Cable-driven Tachometer Accuracy
Thread Starter
Instructor
Joined: Mar 2017
Posts: 131
Likes: 3
From: Beaverton OR
Cable-driven Tachometer Accuracy
Hello, I'm new here and this is my first inquiry. Great forum BTW 
My '69 has the original cable driven tach and I don't believe the readings. At a nice idle, as seen as 700 RPM on a hand-held electronic unit, the dash tach reads under 500 RPM. While driving through the engine's speed range, the dash tach appears to read low (I didn't have the hand-held connected at that time) based on my ear expectations. Are the OEM dash notorious for not being accurate? Is there anything to be done about it, if mine is indeed reading low? Thanks for any help or insight.
My '69 has the original cable driven tach and I don't believe the readings. At a nice idle, as seen as 700 RPM on a hand-held electronic unit, the dash tach reads under 500 RPM. While driving through the engine's speed range, the dash tach appears to read low (I didn't have the hand-held connected at that time) based on my ear expectations. Are the OEM dash notorious for not being accurate? Is there anything to be done about it, if mine is indeed reading low? Thanks for any help or insight.
Safety Car
Joined: May 2004
Posts: 4,352
Likes: 1,579
From: los altos hills california
I believe the mechanism is something like a spinning magnet. The faster it spins the more the needle moves. Not much to tinker with.
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Team Owner
Joined: Jun 2001
Posts: 23,367
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From: Virginia USA
Mine is spot on according to a RPM meter I have that we used to use at work (no longer use it and they were going to throw it away so I asked if I could have it). We used this meter to setup some of our semiconductor equipment that needed precise RPM. How it works is you place a piece of reflective tape to the unit to measure and point the meter at it. Highly accurate.
My guess is if your gears, distributor and cable are worn you would see a difference and a bouncy needle. My speedometer was bouncing big time before I replaced the cable and now it's perfect.
Does your needle bounce?
My guess is if your gears, distributor and cable are worn you would see a difference and a bouncy needle. My speedometer was bouncing big time before I replaced the cable and now it's perfect.
Does your needle bounce?
Drifting
Joined: Jul 2015
Posts: 1,765
Likes: 254
From: Catlett VA
tach
Mine is spot on according to a RPM meter I have that we used to use at work (no longer use it and they were going to throw it away so I asked if I could have it). We used this meter to setup some of our semiconductor equipment that needed precise RPM. How it works is you place a piece of reflective tape to the unit to measure and point the meter at it. Highly accurate.
My guess is if your gears, distributor and cable are worn you would see a difference and a bouncy needle. My speedometer was bouncing big time before I replaced the cable and now it's perfect.
Does your needle bounce?
My guess is if your gears, distributor and cable are worn you would see a difference and a bouncy needle. My speedometer was bouncing big time before I replaced the cable and now it's perfect.
Does your needle bounce?
"My speedometer was bouncing big time before I replaced the cable and now it's perfect. " The needle on my 68 lags behind and bounces. I temporarily installed an electronic tach to help dx a high rpm problem on the car. Was thinking about replacing the cable- lubed it but not much difference. Did you replace the entire cable and exterior or just the actual cable? It's a PIA to get behind the dash if you can "snake" the cable in from the engine compartment side instead.
Team Owner
Joined: Jan 2006
Posts: 37,637
Likes: 3,116
From: Crossville TN
Most analog gauges have better accuracy at the upper end of the scale than they do at the low end of the scale. Seeing the tach off at idle really isn't of any concern...unless you use the car tach to set idle speed (not usually done). But, the upper end of the scale was usually within a 100 rpm (or so) of actual when the gauge was new.
It would be best to have a certified tach unit that you could monitor while driving at highway (or above) speeds so that you can assess your tach's accuracy after this many years of use.
It would be best to have a certified tach unit that you could monitor while driving at highway (or above) speeds so that you can assess your tach's accuracy after this many years of use.
Race Director
Joined: Jan 2000
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From: Corsicana, Tx
2020 C2 of the Year - Modified Winner
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You can have them rebuilt. The spring gets weak over time as well as the magnet. Mine was pretty accurate after rebuilding as long as I was normally driving. But when racing it lagged a good bit. Once on a chassis dyno we compared readings and it was about 600 RPM low at high speed. (6800 vs 7400)
JIM
JIM
Thread Starter
Instructor
Joined: Mar 2017
Posts: 131
Likes: 3
From: Beaverton OR
Mine is spot on according to a RPM meter I have that we used to use at work (no longer use it and they were going to throw it away so I asked if I could have it). We used this meter to setup some of our semiconductor equipment that needed precise RPM. How it works is you place a piece of reflective tape to the unit to measure and point the meter at it. Highly accurate.
