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12/6/14 1:22am - Original Message: 'Tach filter'
bcousins
Former Member
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Jacksonville, FL - USA

Vette(s):
79 L-82 4-speed. Engine stock except long tube headers. Previous owner replaced upholstery and carpet with OEM. I replaced and upgraded steering and suspension.

Joined: 12/15/2009
Posts: 36
Swapped my tired 79 distributor yesterday for a new MSD Street Fire and noticed what I had thought was a radio suppressor, attached to the tach power cable but is actually the tach filter, GM part 14020025 (discontinued). It still works but I figured it is 35 years old so should be replaced. I went to three parts stores to find a new one, but not available. I found one on eBay for $100 and some other places for around $50.
Here is a photo of the filter I found on the web.

I called Summit and talked with tech Tom who told me they don't have the filter but he is very familiar with the distributor and the tach and to disconnect the filter and throw it away because the distributor has inside what the tach needs to run. Not true. 
Without the tach filter hooked up, the tach doesn't work. Plugging in the tach filter, the tach works fine.
I went on several forums and read that the filter steps down the voltage from 12 to 9 for the tach but I don't think that is true either.  
Does anyone know what that device does, exactly? Does it step down the voltage or can I bypass it and just connect the tach power cable directly to the distributor? Notice there are two wires. I believe this filter completes the circuit between the tach ground and power. As I said, it still works but I am going to need a new one sometime in the future. Thanks.

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12/6/14 7:56pm - Reply: 'Re: Tach filter'
Sarge81 Lifetime Member
Lifetime Member
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WEST SENECA, NY - USA

Vette(s):
1981 Blue Ice Met. w/pearl ghost flames TKO500 5speed. LS1 Swap 3.45 Dana 44.

Joined: 12/3/2001
Posts: 2376
I would recommend you run a tach filter. A tach filter smooths out the occasional voltage spikes which causes erratic or false tach readings.  Also without it you may ruin your tach printed circuit board. Your tach with an OEM distributor should still work without the filter. Aftermarket distributor I'm not sure of.

I know even after doing my "LS1" engine swap with the digital to analog tach converter box my tach works without the filter but I get an occasional spike. So next time I tear the dash apart I have to wire a filter into the system.  

You can buy a tach filter from our one sponsor Mid America Motorworks.

Good quality plus a great price at $30. 


____________________________________

GEN III 5.7L "LS6" Engine swap
TKO500 5 spd.
3.54 Dana 44
'69 "N11" Sidepipes
Borgeson Steering Box
Born 8/1981
Sequence #3975




Click here to see more pics of my Vette on CarDomain.


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12/7/14 12:05am - Reply: 'Re: Tach filter'
bcousins
Former Member
- Send Private Message


Jacksonville, FL - USA

Vette(s):
79 L-82 4-speed. Engine stock except long tube headers. Previous owner replaced upholstery and carpet with OEM. I replaced and upgraded steering and suspension.

Joined: 12/15/2009
Posts: 36
Thank you very much for the tip about the filter at Mid-America. That didn't show up on my Google search. The way the filter is wired in on my car the tach wouldn't work without the filter or a jumper wire. Each end of the filter is plugged in between the wire coming from the tach and the wire going to the distributor, so it is part of the circuit. 
I will definitely order a new one from MAM. Enjoy that LS6! 

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