Tach.Problem!!!
Melting Slicks
Joined: Oct 2002
Posts: 2,314
Likes: 111
From: St Louis MO
If the RC filter fails, shorting the tach wire to ground, WILL kill the ignition, and therefore the tach WON'T work! But it should work again after the ignition is fixed.
I agree that there pink wire is hooked directly to the ign cap, but I have never seen a radio noise cap between the pink wire and the cap. Quite possible, the wiring diagram is incorrect. Further, the radio noise capacitor is a simple capacitor and is not like the RC network shown for the tach filter: http://forums.corvetteforum.com/show...lter+off+broke, some 5 posts in.
In any event, the ignition is the most significant source of noise. It makes sense to have a tach filter in the line from the tach terminal on the dist cap to the tach wire.
Racer
Joined: Jul 2005
Posts: 499
Likes: 1
From: Winnipeg Manitoba
Race Director
Joined: May 2006
Posts: 16,528
Likes: 52
From: Dayton, Ohio
So your saying that hooking it up like the drawing in post 9 is the best way?
Thread Starter
Safety Car
Joined: Oct 2005
Posts: 3,570
Likes: 1
From: Algonac Michigan
The tach filter is located on the drivers side back of the intake manifold as per the circled area below
I doubt the filter is causing your bouncing needle issue, but temporarily bypass it to be sure. (plug the wire from the output side of the filter directly into the distributor) A bouncing needle is usually an issue w/the tach board, but it appears you have already changed that via swapping in a different tach. However did you have a differnet tach board, or did you swap the same board back and forth between the two different tachs?
Good Luck!
I doubt the filter is causing your bouncing needle issue, but temporarily bypass it to be sure. (plug the wire from the output side of the filter directly into the distributor) A bouncing needle is usually an issue w/the tach board, but it appears you have already changed that via swapping in a different tach. However did you have a differnet tach board, or did you swap the same board back and forth between the two different tachs?
Good Luck!
Please guys refer to my original post. I have no problem with fixing any other problems in this thread...I just want my tach to read steady on my boat.
Mark
Mark
Racer
Joined: Jul 2005
Posts: 499
Likes: 1
From: Winnipeg Manitoba
Yes, we have digressed.....
From your description, my understanding is that your tach is sort of
working, but that it bounces when the engine is at low speed.
This could be due to ripples on the 12V supply which smooth out when the engine is at high RPM.
These tachs are often susceptable to fluctations on the 12V supply.
I would try putting a 100uF/63V cap across the power lines on the back of the tach.
N.B. Watch the polarity as Caps of this size will blow up if connected backwards.
Racer
Joined: Jul 2005
Posts: 499
Likes: 1
From: Winnipeg Manitoba
However, RF is funny stuff and the only way to determine the 'best' way is to actually measure the radiated emissions with and without the capacitor.
That being said, my bet would be that cap on the ignition coil would do a better job than the RC filter.
The RC filter will reduce the noise on the tach signal line (the wht wire in diagram 8A-14).
It doesn't really address the noise generated between the battery and the distributor (the pink wire).
Last edited by Kilroy1024; Aug 23, 2008 at 03:20 PM.
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Melting Slicks
Joined: May 2007
Posts: 3,246
Likes: 4
From: Bay Area CA
Yes, we have digressed.....
From your description, my understanding is that your tach is sort of
working, but that it bounces when the engine is at low speed.
This could be due to ripples on the 12V supply which smooth out when the engine is at high RPM.
These tachs are often susceptable to fluctations on the 12V supply.
I would try putting a 100uF/63V cap across the power lines on the back of the tach.
N.B. Watch the polarity as Caps of this size will blow up if connected backwards.
From your description, my understanding is that your tach is sort of
working, but that it bounces when the engine is at low speed.
This could be due to ripples on the 12V supply which smooth out when the engine is at high RPM.
These tachs are often susceptable to fluctations on the 12V supply.
I would try putting a 100uF/63V cap across the power lines on the back of the tach.
N.B. Watch the polarity as Caps of this size will blow up if connected backwards.
Tach could be reading spikey alternator noise,,, other then installing a 50V 1000uF cap on the back of the tach (Radio shack), I would check the 2 main ground straps in the vehicle, if they were corroded, not making good contact, or if any of the alternator + wiring was not making good contact, it would generate spikes. I would inspect and clean both my main ground cable contact points and the Alternator + power wire contacts. C.
