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  #1  
Old Mar 23rd, 08, 08:16 AM
67motorcat 67motorcat is offline
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Steve O.
 
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Default Tach filter revisited

OK..I ordered a new OER tach to replace the OER tach that has been in my car for a couple years as it quit working.The new one worked for about a minute then quit as well.I then began to search the forum here and found that a filter should be installed since I have a breakerless Accell ignition distibutor and it sends a full 12 volts as opposed to 9 volts with the stock points system.I bought the stuff from Radio shack and made it and installed it but can't test it till I recieve my new printed circuit board(I decided to replace it since it looked a little thrashed and the dash was out,although it worked fine) and put the dash back in.My question is this,do you think I fried my brand new Tach or will it work again and why did my other OER Tach work fine for a long time.There seems to be a no return policy on electrical parts,but why don't any vendors state the tach will only work with OEM ignition systems..I'm a little pissed and possibly out $200.....The filter I made seems logical and how has it worked for other people.......
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  #2  
Old Mar 23rd, 08, 08:58 AM
LCAC_Man LCAC_Man is offline
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Default Re: Tach filter revisited

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Originally Posted by 67motorcat View Post
I bought the stuff from Radio shack and made it and installed it but can't test it till I recieve my new printed circuit board(I decided to replace it since it looked a little thrashed and the dash was out,although it worked fine) and put the dash back in.
There seems to be a no return policy on electrical parts,but why don't any vendors state the tach will only work with OEM ignition systems..I'm a little pissed and possibly out $200.....The filter I made seems logical and how has it worked for other people.......
I don't have any answers for you but I'm interested in this as well, I have an MSD Ready to Run billet distributor and an OER Tach, still a couple weeks from firing up so if I need a filter or some other modification to make this work I want to do it now.
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Old Mar 23rd, 08, 09:26 AM
JimM JimM is offline
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Default Re: Tach filter revisited

If the filter in my page in the electrical basics sticky is the one you made...

The "need" has more to do with the amount of electrical noise in your system than anything else. I orignally designed and built it when I switched from a sun mini tach to the oer tic toc, ignition was an accell billetproof. I still used it (and needed it) with an msd 8360. I now have a late model 8 pin computer controlled distributor (efi) and the filter is still on there. The tach is accurate and the needle steady, I have no intention of testing without the filter.

For a while I was making them for people, I probably shipped 15, and 12 of them ended up with tach's that worked.

I have no idea exactly how many built them on thier own, quit a few I think. These has been some feedback over the years, mostly but not always positive.

The filter works by shunting high frequencies to ground. It "smooths" the signal from the ignition to the tach. It prevents the tach from couning false pulses. The cutoff frequency works out to about 8500 rpm.

I would hope the circuitry in these tachs is done so the tach can't possibly be damaged from a bad input signal, but I have no way of knowing this for fact. Get it going on your existing known good tach, then try the others.
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Old Mar 23rd, 08, 11:28 AM
67motorcat 67motorcat is offline
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Default Re: Tach filter revisited

Thanks Jim..I'll let you guys know when my printed board arrives next week and hopefully my new Tach isn,t fried...Your intructions for the filter was great and easy to build.I hope vendors will disclose the fact that there is a problem with OER tachs and aftermarket ignition.I spoke with a tech guy at MSD and they said to install a 1 amp-400v diode inline to the tach feed to reduce voltage as another possble fix,also available from Radio Shack.
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Old Mar 23rd, 08, 03:11 PM
dnult dnult is offline
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Default Re: Tach filter revisited

Tachs are a mystery component. These archives are full of tachometer stuff. It's got to be a top 10. Noise is usually temporary and the filter will reverse the effect. But there is another possibility that would leave the tach permanently damaged.

A tach is a frequency to voltage convertor. At the heart of it is a diode on the input to rectify the signal from the primary ignition circuit. Diodes are susceptable to breakdown at higher voltages. I suspect the input diode is failing. Semiconductor technolgoy wasn't all that great back in the 60's anyway. I'd hope the replacements are a bit more robust.

The tach filter will remove some of the noise as Jim said, and reduce the amplitude of the signal from the coil which protects the diode. It's good insurance.
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Last edited by dnult; Mar 24th, 08 at 11:54 AM.
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Old Mar 23rd, 08, 04:04 PM
67motorcat 67motorcat is offline
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Default Re: Tach filter revisited

I hope the new replacments are a little more tolerent of increased Voltage as well,but from what Iv'e heard they are not..I hope so or I'm out $200 bucks.Ouch!
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Old Mar 25th, 08, 03:12 PM
67motorcat 67motorcat is offline
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Default Re: Tach filter revisited

Well..I have some good news and some bad news.The good news is Tamraz is going to exchange the tach I bought from them even though I'm not sure if I fried it or not.(Actually it seems I did,quiet now)They seem to be a very good company and their people know what they are talking about..kudos to them.the bad news is my tach started smoking when I installed it with the filter I made that Jim designed..I'm not sure what went wrong but I made it verbatum per instuctions???.I then installed my old tach and it started to smoke the same way(the same capacitor on the circuit board turned black).So I don't know what to do except when I do recieve my 2nd Tach from Tamraz I'm going to remove the filter and install the 1amp-400v diode that MSD recomended to reduce the Voltage to 9 volts.My first OER tach did not flutter before it quit and the filter that Jim has says it cures that,but does it reduce the Voltage as well?I'm kinda lost here because if the 3rd one smokes I'm certainly out 200 bucks and this could get expensive quickly.All I have is a breakerless Accel billet Dist with a standard coil and no box...Ideas why it smoked,has this happened to anyone else?......
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Old Mar 25th, 08, 04:06 PM
JimM JimM is offline
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Default Re: Tach filter revisited

well, the down and dirty is things smoke when they try to pass more current than they are capable of... and that usually happens when voltage is applied in unintended places.

