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08-10-2020, 01:36 PM
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CC Member
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Join Date: Jan 2008
Cobra Make, Engine:
Posts: 351
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Not Ranked
tach and speedo problem
Tach- at running speed when tach should be at 2500 it vacillates between 2000 and 3000. if at 3000 goes between 2000 and 4000. then stops for awhile then does it again.
Speedo- will be running steady at 60mph then swings down to 20mph then up to 100 back to 20 then will steady out. does this periodically.
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08-10-2020, 04:53 PM
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CC Member
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Join Date: Nov 2003
Location: New Britain,
CT
Cobra Make, Engine: Size 10 Feet
Posts: 3,019
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Not Ranked
What brand instruments are they? Mechanical? Electrical?
What ignition system are you using?
Is the car stuttering when the gauges are malfunctioning?
What is the ammeter/voltmeter showing?
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08-10-2020, 04:57 PM
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CC Member
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Join Date: Nov 2003
Location: New Britain,
CT
Cobra Make, Engine: Size 10 Feet
Posts: 3,019
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Not Ranked
duplicate post...
Not my fault.
Last edited by strictlypersonl; 08-10-2020 at 05:00 PM..
Reason: Duplicate post
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08-10-2020, 07:23 PM
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CC Member
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Join Date: Aug 2007
Location: Carlsbad,
Ca
Cobra Make, Engine: SPF 2932 with 438 Lykins Motorsports engine. Previous owner of FFR 5452.
Posts: 2,616
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Not Ranked
If you have an electric speedo and tach, you’ve got a wiring problem. Probably loose connection in the fuse box.
When my tach fluctuated, that solved the problem.
If you’ve got a mechanical speedo, the problems are separate and unrelated.
__________________
Jim
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08-10-2020, 08:21 PM
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CC Member
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Join Date: Feb 2014
Location: White City,
SK
Cobra Make, Engine: West Coast, 460 CID
Posts: 2,908
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Not Ranked
Quote:
Originally Posted by Steve Gallegos
Tach- at running speed when tach should be at 2500 it vacillates between 2000 and 3000. if at 3000 goes between 2000 and 4000. then stops for awhile then does it again.
Speedo- will be running steady at 60mph then swings down to 20mph then up to 100 back to 20 then will steady out. does this periodically.
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Mine used to fluctuate as well, but that went away when I replaced my Ford large frame 1G alternator with a Ford 3G alternator. I don't know whether I had a bad alternator diode, bad grounds, poor wire connections or a flaky voltage regulator, but the problems all disappeared after changing the alternator and re-wiring for the 3G and its internal voltage regulator.
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Brian
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08-10-2020, 08:41 PM
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Full Blown Member
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Join Date: Sep 2008
Cobra Make, Engine: KMP 427 S/C, Twin Paxton 511 FE
Posts: 2,594
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Not Ranked
I had the same problem over 10 years ago in my (then) SPF. Blas told me to clean the contact on the distributor cap. Solved.
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rodneym
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08-11-2020, 06:15 AM
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CC Member
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Join Date: Sep 2002
Cobra Make, Engine: SPF#0760
Posts: 3,405
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Not Ranked
(Up-Dated)
For Tach: Erosion of the center contact graphite on the cap caused by wear or improper adjustment of the spring contact on the rotor.
For Speedo: Cable routing and/or cable lubrication. If replacement cable, could be a bit too long. If original cable in same position, untouched, add gauge to list.
Chassis numbers would be nice.
Blas
__________________
Wiring Diagrams: SPF MKII, MKIII, GT40, CSX7000, CSX8000, Corvette Grand Sport, and Shelby Sebring, Bondurant & Cinema Tribute Cars.
Owner’s Manuals: SPF MKII, CSX7000, CSX8000, Sebring, Bondurant, Cinema Tribute Cars $ GT40’s..
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SPFWiringDiagrams@Comcast.net
Last edited by Blas; 08-11-2020 at 07:31 PM..
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08-11-2020, 05:57 PM
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CC Member
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Join Date: Sep 2003
Location: Colgate,
WI
Cobra Make, Engine: FFR 4071, SHP 408 Carb dry sump, G-Force T-5, 4 into 4 headers, 3-link 8.8 with 3.55 gears in the rear
Posts: 56
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Not Ranked
If you are running mechanical gauges check to make sure they aren't bent to much. If the bend radius of the cables is too tight they will chatter and fluctuate as you described.
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08-14-2020, 07:04 AM
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Registered User
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Join Date: Feb 2018
Location: Pocatello,
ID
Cobra Make, Engine: Shell Valley, 428 SCJ
Posts: 2
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Not Ranked
Check your grounds. I had a similar issue.
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