Anyone dealt with Tach filters???

Guys, I'm having problems with my tach. Chevy 305 with HEI distributor. I got a Iequus tach that is supposed to work either on points or electronic. I have thoroughly checked my wiring time and again. I went into my distributor and found some connections that were iffy and repaired them but had ideas that the ignition module or other components might have been harmed running it with a few of the defects I found. Under the coil, I found the ground strap mis-positioned and by marks found on the strap, it had been arcing. When I opened it up it was right after a run and could tell the parts inside were pretty hot. Ok, so I swapped the distributor out of my old motor to the new and nothing changed. Same exact distributor and same age. I get very erratic needle movements. Could start out accurate then goes to 8000rpm, then back to zero, and at times, at speed, it may read correctly. So I've been reading all I can and found info on tach filters. From the mid 70s on to the late 80s, GM put them on lots of stuff. I've never messed with them but I'm wondering if that's what I need to correct my problem. I've found a few places to buy them so that's not the problem. I'd like to hear about anybody else's experience with these things.

Most time it's an iffy ground doing what you're explaining.

Yeah Iron, those were my thoughts too! I spent 30+ years in electronics and know the importance of a good ground and that is what I tried to prove first. I had my whole harness apart today physically following wires to where they went and testing with volt ohm meters and test sets. I can find no errors in the wiring. What I'm reading is that the HEI signals can be cleaned up before hitting aftermarket devices with these filters. I ran a different tach on this same motor before without any problems but I was also using a points type ignition. I wired the old tach up today but get nothing out of it and I ran the wires straight to different places for ground...no change.

I got nothing but sympathy for you. Wish I could help but you've already
checked everything I would have. Does it run alright with the new distributor?
Hope you get it figured out.

I-R-POPS said:

I got nothing but sympathy for you. Wish I could help but you've already
checked everything I would have. Does it run alright with the new distributor?
Hope you get it figured out.

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Pop's, The only problem I've had with either distributor is the tach issue. Motor runs great with either.

smallfoot said:

Pop's, The only problem I've had with either distributor is the tach issue. Motor runs great with either.

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I'd hook up another tach. Maybe that one is cooked.

Small, I have an issue with my new Dolphin tach so, I'll be wanting to try what you're doing too.
If you'll post the filter you find to work I'd be grateful.

This is the one I'm ordering:
http://www.speedwaymotors.com/Classic-Instruments-SN20-Tach-Filter,5613.html

I'll post my results.

Thanks!!!!!!!!!!!!!!!!

Have you changed distributor cap when you changed distributors, or did you use the same one with both? It could be a bad cap, bad from factory.

Bamamav said:

Have you changed distributor cap when you changed distributors, or did you use the same one with both? It could be a bad cap, bad from factory.

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Bama, I bought 2 of the same exact distributors from Speedway. Listed as Chevy 100% New blueprinted HEI distributor.
Hardened gear
High performance rotor
High performance ignition module
50,000 V coil
Both are complete to the cap
I used one in the original 305 I started with and one in this crate motor. They were completely swapped when I changed motor and tranny. Because of some small connection problems I found in the one that wound up in this new motor, I changed the distributor back to the first one to see if it effected the tach...no change. Motor hasn't skipped a beat on either distributor but neither will operate the tach other than erratic needle movements. Not a real big deal but I'd like to get it working. I'm hoping it's just a "noisy" condition I keep seeing referred to on problems I've read about on the web. Chevrolet placed them in cars when they came out with HEI. Chevrolet parts numbers for this were discontinued. Several are being offered aftermarket. I'll get the new parts in a few days and see if it does any good. Although the motor is still being babied trying to break it in easy, I'm getting a multi-blade fan to maybe help with the cooling some and a shroud may follow. I've never had one of these motors to run hot. They are pretty sweet for me. This one is already settling in and doesn't gain the heat like it was for a day or two. The new tranny feels like the seat slides forward a little under your hiney when it shifts....nice!

OK bare with me.
Are you sure the wire is plugged into where it says tach on the distributor?
I'm not questioning your knowledge just your eye sight [ddd

Old Iron said:

OK bare with me.
Are you sure the wire is plugged into where it says tach on the distributor?
I'm not questioning your knowledge just your eye sight [ddd

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Haha! yessir, always try to make sure of my efforts before I blame the manufacturer. To the point of pulling the cap off and making sure of good contact at my wire clip to the spade tip in the cap and then to the point of pulling the coil cover to ascertain that the proper wire(yellow -) was actually leading from the coil to the spade connector marked as Tach.
One of these days shortly, I hope, this old magic box will get done trying to download all the updates it poured to me when I tried to order my parts the other night. Two days now and can't get the order completed online because of that. Dial-up sux!!! Tomorrow I'll call them live and get them on the way.
In this case, I don't really feel there is any defect. Just maybe a noisy signal because of high energy parts. If I was getting just a bump to the needle once in a while like something lacked a good ground to measure the signal I'd still be checking grounds. This tach gets a constant signal when the motor is running but it never stays stable. The test calls for wiring a test lamp to the tach connection and seeing if you get a blinking light while trying to crank the motor which worked fine. I can just turn the key with the motor cold and it fires before a complete turn and runs real nice. The needle will be about correct as far as I can tell. Rev the motor once and the signal splatters. May be high, may be low, sometimes pinned to the needle stop at either end of the scale on the tach. I have made long test leads where I can clip to the wiring and go right to battery ground from the ground lead of the tach with no change on the tach. Also done on the + feed too. Continuity tests done on all leads, both mine and the factory pigtails. Every test I'm capable of doing here has checked out ok. I've had troubles before with some tachs not working when going from points style ignition to HEI but usually that is the tach. I made sure before buying this one that it was designed to work with electronic ignitions. I got this one also because of its small size as I have it mounted to my steering column and trying to keep it compact. I've never used this brand before but I did change all my other gauges to the same brand and have not had the problems I experienced with the set I started with. The Sun Pro set I had from day one, the volt gauge needle stuck on start-up. You could tap the gauge face and get correct readings. The temp guage with mechanical not electrical connections quit working altogether before my engine swap. New gauges all worked on changing them out with no troubles since except for the tach readings. I'm grasping at straws to make it work now and it is not really something absolutely necessary to me. Tach just eases some testing procedures I like to see while tweeking this new motor out to run like it can. I'm not too worried and if it leaves my column, it may hit the barn wall......SMACK!!! After all, my "smart phone" wasn't even smart enough to keep from making me mad. It didn't survive that hurl at the log wall that would have made a major league pitcher proud...about 94 mph I figure...[ddd

You have done a lot of diagnostics, looking for that grimlen [cl

Have to...can't see...

Did get my order in about 425 am, I'll let ya know if the filter works.