tach filter probs....help! - Team Camaro Tech
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post #1 of 10 (permalink) Old Mar 3rd, 04, 03:22 PM Thread Starter
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I have an MSD ready to run distributor and have just installed factory guages and tach. After the tach read erratic I did a archive search and found out about the tach filter issue. Made one and it worked for a while then the tach quit. Pulled the dash apart (again!!!) and the circuit board was burned out on the path to tach wire. Ordered new circuit board, put it togather (again!!!) still didn't work. Spent a few days tearing back into wiring, couldn't find anything wrong. On a whim I removed the filter and tach worked erratic like before. My question is how do you check the filter. I don't want to smoke another $70.00 board! Having the dash apart and the wiring like speghetti, it might not have even been the filters fault. Dang, this is a really long post...sorry! Thanks for any help.

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post #2 of 10 (permalink) Old Mar 3rd, 04, 03:54 PM
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Wow...burned the foil? Is this the flexible mylar PC board? The tach should not have allowed enough current into it to burn the foil. I have a 69 tach I'm working on now. It has a large enough resistor in series with the tach line that it could not possibly draw enough current to burn the trace even if it was connected to 12 volts directly. I can't explain it.

I have learned a few things about 1st gen tachometer technology recently though. They depend on a flyback voltage from the coil to make the meter read. In other words, they must see more than 12 volts on the tach lead for a short period of time to get a reading. What I suspect happens with HEI and MSD ignitions is that the flyback voltage is large enough that when it goes negative it causes and input diode to break down and makes the tach read irratically.

The filter does a couple of things. It suppresses high frequency noise from the coil and reduces the amplitude of the signal to prevent the diode from breaking down. The optimum filtering would probably be just above the point where the tach stops reading. A potentiometer would be nice since you could adjust it for the point of no reading, back up a bit and then build a filter using the same value.

I appologize if this isn't clear. What I'm trying to say is that the GM engineers probably did a cut and try job with the tach. They kept tweaking component values and when it worked they shipped it. Aftermarket ignitions change the formula making it necessary to add filters. Problem is, that each type of ignition could benefit from a different filter. So I think you will need a filter, but it is possible that your filter is working a bit too good.

Try this. Put a variable resistor in paralell with your filter (say a 20K ohm pot). Adjust the filter until the tach reads right and then replace the pot with a fixed value resistor. The extra resistor will reduce the filters resistance and allow a bit more signal through.

Dave
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68 Coupe, 350 w/ Edelbrock Performer RPM heads, cam, intake, 700R4, Dave's small body HEI
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post #3 of 10 (permalink) Old Mar 3rd, 04, 04:35 PM Thread Starter
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Thanks dnult. Yeah it's the printed circuit board that attaches to the back of the speedometer/tach assembly. Tracing the burned path it goes to the double pink wire. Like I said I had everything splayed out and I'm not positive it was the filter that did the damage. Is there any way to test the filter? When I bought the stuff to make it there were I think ten resistors to each pack, so I have those. I only have the one capacitor though. How do you check a capacitor with a multi meter? Also they talked about a potentiometer in the archive post so I bought one of those as well. I'm not real swift on electronics though, how do you tell what the value is when you dial it in? If I understand you correctly, there's no way that it could cause any damage while playing with it, it would just read inaccurately, right? Oh one more thing, the potentiometer has 3 lugs on it, how do I wire it up? I appreciate your help partner. In reading past posts I see you to electronics as David Pozzi is to suspension or MARTINSR is to body work.

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post #4 of 10 (permalink) Old Mar 3rd, 04, 04:46 PM
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Or, you could call MSD @ 915-855-7123 and ask for Vic, he's the one that helped me out. The little filter he sent me just popped right in and worked perfectly. Maybe not as cool as doing it yourself, but maybe it'll keep you from burning out another circuit board. (Although I can't understand how that happened.)

Steve W
1968 Camaro Convertible
1966 GTO Convertible
1995 Harley Road King
"You can't always get what you waaant..."

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post #5 of 10 (permalink) Old Mar 3rd, 04, 05:14 PM Thread Starter
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How much do they charge for their filter? Is it accurate? You would think that if it was accurate it would be a set value, huh? Thanks for the input bud.

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post #6 of 10 (permalink) Old Mar 4th, 04, 07:53 AM
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They sent one to me for NOTHING, not even postage.
I plugged it in, and then hooked up my timing light (with built-in tach) and they were within 100 rpms of each other. Thats close enough for me!

Steve W
1968 Camaro Convertible
1966 GTO Convertible
1995 Harley Road King
"You can't always get what you waaant..."

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post #7 of 10 (permalink) Old Mar 4th, 04, 09:15 AM
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Sounds like you've got a couple of options here. There are a couple of ways to make those filters. Most likely you will be able to measure some resistance accross it with a ohm meter (digial or analog). I would guess the resistance would be about 5K ohms or so. I'm very perplexed about the burned foil. No way your filter could do that unless you hooked it to 120 volts

Potentiometers are a resistor with a wiper that varies the resistance. The outer terminals are the ends of the resistor and the middle terminal is the wiper. In your case, hook the wiper (middle terminal) to one of the two ends. Then connect the end terminals accross your filter. Adjust it for best performance.

You can either leave it that way, or removing it, measure the resistance of it as adjusted (with your digital or analog meter) and replace it with a permanant fixed value resistor of approximately the same size.

Capacitors are hard to check with a meter. There are some Digital meters with a capacitance checker built in. Other than that, about the only thing you can do is make sure they are not shorted by measuring their resistance.

If this doesn't sound like much fun, maybe the freebe is the way to go. But like I say, each application is different so I would expect MSD would have had to design a filter for their box running on a factory (or repro) 1st gen tachometer or something like that.

Dave
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post #8 of 10 (permalink) Old Mar 4th, 04, 03:26 PM Thread Starter
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Thanks for the input Steve. I'll probably do that. Hey dnult, you've got mail.

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post #9 of 10 (permalink) Old Mar 5th, 04, 01:30 PM Thread Starter
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Just a follow up Steve, I called Vic at MSD today. Really nice guy. He said he would send me a filter ASAP. Thanks for the heads up buddy! And to you too dnult. I appreciate the help!

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post #10 of 10 (permalink) Old Mar 5th, 04, 08:09 PM
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Cool!

Steve W
1968 Camaro Convertible
1966 GTO Convertible
1995 Harley Road King
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