Tachometer Testing
Thread Starter
Instructor
Joined: Jun 2010
Posts: 120
Likes: 1
From: CT
Tachometer Testing
Sold my 75 L-82 and bought a nice 71 4-spd base, motor back in August.
I have the interior disassembled (for the most part)to redo some items. When I remove the right side dash panel, I plan to test the tachometer, since it bounces around so erratically, even at idle.
I thought about attaching a drill to the back of the tach for testing:
1. I would guess it is a 1:1 ratio off of the distributor?? Am I correct?
2. What direction would it rotate - looking from the back?
Seems like I could find a way to adapt something to the back of the tach with some resourceful ingenuity in the workshop. Anyone ever try this before? I think it would help me isolate to the culprit. (tach or cable)
I have the interior disassembled (for the most part)to redo some items. When I remove the right side dash panel, I plan to test the tachometer, since it bounces around so erratically, even at idle.
I thought about attaching a drill to the back of the tach for testing:
1. I would guess it is a 1:1 ratio off of the distributor?? Am I correct?
2. What direction would it rotate - looking from the back?
Seems like I could find a way to adapt something to the back of the tach with some resourceful ingenuity in the workshop. Anyone ever try this before? I think it would help me isolate to the culprit. (tach or cable)
Le Mans Master
Joined: Dec 2006
Posts: 8,837
Likes: 1,028
It would be a good idea to pull it out, clean it with brake cleaner, then grease it and slide it back in. The old grease can bind it and make it "flippy."
Thread Starter
Instructor
Joined: Jun 2010
Posts: 120
Likes: 1
From: CT
Tachometer Testing
Maybe I can simply test the cable that way by itself.
Racer
Joined: Dec 2008
Posts: 297
Likes: 19
From: arlington TX
Sold my 75 L-82 and bought a nice 71 4-spd base, motor back in August.
I have the interior disassembled (for the most part)to redo some items. When I remove the right side dash panel, I plan to test the tachometer, since it bounces around so erratically, even at idle.
I thought about attaching a drill to the back of the tach for testing:
1. I would guess it is a 1:1 ratio off of the distributor?? Am I correct?
2. What direction would it rotate - looking from the back?
Seems like I could find a way to adapt something to the back of the tach with some resourceful ingenuity in the workshop. Anyone ever try this before? I think it would help me isolate to the culprit. (tach or cable)
I have the interior disassembled (for the most part)to redo some items. When I remove the right side dash panel, I plan to test the tachometer, since it bounces around so erratically, even at idle.
I thought about attaching a drill to the back of the tach for testing:
1. I would guess it is a 1:1 ratio off of the distributor?? Am I correct?
2. What direction would it rotate - looking from the back?
Seems like I could find a way to adapt something to the back of the tach with some resourceful ingenuity in the workshop. Anyone ever try this before? I think it would help me isolate to the culprit. (tach or cable)
Hi,
The cable is usually the problem. In the stock position, the tach cable has to make a pretty severe bend coming off the back of the distributor. 2 cures for this -
- the Corvette vendors used to sell a 'right angle' adapter for the output from the distributor to connect the tach cable to eliminate the bend.
- I solved my bend issue by running the plug wires one position over on the top of the dist cap and then re-timing the car by rotating the dist around. The bend was gone and no money spent.
Good luck.
walt z.
Advanced
Joined: Aug 2016
Posts: 99
Likes: 10
From: Denver CO
The cable will basically be straight when you have the drill on it instead of the distributor, and the drill speed will be more constant so if its working right you will know its not either of those things, so do that before trying to test them individually. I read alot of people having issues with the bend going into the distributor, and make sure the square end on the cable is good.
When I fixed the cable on mine (was disconnected by previous owner) I also put the distributor in the position where the cable was least bent and then set the timing.
I would do that before getting a 90 degree adapter. Mine did bounce around a little bit too. Recently I put in a '76 electric tach (total pain) but now its much more accurate and smoother.
When I fixed the cable on mine (was disconnected by previous owner) I also put the distributor in the position where the cable was least bent and then set the timing.
I would do that before getting a 90 degree adapter. Mine did bounce around a little bit too. Recently I put in a '76 electric tach (total pain) but now its much more accurate and smoother.
Last edited by Ali Vette; Jan 12, 2017 at 07:28 PM.
Race Director
Joined: Oct 2008
Posts: 19,147
Likes: 1,667
From: Long Island
I just put a 90 degree fitting on a friends big block 72. I got it from Ricks Corvette for around $40 , It has a grease fitting. I believe Wilcox has them also.
You can "align" the tech drive if you play with the distributor and how the wires / rotor are in the cap - it's a PIA but I've done it.
Another reason can be worn gears or a bad fit into the distributor of the cable drive. The next reason is the ends of the cable get rounded and sometimes slip..
You can "align" the tech drive if you play with the distributor and how the wires / rotor are in the cap - it's a PIA but I've done it.
Another reason can be worn gears or a bad fit into the distributor of the cable drive. The next reason is the ends of the cable get rounded and sometimes slip..
Last edited by BLUE1972; Jan 12, 2017 at 09:13 PM.
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Le Mans Master
Joined: Jul 2006
Posts: 8,117
Likes: 104
From: charlotte north carolina
The ratio is less than 1:1. If I remember correctly, the speedos indicate 60 MPH at 1000 RPM- assuming an average car will turn 2700 RPM at 60 MPH, that gives an approximate 1:1.5 ratio (distributor turns half as fast as the crank) ratio. As posted, keep the cable straight to reduce wear on the cable AND the distributor cross drive gears. I would pull the distributor and check the cross gear, main shaft gear and bushings for wear-it's probably time for a rebuild.
