Tach filter question
Thread Starter
Racer
Joined: Jul 2009
Posts: 336
Likes: 1
From: Asheville N.C.
Well as the tach seems to work fine, the radio has engine rev noise. It's been this way for 4 yrs. now and I've had enough. After doing the forum search I read how the filter could cause radio interference and was wondering if this could be my problem? 
ED
ED
Tech Contributor
Joined: Jun 2004
Posts: 20,847
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From: I tend to be leery of any guy who doesn't own a chainsaw or a handgun.
Does the noise correlate to ignition/spark frequency, or alternator ripple frequency?
Tech Contributor
Joined: Jun 2004
Posts: 20,847
Likes: 959
From: I tend to be leery of any guy who doesn't own a chainsaw or a handgun.
Removing the alternator belt for a quick test should allow you to determine if the noise is alternator ripple. If the noise is gone the culprit is the alternator.
If the noise persists, unplug the Tach filter line and run the engine again. This will give you an indication if the filter is the noise conducting medium. If the noise is still there then the conduction medium is elsewhere (perhaps in closely run wires in the harness).
Let us know what you find.
If the noise persists, unplug the Tach filter line and run the engine again. This will give you an indication if the filter is the noise conducting medium. If the noise is still there then the conduction medium is elsewhere (perhaps in closely run wires in the harness).
Let us know what you find.
Thread Starter
Racer
Joined: Jul 2009
Posts: 336
Likes: 1
From: Asheville N.C.
Removing the alternator belt for a quick test should allow you to determine if the noise is alternator ripple. If the noise is gone the culprit is the alternator.
If the noise persists, unplug the Tach filter line and run the engine again. This will give you an indication if the filter is the noise conducting medium. If the noise is still there then the conduction medium is elsewhere (perhaps in closely run wires in the harness).
Let us know what you find.
If the noise persists, unplug the Tach filter line and run the engine again. This will give you an indication if the filter is the noise conducting medium. If the noise is still there then the conduction medium is elsewhere (perhaps in closely run wires in the harness).
Let us know what you find.
Thanks again for your suggestions...
Ed
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Instructor
Joined: May 2010
Posts: 172
Likes: 3
From: Hesperia California
Could I please but in on this tach info, I am a previous 69 corvetter and looking for my next 69 big block but right now I have a question on my 72 z28. The other day traveling to a show tach reading OK car sat for 6 hours then on the way home noticed tach was slow and about 1500 to 2000 rpms off, normal idle 800 to 850 now 500 to 550 car has m22 and 4.10 I know at 70 mph are 3500+ and now 2500+- at 60 2100, I know that's not right. I checked the wire under the points plate to the advance vacuum screw ground and it is tight and uncorroded so could my tach filter be bad ? and where can I get a new tach filter before pulling the gauge cluster out for repair, any suggestions would be appreciated, THANKS ........................ Garry
Thread Starter
Racer
Joined: Jul 2009
Posts: 336
Likes: 1
From: Asheville N.C.
Could I please but in on this tach info, I am a previous 69 corvetter and looking for my next 69 big block but right now I have a question on my 72 z28. The other day traveling to a show tach reading OK car sat for 6 hours then on the way home noticed tach was slow and about 1500 to 2000 rpms off, normal idle 800 to 850 now 500 to 550 car has m22 and 4.10 I know at 70 mph are 3500+ and now 2500+- at 60 2100, I know that's not right. I checked the wire under the points plate to the advance vacuum screw ground and it is tight and uncorroded so could my tach filter be bad ? and where can I get a new tach filter before pulling the gauge cluster out for repair, any suggestions would be appreciated, THANKS ........................ Garry
Former Vendor
Joined: Aug 2006
Posts: 76,656
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From: Jeffersonville Indiana 812-288-7103
St. Jude Donor '08-'09-'10-'11-'12-'13-'14-'15
http://willcoxcorvette.com/advanced_...=tach%20filter 37.00 in the forum special section of the site.
The filter actually smooths the square signal before hitting the tach, it just knocks the edge off of it.