The filter should not be attached to anything with much in the way of voltage. The resister goes to the negative coil terminal. The capacitor goes to ground. The junction of the capacitor and resister go to the tach. If your tach didn't smoke without it, it can't smoke with it... unless perhaps you connected something to the POSITIVE side of the coil? I'm not sure what would happen if you did that.
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Old Mar 25th, 08, 04:39 PM
67motorcat 67motorcat is offline
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Default Re: Tach filter revisited

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Originally Posted by JimM View Post
well, the down and dirty is things smoke when they try to pass more current than they are capable of... and that usually happens when voltage is applied in unintended places.

The filter should not be attached to anything with much in the way of voltage. The resister goes to the negative coil terminal. The capacitor goes to ground. The junction of the capacitor and resister go to the tach. If your tach didn't smoke without it, it can't smoke with it... unless perhaps you connected something to the POSITIVE side of the coil? I'm not sure what would happen if you did that.
Thanks Jim,no I did not connect it to the positive side of the coil and it was installed as you said exactly and I'm sure the ground connection was good...here is something I remember,when I was testing the feed from the dash connector with a bench Tach to see if the feed wire was ok,and it is,I got a little shock from the tach feed wire when it touched my bare fingers,kinda like when grabbing a spark plug wire but not quite as strong,I did not think that much amperage went thought the tach feed?..just a thought...the car runs perfectly. what do you think?
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Old Mar 25th, 08, 04:49 PM
LCAC_Man LCAC_Man is offline
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Default Re: Tach filter revisited

Bad Coil? Passing to much voltage to the negative terminal?
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  #11  
Old Mar 25th, 08, 05:08 PM
67motorcat 67motorcat is offline
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Steve O.
 
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Default Re: Tach filter revisited

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Originally Posted by LCAC_Man View Post
Bad Coil? Passing to much voltage to the negative terminal?
I'm not sure,coil is fairly new and it's an accell coil made for the application.How do you test a coil other than swapping it out,car runs great.The coil is mounted horizontaly on the firewall,maybe it should be right side up?,see then mounted that way all the time..is it possible to have to much voltage from the neg side?..it does seem like alot is coming out although I think it's to much amps not voltage..doesn't make any sense to me why the tach would smoke as nothing has changed with the car until the tach just quit working one day?
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Old Mar 25th, 08, 06:01 PM
JimM JimM is offline
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Default Re: Tach filter revisited

The plus side of the coil gets 12 volts from the ignition switch.

The minus side either floats to 12 volts, or is shorted to ground by the module or points in the distributor. The spark fires when the ground is released.
There is always a certain amount of "ringing" going on when the spark fires. In fact, this "ringing" is what the filter is trying to keep out of the tach.

I really don't have a clue what's going on. It's very weird.

The coil (the round stock looking ones) really should be mounted coil lad up.
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Old Mar 25th, 08, 07:26 PM
dnult dnult is offline
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Default Re: Tach filter revisited

Quote:
I then installed my old tach and it started to smoke the same way(the same capacitor on the circuit board turned black)
Perhaps it was a resistor that smoked? Capacitors don't pass DC and won't burn up unless they are shorted (bad) internally. If you got the tach hooked up to the 12V side of the coil, that would explain it.
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Old Mar 26th, 08, 05:47 AM
booman55 booman55 is offline
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Default Re: Tach filter revisited

Where does this 1amp-400v diode get installed? I'm in the process of installing the OER Tic-Toc-Tach w/ MSD6AL ignition and coil.

Here's a link MSD install instructions. It says a GM tach you should bypass inline filter.
http://www.msdignition.com/pdf/6%20series/6series.pdf

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Old Mar 26th, 08, 06:19 AM
67motorcat 67motorcat is offline
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Default Re: Tach filter revisited

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Originally Posted by booman55 View Post
Where does this 1amp-400v diode get installed? I'm in the process of installing the OER Tic-Toc-Tach w/ MSD6AL ignition and coil.

Here's a link MSD install instructions. It says a GM tach you should bypass inline filter.
http://www.msdignition.com/pdf/6%20series/6series.pdf

Thanks!
From what I understand the diode is simply installed inline between the neg coil lead and the tach.It seems if you run a external box it would be the same way from the tach feed putlet but I could not tell from the diagram.has anyone experienced the faint shock I did touching the tach feed wire bare handed...maybe I should buy a new coil before I install the new tach?This is really wierd...
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