Does the Tach in a 1980 require a filter? - The 1947 - Present Chevrolet & GMC Truck Message Board Network

hayesperformanceplus · 04-08-2011, 12:06 AM

Hi Everyone,

I was wondering if anyone knew whether or not a filter is need on a 1980 and up tach cluster on a 1980 pick-up.

Thanks!

David

allengator · 04-08-2011, 10:45 AM

I have never seen any kind of filter on a tach harness for the 79-80

*cr34t3d_by_d4rkd3n* · 04-09-2011, 06:34 AM

yea, a fram is the way to go!

hayesperformanceplus · 04-09-2011, 11:46 AM

Come on now, I need to use a good K&N filter for that tach.... LOL hahaha...

hayesperformanceplus · 04-09-2011, 11:54 AM

I didn't think I remember seeing any filter on a 79' - 87' Tach, but I just wanted to be sure.

So on the 73' to 78' Tach clusters a Filter IS Required???

And if you don't use one, will it wreck the tach? The reason why I ask is I have a quite a few Tach clusters I was going to check but I want to know ahead of time if I need to test them using a filter. Accually one of the clusters I've had stored forever I think is a NOS 77' Tach cluster, if not it must be a reman. or something because it looks brand new.

ray_mcavoy · 04-09-2011, 07:55 PM

I believe the tach filters were used simply to prevent radio interference. So you won't damage the tach by not using a filter.

By the way, the tach filter's internal circuit is shown in some of the factory wiring diagrams and a functional equivalent could easily be built using parts from RadioShack (or other electronics stores).

The circuit consists of two resistors in series, and a couple capacitors to ground. The "filter input" wire coming from the negative side of the coil (TACH terminal on HEI) goes to a 5.6K resistor, followed by a 10K resistor (in series), which then goes to the "filter output" wire that plugs into the tach harness. A .15MFD capacitor connects from ground to the junction point between the two resistors. And a 100pF capacitor connects from ground to the junction point between the 10K resistor and the "filter output" wire.

Jonboy · 04-10-2011, 06:12 AM

I didn't use one on mine when I installed it. However, they used to be pretty easy to find in 70's & 80's cars factory equipped with a tach. May be worth statching one if you see it.

04-08-2011, 12:06 AM	#1
hayesperformanceplus Performance Shop Owner Join Date: Oct 2009 Location: Edmonton, Alberta, CANADA Posts: 332	Does the Tach in a 1980 require a filter? Hi Everyone, I was wondering if anyone knew whether or not a filter is need on a 1980 and up tach cluster on a 1980 pick-up. Thanks! David __________________ 1980 GMC C25 Sierra Classic with 20,000 ORIGINAL MILES (32,000Km's) & Matching numbers 1977 GMC C35 Sierra Classic Crew Cab 454 A/C with 21,000 Miles & Matching numbers 1980 Chevrolet Silverado K20 400 4spd 4X4 74,000km's 1980 GMC C25 Sierra Classic CREW CAB, All Options, with 49,000 Miles 1979 GMC C15 Sierra Classic 454 1989 GMC R3500 CREW CAB 2003 Grand Prix GTP 1998 Regal GS

04-08-2011, 10:45 AM	#2
allengator Registered User Join Date: Dec 2010 Location: Burleson, Texas Posts: 596	Re: Does the Tach in a 1980 require a filter? I have never seen any kind of filter on a tach harness for the 79-80 __________________ Allengator Outta the Blue (my band - available for hire in the DFW area!) 1980 Chevy SWB Pickup (build thread) 1951 Chevy Sedan Delivery 2004 Chevy Tahoe Z71 2011 Harley Davidson Street Glide 2018 Silverado Z71 Crew Cab

04-09-2011, 06:34 AM	#3
cr34t3d_by_d4rkd3n Account Suspended Join Date: Jan 2011 Location: Allen, TX Posts: 188	Re: Does the Tach in a 1980 require a filter? yea, a fram is the way to go!

04-09-2011, 11:46 AM	#4
hayesperformanceplus Performance Shop Owner Join Date: Oct 2009 Location: Edmonton, Alberta, CANADA Posts: 332	Re: Does the Tach in a 1980 require a filter? Come on now, I need to use a good K&N filter for that tach.... LOL hahaha... __________________ 1980 GMC C25 Sierra Classic with 20,000 ORIGINAL MILES (32,000Km's) & Matching numbers 1977 GMC C35 Sierra Classic Crew Cab 454 A/C with 21,000 Miles & Matching numbers 1980 Chevrolet Silverado K20 400 4spd 4X4 74,000km's 1980 GMC C25 Sierra Classic CREW CAB, All Options, with 49,000 Miles 1979 GMC C15 Sierra Classic 454 1989 GMC R3500 CREW CAB 2003 Grand Prix GTP 1998 Regal GS

04-09-2011, 11:54 AM	#5
hayesperformanceplus Performance Shop Owner Join Date: Oct 2009 Location: Edmonton, Alberta, CANADA Posts: 332	Re: Does the Tach in a 1980 require a filter? I didn't think I remember seeing any filter on a 79' - 87' Tach, but I just wanted to be sure. So on the 73' to 78' Tach clusters a Filter IS Required??? And if you don't use one, will it wreck the tach? The reason why I ask is I have a quite a few Tach clusters I was going to check but I want to know ahead of time if I need to test them using a filter. Accually one of the clusters I've had stored forever I think is a NOS 77' Tach cluster, if not it must be a reman. or something because it looks brand new. __________________ 1980 GMC C25 Sierra Classic with 20,000 ORIGINAL MILES (32,000Km's) & Matching numbers 1977 GMC C35 Sierra Classic Crew Cab 454 A/C with 21,000 Miles & Matching numbers 1980 Chevrolet Silverado K20 400 4spd 4X4 74,000km's 1980 GMC C25 Sierra Classic CREW CAB, All Options, with 49,000 Miles 1979 GMC C15 Sierra Classic 454 1989 GMC R3500 CREW CAB 2003 Grand Prix GTP 1998 Regal GS

04-09-2011, 07:55 PM	#6
ray_mcavoy Registered User Join Date: Nov 2009 Location: Sherman, ME Posts: 2,359	Re: Does the Tach in a 1980 require a filter? I believe the tach filters were used simply to prevent radio interference. So you won't damage the tach by not using a filter. By the way, the tach filter's internal circuit is shown in some of the factory wiring diagrams and a functional equivalent could easily be built using parts from RadioShack (or other electronics stores). The circuit consists of two resistors in series, and a couple capacitors to ground. The "filter input" wire coming from the negative side of the coil (TACH terminal on HEI) goes to a 5.6K resistor, followed by a 10K resistor (in series), which then goes to the "filter output" wire that plugs into the tach harness. A .15MFD capacitor connects from ground to the junction point between the two resistors. And a 100pF capacitor connects from ground to the junction point between the 10K resistor and the "filter output" wire.

04-10-2011, 06:12 AM	#7
Jonboy Slots go on anything! Join Date: May 2003 Location: Richardson, TX Posts: 5,957	Re: Does the Tach in a 1980 require a filter? I didn't use one on mine when I installed it. However, they used to be pretty easy to find in 70's & 80's cars factory equipped with a tach. May be worth statching one if you see it. __________________ 1974 Jimmy- 5.3/4L80e/NP241



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