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Topic: 79 speedo and tach problems

in Forum: C3 Electrical


79 speedo and tach problems

Posted: 2/24/04 12:19pm Message 1 of 8
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Posts: 28
I have recently purchased a 79 350 manual vette. The Tach doesn't work at all and the speedo reads very high. I don't know if a previous owner has changed anything as I am still sorting through the numbers book. The speedo is very frustrating as we have a lot of speed cameras where I live so knowing the actual speed would be helpfull. Anyone have any ideas as to where to start. Thanks. |ohwell|


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79 speedo and tach problems

Posted: 2/24/04 3:41pm Message 2 of 8
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CUYAHOGA FALLS, OH - USA
Joined: 12/2/2003
Posts: 6424
Vette(s): 1975 C3 Red, T-Tops, Black Interior. All I need is time and money! Getting there!
The tach and speedo don't have anything to do with each other. I guess that's the bad news.
The speedo is gear driven from the transmission, through a cable, to the speedo head in the dash. It may go to the cruise control then the speedo head. None of these will make it read too fast. The gear in the transmission may make it read too fast if it's the wrong gear. This can be changed or checked by taking out one bolt that holds gear holder into the trans, where the cable connects. Just pull it out and count teeth or check the color of the gear. Color is not reliable after several years. Now check the tire size and gear ratio of the diff. Go the the parts book charts at Chevy and see if it's the right gear. If it is, then the problem would be in the speed head itself. It can be removed and serviced. You may be able to clean it and be okay. Don't get anything on the face of the speedo or the odometer numbers. A light penetrant like WD40 will work fine. Then gently blow off the excess. If this doesn't work, it will need to go to a specialist. Electrosound on Brookpark Rd in Cleveland OH is one place. Hopkins and Kipps in Akron OH used to do this, I don't know if they still do. There are others.

The tach is fed from the tach lead (brown wire) that comes out of the distributor, at the side of the cap. It goes to a tach filter, the goes to the tach in the dash. The tach is fed with ignition power and ground, and the tach signal. It could be disconnected, or the tach filter could be bad, or an open circuit to the tach. If the signal is okay and you have power and ground, the tach need repaired. Same places. There is probably one close to you somewhere.

Ken Styer


79 speedo and tach problems

Posted: 2/24/04 8:52pm Message 3 of 8
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Toms River, NJ - USA
Joined: 3/24/2003
Posts: 105
Vette(s): '79 L82 4spd
kstyer >> you mentioned the tach filter,,, do you know the "proper" mounting place for it? Mine is just laying loose next to the distributor. Thanx


EdelBrock Performer w/ 750 doub pumper,port/polished heads, triple cut valves w/4th cut down the throat of the exhaust valves, 286 Comp Magnum cam, 1.6 roller rockers, ball-peen pushrods w/guides, screw in rocker studs w/girdles, Comp double-roller chain, , hi-rise valves covers w/ spacer (needed for the girdles), Flowtech by Holley long-tube ceramic coated headers into Stage II pipes (no cats), chrome wire looms for the MSD 8.5 mm wires, MSD 50,000v coil kit w/advance springs, cap, rotor, module, steel-braided radiator,heater,vacuum hoses, Infinity Kappa 4x6 plates (dash), Sony Xplode 6 3/4" (rear), chrome T-stat housing, A-arm shields, B&M Hammerhead shifter |IMG||/IMG|

79 speedo and tach problems

Posted: 2/25/04 1:46am Message 4 of 8
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Ken, Thanks for the information. I will start with the gear ratio first as taking the dash out looks a bit more fiddly. With regards the tach, what does the filter look like and where would I find it? I have noticed that the brown wire from the distributor has be cut and re-connected at sometime in the past. |ohwell|


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79 speedo and tach problems

Posted: 2/25/04 8:23am Message 5 of 8
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CUYAHOGA FALLS, OH - USA
Joined: 12/2/2003
Posts: 6424
Vette(s): 1975 C3 Red, T-Tops, Black Interior. All I need is time and money! Getting there!
The tach filter is in line in the brown tach wire. It is usually near the distributor or mounted on the firewall. I have found one under the dash.

Ken Styer


79 speedo and tach problems

Posted: 2/25/04 7:21pm Message 6 of 8
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United Kingdom
Joined: 10/4/2002
Posts: 122
Vette(s): Red 1980 350 auto w/ quickshsift Edelbrock intake / carb Crane Fireball cam 0 - 60 6.3s
If the filter IS mounted on the firewall it must be grounded somehow. It does precisely jack if it's not grounded. The proper place for it is bolted down to the intake manifold, just beneath the distributor. then it's grounded thru its mounting tab..
|cheers|


|IMG|http://www.nottingham.ac.uk/~eaztl1/images/vette_4_sig.jpg |/IMG| Red 1980 350 auto w/ quickshsift Edelbrock intake / carb Crane Fireball cam 0 - 60 6.3s

79 speedo and tach problems

Posted: 3/2/04 2:32am Message 7 of 8
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Ken/Theo, Thanks for your help. I haven't started on the speedo yet but there definitely is no filter on the tach so I need to get one before I can say there is anything else wrong, hopefully it was removed by a previous owner who didn't know what they were doing and fitting one will resolve the problem. Again thanks for your efforts. |thumb|


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79 speedo and tach problems

Posted: 3/2/04 6:07am Message 8 of 8
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SOUTHWICK, MA - USA
Joined: 6/26/2003
Posts: 336
Vette(s): 1975, L-48 coupe, 4-speed, light mods
There is an older instruction from Corvette Fever on this. It doesn't say too much about the Speedo, but does give a detailed description about rebuilding your tach.

http://www.3will.net/~jawillis/vette/tech/corvette_fever/jul97/eN.htm

Hope it helps.
Glenn


Glenn's Bright Blue 75 T-Top
L48, 4 Speed, Dual Exhaust

in Forum: C3 Electrical


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