How do you tell if it's bad? Noise in the stock radio would be one way, but usually when they go bad it is the wire pulling out of the end and not the actual filter going bad. If the tach is not working bypass the filter to see if this cures the problem.
The filter actually smooths the square signal before hitting the tach, it just knocks the edge off of it.
How do you tell if it's bad? Noise in the stock radio would be one way, but usually when they go bad it is the wire pulling out of the end and not the actual filter going bad. If the tach is not working bypass the filter to see if this cures the problem.
Instructor
Joined: May 2010
Posts: 172
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From: Hesperia California
WILLCOX
Thanks for the info and the diagram. The car is a 72 z28 but i assume a tach filter is a tach filter. So If i disconnect the filter what will be the reaction of the tach, will it still work, not work or just still be incorrect on it readings. I have checked all the wiring and all are connected and tight, except for the tach connection on the back of the tach, i'm not familiar with the back of the gauge cluster, Ill look at my manual when I get home, The tach works but it is slow on rev and its reading is slow by 1500 to 2000 rpms at idle it is 500/550 should be 800/850 when I rev from idle it goes to around 3500 and I know it should be going up to 5000 or so, at 60 mph it shows 2100, I have m22 with 4.10s so i know it should he around 2800 or so, just from memory. I think ill order the filter anyway mine is rusty and discolored. Again THANKS for the input OH and sorry I forgot all about willcox glad you chimed in...........................Garry
Thanks for the info and the diagram. The car is a 72 z28 but i assume a tach filter is a tach filter. So If i disconnect the filter what will be the reaction of the tach, will it still work, not work or just still be incorrect on it readings. I have checked all the wiring and all are connected and tight, except for the tach connection on the back of the tach, i'm not familiar with the back of the gauge cluster, Ill look at my manual when I get home, The tach works but it is slow on rev and its reading is slow by 1500 to 2000 rpms at idle it is 500/550 should be 800/850 when I rev from idle it goes to around 3500 and I know it should be going up to 5000 or so, at 60 mph it shows 2100, I have m22 with 4.10s so i know it should he around 2800 or so, just from memory. I think ill order the filter anyway mine is rusty and discolored. Again THANKS for the input OH and sorry I forgot all about willcox glad you chimed in...........................Garry
Former Vendor
Joined: Aug 2006
Posts: 76,656
Likes: 1,850
From: Jeffersonville Indiana 812-288-7103
St. Jude Donor '08-'09-'10-'11-'12-'13-'14-'15
Could I please but in on this tach info, I am a previous 69 corvetter and looking for my next 69 big block but right now I have a question on my 72 z28. The other day traveling to a show tach reading OK car sat for 6 hours then on the way home noticed tach was slow and about 1500 to 2000 rpms off, normal idle 800 to 850 now 500 to 550 car has m22 and 4.10 I know at 70 mph are 3500+ and now 2500+- at 60 2100, I know that's not right. I checked the wire under the points plate to the advance vacuum screw ground and it is tight and uncorroded so could my tach filter be bad ? and where can I get a new tach filter before pulling the gauge cluster out for repair, any suggestions would be appreciated, THANKS ........................ Garry
Your camaro tachometer is different... you can toss out what I've said about the tach and the filter. The early F body tach worked differently than the Corvette electronic tach.
It's been a very long time since I've even seen one of those but if I remember correctly it was a four pole analog movement with two common's vs the Corvette shared three pole, one common movement. Probably lost you on that one..
But take a picture of the back of the tach if possible and email it to me.
What would make it slow all of a sudden.. My guess is the tach is taking a dump on you. Wish I had better news but I don't. I wish I knew more about that movement but again, I've not studied it just going off what I think I saw years ago.
