monte would not work without the filter, that I can tell you. Hope this helps. I'll copy and post this there as well."

by Oldschool454 Fri Aug 01, 2014 11:34 am

by Oldschool454 Fri Aug 01, 2014 2:39 pm

I asked "tach man" about the tach filter. I sent this....."Ok, Thank you. But just curious, have you ever heard of a lack of tach filter causing a tach not to work? The dash is from a 73 el camino so I'm pretty sure that was pre-HEI." His response was......."no, that will not cause the tach to malfunction"

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? 75_tac10

75 Tach Face
Dead tach....need tach filter? 75_tac11

75 Tach Circuit Board
Dead tach....need tach filter? 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.

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Tach Differences Between 73-74 and 75-77

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