Topic: need help with tach
in Forum: C3 Electrical
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West Bend, WI - USA
Joined: 3/18/2010
Posts: 46
Vette(s): 1985 silver coupe & 1975 red coupe
Help! I have a 1975 car with a tach problem. The thing was acting strange so I replaced the circuit board. That helped somewhat but now the faster I go the more the tach reads high. At 70 MPH I show 3500RPM and it should really be about 2900RPM. Should I try another circuit board or what? Tearing the dash board apart again is not something I'm looking forward to doing. Could the problem be with the Tach filter? What does the filter do and where the heck is the thing located? I should add that when I turn the key to ON, the Tach reads 0 and I think that is correct. THANKS much for any help you guys can give me! Darrell
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Duncanville, TX - USA
Joined: 11/8/2003
Posts: 20214
Vette(s): #1-1974 L-48 4spd Cp Med Red Metallic/Black deluxe int w/AC/tilt/tele./p/w-p/b/
Am-Fm/map light National/Regional/Chapter NCRS "Top Flight"
#2-1985 Bright Red/Carmine Cp.L-98/auto
Member: NCRS, NCRS Texas, Corvette Legends of Texas
Not sure if the '75 have a tach filter like the later cars, but if it does, it will be in the "Tach" circuit coming from the dist cap connection, which should be a brown wire. Tach filters are normally in that circuit not too far from the cap connection, and may be bolted to the top bellhousing bolt.
Joel Adams
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Tach filter is mounted on the intake manifold very close to the distributor, visible from the driver's side. 1975 is the first year with the electric tach due to the switch to HEI that year and thus the tach filter. You can check to see if the tach filter is the culprit by bypassing it. Unplug the tach filter from the distributor, unplug the brown wire coming through the firewall from the tach to the tach filter, plug the brown wire from the tach directly into the distributor. If the tach works properly, you have a bad filter. You won't hurt the tach by bypassing the filter. It is only there to filter out electical noise from the circuit which can give wonky tach readings.
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Build Date: May 7, 1975. 383 w/267 RWHP/310 RWTQ
Doesn't sound like the filter to me. I replaced my board with one from Wilcox and they have good videos on YouTube about how to calibrate. Which vendor did you get yours from?
When I started my car two weeks ago after rebuilding the motor (and tach and a whole lotta other stuff) the tach was bouncing around like crazy. I was worried that I did the board wrong or it was bad, but I later discovered that the filter was connected electrically but I didn't screw it down to the intake like it's supposed to be - it was just hanging there suspended by its wires. As soon as I bolted it down to the manifold the tach settled down and is working better than it ever has in 11 years of owning this car. Must need to be grounded for it to work properly.
1976 Silver/Firethorn. L48, 4spd. Original 2 bolt, vortec heads, 9.4:1 CR, Speed Pro Cam: 224/224@0.050, 112 LSA, Eagle Steel Crank.
daveo76 said: Doesn't sound like the filter to me. I replaced my board with one from Wilcox and they have good videos on YouTube about how to calibrate. Which vendor did you get yours from?
When I started my car two weeks ago after rebuilding the motor (and tach and a whole lotta other stuff) the tach was bouncing around like crazy. I was worried that I did the board wrong or it was bad, but I later discovered that the filter was connected electrically but I didn't screw it down to the intake like it's supposed to be - it was just hanging there suspended by its wires. As soon as I bolted it down to the manifold the tach settled down and is working better than it ever has in 11 years of owning this car. Must need to be grounded for it to work properly.
I was thinking the same thing Dave; might be the calibration of the board. I just wanted OP to know how to bypass filter. Good point about grounding the filter. Very important. I got my board from Wilcox as well.
Build Date: May 7, 1975. 383 w/267 RWHP/310 RWTQ
Let us know how this one turns out. Maybe if you don't have luck with calibration you could try to call Wilcox. I've been wondering how their customer service is. Good luck!
1976 Silver/Firethorn. L48, 4spd. Original 2 bolt, vortec heads, 9.4:1 CR, Speed Pro Cam: 224/224@0.050, 112 LSA, Eagle Steel Crank.
Lifetime MemberHOWELL, NJ - USA
Joined: 5/18/2004
Posts: 6812
Vette(s): 1979, Targa Blue (72 Color), Pace Car rear spoiler, L88 hood, Dark blue factory interior, 525HP 406, HD 700R4, 370 gears,Steeroids, composite rear spring, TT IIs wrapped in T/A Radials.
With the key in the run position is the tach at 0 RPM? If not, you will need to re-zero the tach. I had to do that with mine after I changed my PC board.
Has to be done with the tach out of the dash (guess you know how to get it out...)
It basically requires carefully removing the needle, applying a 12V source to 2 out of the 3 studs on the back (I have to find the article for you that tells which 2) and carefully pressing the needle back on.
Be very careful if you try this. The needles tend to get brittle with age, but you can find them if you need one.
Good luck
Paul
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I suggeest that you make sure that the gauge and the filter are well grounded before attempting to recalibrate or re-zero the tach.
Lifetime MemberModerator
Duncanville, TX - USA
Joined: 11/8/2003
Posts: 20214
Vette(s): #1-1974 L-48 4spd Cp Med Red Metallic/Black deluxe int w/AC/tilt/tele./p/w-p/b/
Am-Fm/map light National/Regional/Chapter NCRS "Top Flight"
#2-1985 Bright Red/Carmine Cp.L-98/auto
Member: NCRS, NCRS Texas, Corvette Legends of Texas
Joel Adams
C3VR Lifetime Member #56
My Link
(click for Texas-sized view!) NCRS
"Money can't buy happiness -- but somehow it's more comforting to cry in a CORVETTE than in a Kia"
in Forum: C3 Electrical
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