Willcox
Instructor
Joined: May 2010
Posts: 172
Likes: 3
From: Hesperia California
Yes I understand, no problem, I am presently pulling the intake for cleaning so I can reinstall the original Holley 780, may be a couple weeks before i get to the gauge cluster but i will accommodate a pic or two and let you know the out come, first when i get the car running again ill delete the filter to see if there is a difference, I guess there is a guy on nastyz 28 that rebuilds gauges and clusters and is top notch, may have to send it to him or just replace the tach myself and or see just what it looks like behind the tach, maybe ill find something that just jumps out and I can say,OH there's the problem LOLOL. THANKS again WILLCOX
Thread Starter
Racer
Joined: Jul 2009
Posts: 336
Likes: 1
From: Asheville N.C.
Just an update on the radio whinning noise:
I ordered and installed the noise suppression piece from Corvette Central annnnd
no go, didn't change a thing so I guess I'll order an alternator rebuild kit and go from there...
I ordered and installed the noise suppression piece from Corvette Central annnnd
no go, didn't change a thing so I guess I'll order an alternator rebuild kit and go from there...
Tech Contributor
Joined: Jun 2004
Posts: 20,847
Likes: 959
From: I tend to be leery of any guy who doesn't own a chainsaw or a handgun.
You might do a search for alternator issues. IIRC, sometimes when one of the rectifier diodes is shorted that it will induce noise into the harness.
Thread Starter
Racer
Joined: Jul 2009
Posts: 336
Likes: 1
From: Asheville N.C.
Update;
So after the Corvette Central noise suppression capacitor didn't work I ordered an alternator rebuild kit including; brushes and springs, front and rear bearings, diode trio, rictifier, capacitor and installed them. The rpm noise still exists so here's what else I've done with no success...
1. unplugged the tach filter
2. regrounded the distributor sheild to the frame
3. regrounded the radio side ground lug straight to the battery
There again if I unplug the 2 wire leads to the alternator, there is no rpm noise. So I then rechecked my plug wires and I have AC Delco Premium Silicone 8mm, plugs are AC Delco 45RTS...damn this is an all stock car and system...just at a loss here, any new leads would be helpful...
Ed
So after the Corvette Central noise suppression capacitor didn't work I ordered an alternator rebuild kit including; brushes and springs, front and rear bearings, diode trio, rictifier, capacitor and installed them. The rpm noise still exists so here's what else I've done with no success...
1. unplugged the tach filter
2. regrounded the distributor sheild to the frame
3. regrounded the radio side ground lug straight to the battery
There again if I unplug the 2 wire leads to the alternator, there is no rpm noise. So I then rechecked my plug wires and I have AC Delco Premium Silicone 8mm, plugs are AC Delco 45RTS...damn this is an all stock car and system...just at a loss here, any new leads would be helpful...
Ed
Tech Contributor
Joined: Jun 2004
Posts: 20,847
Likes: 959
From: I tend to be leery of any guy who doesn't own a chainsaw or a handgun.
Update;
So after the Corvette Central noise suppression capacitor didn't work I ordered an alternator rebuild kit including; brushes and springs, front and rear bearings, diode trio, rictifier, capacitor and installed them. The rpm noise still exists so here's what else I've done with no success...
1. unplugged the tach filter
2. regrounded the distributor sheild to the frame
3. regrounded the radio side ground lug straight to the battery
There again if I unplug the 2 wire leads to the alternator, there is no rpm noise. So I then rechecked my plug wires and I have AC Delco Premium Silicone 8mm, plugs are AC Delco 45RTS...damn this is an all stock car and system...just at a loss here, any new leads would be helpful... Ed
So after the Corvette Central noise suppression capacitor didn't work I ordered an alternator rebuild kit including; brushes and springs, front and rear bearings, diode trio, rictifier, capacitor and installed them. The rpm noise still exists so here's what else I've done with no success...
1. unplugged the tach filter
2. regrounded the distributor sheild to the frame
3. regrounded the radio side ground lug straight to the battery
There again if I unplug the 2 wire leads to the alternator, there is no rpm noise. So I then rechecked my plug wires and I have AC Delco Premium Silicone 8mm, plugs are AC Delco 45RTS...damn this is an all stock car and system...just at a loss here, any new leads would be helpful...
Ed
Thread Starter
Racer
Joined: Jul 2009
Posts: 336
Likes: 1
From: Asheville N.C.