Tachometer help
Thread Starter
Intermediate
Joined: Aug 2009
Posts: 30
Likes: 0
Tachometer help
I have an '81 Vette that I'm rebuilding and have had the Tach looked at and "worked on" by supposed Experts. They installed a new cable/drive and it's all still irregular.
The Tach will lay dead and stay there OR sometimes work, sort of, where it will limp up a couple of times then die. OR seem to run at 2 - 3000 rpms. - But that doesn't seem to be the true rpm.
Those experts say I need a new Tach.
What does that really sound like is needed?
Greatly appreciate all thoughts.
Thank you.
G
The Tach will lay dead and stay there OR sometimes work, sort of, where it will limp up a couple of times then die. OR seem to run at 2 - 3000 rpms. - But that doesn't seem to be the true rpm.
Those experts say I need a new Tach.
What does that really sound like is needed?
Greatly appreciate all thoughts.
Thank you.
G
Former Vendor
Joined: Apr 2009
Posts: 766
Likes: 3
From: Charlotte North Carolina 704-394-5150
The filter mounted on the intake manifold that plugs into your distributor could be a possible cause....broken wires at it. OR more likely the printed circuit board on the back of your tachometer itself ( needs to be removed and checked) is burnt...which they often times do. OR you have a problem in the flexible printed circuit board that is mounted on the backside of your speedometer/tachometer assembly is damaged.
"DUB"
"DUB"
Last edited by grandmastercorvette; Oct 1, 2009 at 07:39 PM. Reason: typo
Le Mans Master
Joined: Jul 2009
Posts: 6,647
Likes: 1
From: East Aurora N.Y.
St. Jude Donor '10
....
Last edited by jeff79'; Oct 1, 2009 at 07:57 PM.
Former Vendor
Joined: Aug 2006
Posts: 76,656
Likes: 1,853
From: Jeffersonville Indiana 812-288-7103
St. Jude Donor '08-'09-'10-'11-'12-'13-'14-'15
G,
The tachometer in your car works off 12 volts, a ground and what is called a square wave signal generated by the distributor. The 12 volts and the ground are supplied to the tachometer by the printed circuit on the back of the speedometer and tachometer housing. The signal is supplied from the distributor through a filter and then straight to the tach.
The wire locations can be found below.
18243
This signal passes through a tach filter as mentioned above, mounted on the intake manifold. This filter is only there to keep electrical static from the ignition system from interfering with the signal to the tachometer.
The tach filter should have a ohms reading from end to end around 500 ohms, so you can test it and see if the filter is good from end to end by using this figure. If you test the filter and it’s bad replace it. You can also do a simple by-pass of the tach filter by unplugging it from the distributor and the main signal feed wire. Then plug the main feed wire up to the distributor direct. This will not hurt anything for testing and if the tach comes to life you’ll know the filter is the problem. If the wire in the filter is loose or breaking this would cause the tach to function as you described above.
It sounds to me like either the wire is loose at the filter, or one of the associated connections. If the wire is broken from the end of the tach filter, you can in some instances strip off the end of the wire and drip a tiny bit of solder in there to attach the wire back. If this fails, I do have the filters in stock and you'll find them priced for forum members at this link: http://willcoxcorvette.com/product_i...ducts_id=26320
If you need to pull the tach out, always remember you can do this from the front of the dash without pulling the speedometer/tachometer housing. To remove the tach, simply remove the screws from the front lens and then the three screws that hold the tachometer in housing. When you have these three screws out of the car you can then pull the tach forward and remove it from the dash.
If you need to perform more testing and you have the tachometer out of the dash look inside. You’ll see three metal clips that the studs on the back of the tachometer pass through. You can test on these clips to see if the tachometer is getting power and ground. You can also verify the wire from the clip to where it connects to the tach filter by doing a continuity check from the input signal clip to the connection before the tach filter. The number 1 clip is your power clip, the number 2 clip is your signal clip and the number 3 clip is your ground clip.
If you have power, if you have good ground and you have continuity, then the problem is in your tach board or the tachometer electrical movement.
If you need more help post again!
You might guess from the pictures above.... along with working on the new tach boards, I'm also working on a new help and repair for tachometer diagnostics! It's not far from being done either!
Willcox
The tachometer in your car works off 12 volts, a ground and what is called a square wave signal generated by the distributor. The 12 volts and the ground are supplied to the tachometer by the printed circuit on the back of the speedometer and tachometer housing. The signal is supplied from the distributor through a filter and then straight to the tach.
The wire locations can be found below.
18243This signal passes through a tach filter as mentioned above, mounted on the intake manifold. This filter is only there to keep electrical static from the ignition system from interfering with the signal to the tachometer.
The tach filter should have a ohms reading from end to end around 500 ohms, so you can test it and see if the filter is good from end to end by using this figure. If you test the filter and it’s bad replace it. You can also do a simple by-pass of the tach filter by unplugging it from the distributor and the main signal feed wire. Then plug the main feed wire up to the distributor direct. This will not hurt anything for testing and if the tach comes to life you’ll know the filter is the problem. If the wire in the filter is loose or breaking this would cause the tach to function as you described above.
It sounds to me like either the wire is loose at the filter, or one of the associated connections. If the wire is broken from the end of the tach filter, you can in some instances strip off the end of the wire and drip a tiny bit of solder in there to attach the wire back. If this fails, I do have the filters in stock and you'll find them priced for forum members at this link: http://willcoxcorvette.com/product_i...ducts_id=26320
If you need to pull the tach out, always remember you can do this from the front of the dash without pulling the speedometer/tachometer housing. To remove the tach, simply remove the screws from the front lens and then the three screws that hold the tachometer in housing. When you have these three screws out of the car you can then pull the tach forward and remove it from the dash.
If you need to perform more testing and you have the tachometer out of the dash look inside. You’ll see three metal clips that the studs on the back of the tachometer pass through. You can test on these clips to see if the tachometer is getting power and ground. You can also verify the wire from the clip to where it connects to the tach filter by doing a continuity check from the input signal clip to the connection before the tach filter. The number 1 clip is your power clip, the number 2 clip is your signal clip and the number 3 clip is your ground clip.
If you have power, if you have good ground and you have continuity, then the problem is in your tach board or the tachometer electrical movement.
If you need more help post again!
You might guess from the pictures above.... along with working on the new tach boards, I'm also working on a new help and repair for tachometer diagnostics! It's not far from being done either!
Willcox
Last edited by Willcox Corvette; Jan 30, 2010 at 03:34 PM.