My guess is if your gears, distributor and cable are worn you would see a difference and a bouncy needle. My speedometer was bouncing big time before I replaced the cable and now it's perfect.
Does your needle bounce?
My guess is if your gears, distributor and cable are worn you would see a difference and a bouncy needle. My speedometer was bouncing big time before I replaced the cable and now it's perfect.
Does your needle bounce?
Former Vendor
Joined: Aug 2006
Posts: 76,656
Likes: 1,851
From: Jeffersonville Indiana 812-288-7103
St. Jude Donor '08-'09-'10-'11-'12-'13-'14-'15
As one that has calibrated these gauges for over thirty years, I can say that in a perfect world the gauge can function with about 95% accuracy... In a perfect world. As far as a tach is concerned, once you move off idle speed the needle is constantly changing so a variance if you don't have a calibration machine is pretty hard to detect unless you use a digital gauge to check it.
The tachometer is magnetically driven just like the speedometer and while GM allowed a 3-5 */- margin for error in the calibration chart, you'll find that if you have a new head it'll be pretty darn close if calibrated correctly.
So what happens... Several things can happen over the years to impede the function and the most common issue is the magnetism of the head and the adjoining parts including the speed cup jewel plate bushing and the jewel bushing in the first worm and magnet, and also the condition of the first worm bushing.
I've attached a picture below of a speedometer to help you understand what these items are. A tach won't have the second, third or forth worm gears (they drove the odometer). The set up is exactly the same though.
IMHO
-Willcox
The tachometer is magnetically driven just like the speedometer and while GM allowed a 3-5 */- margin for error in the calibration chart, you'll find that if you have a new head it'll be pretty darn close if calibrated correctly.
So what happens... Several things can happen over the years to impede the function and the most common issue is the magnetism of the head and the adjoining parts including the speed cup jewel plate bushing and the jewel bushing in the first worm and magnet, and also the condition of the first worm bushing.
I've attached a picture below of a speedometer to help you understand what these items are. A tach won't have the second, third or forth worm gears (they drove the odometer). The set up is exactly the same though.
IMHO
-Willcox
Last edited by Willcox Corvette; Mar 15, 2017 at 12:58 PM.
Team Owner
Joined: Jun 2001
Posts: 23,367
Likes: 1,061
From: Virginia USA
"My speedometer was bouncing big time before I replaced the cable and now it's perfect. " The needle on my 68 lags behind and bounces. I temporarily installed an electronic tach to help dx a high rpm problem on the car. Was thinking about replacing the cable- lubed it but not much difference. Did you replace the entire cable and exterior or just the actual cable? It's a PIA to get behind the dash if you can "snake" the cable in from the engine compartment side instead.
Thread Starter
Instructor
Joined: Mar 2017
Posts: 131
Likes: 3
From: Beaverton OR
Well, finally the roads were dry and I took the C3 out for a drive. The dash tach actually reads within 100 RPMs of what it should based on speed (assume the speedo is correct!), gear ratios, tire size, etc. I think it just loses accuracy below about 1000-1500 RPM. I can live with that. Now, the hand tach unit I just bought (Actron CP7677 model) for tuning purposes doesn't read worth a hoot below 2500 RPM! I'm thinking a strobe might be the best method. I appreciated all of the replies.
Drifting
Joined: Mar 2011
Posts: 1,347
Likes: 121
From: Kernersville NC
Well, finally the roads were dry and I took the C3 out for a drive. The dash tach actually reads within 100 RPMs of what it should based on speed (assume the speedo is correct!), gear ratios, tire size, etc. I think it just loses accuracy below about 1000-1500 RPM. I can live with that. Now, the hand tach unit I just bought (Actron CP7677 model) for tuning purposes doesn't read worth a hoot below 2500 RPM! I'm thinking a strobe might be the best method. I appreciated all of the replies.
I have the Actron CP7677 and it works fine from idle and at least 3600 rpms. If yours is not working below 2500 rpm you should return it. I have about a 50-100 rpm difference at idle between the Actron and the mech tach. Not sure about cruise rpm but it seems correct based on calculations.
Thread Starter
Instructor
Joined: Mar 2017
Posts: 131
Likes: 3
From: Beaverton OR
I received a no-hassle replacement Actron CP7677 unit the other today! I sent an inquiry into tech support and they responded quickly stating the unit was most likely faulty. All I did was send in a photo of the unit and the Amazon receipt. No questions asked. Thumbs up to Bosch Automotive Service Solutions.
