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Discussion Starter #1
My car has the in dash tach which is cool but it doesn't work...
I have no idea how they are wired and need some help...

It obviously either is not hooked up or is broken...
What I need to know is can it be checked and how is it wired...

I have had the instrument panel out and have attached pictures of the back of the tach here...

The original engine is long gone and so is most of the original engine wiring
including wires that would have gone to the tach originally...

Any help given would be very appreciated....

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You can test the tach two different ways.

One way I did it years ago was with the tach out of the instrument panel and on a test bench at the shop I work for. I wired it up to a fused 12 volt power/ground supply and then with a signal generator was able to mimic a pulsed signal just like what would happen when the points open and close (or the ignition coil got triggered). I was able to do some math at the time and found out the tach was pretty close to being accurate.

Another test you could do is on the car under the hood BUT be careful. By getting a fused 12 volts and ground off of the cars battery you could then run a third wire to the coils negative terminal and see if the tach works after the motor is started.

You are going to have to trace down the wiring diagram and/or the flexible circuit board to figure out which terminal is the positive 12 volts and which one is the tach signal terminal but I know for sure the case of the tach is the ground.

Just looking at your pictures how the right terminal goes to one strip of the flexible circuit board and then to the main connector I would think that one marked + would be the terminal that connects to the negative side of the coil and then the left one how it jumpers in the flexible circuit board to one of the idiot lights, then I would think that one would be the power terminal. With the tach being bolted to the dash and the 1/4" tab on the right of the photo going to a ground behind the instrument panel, this is how the tach gets grounded.

Jim
 

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Looking at your pictures and how the one is labeled "+" and the other "S" I wonder if either they have the tabs on wrong on the gauge or the gauge is not designed for your dash. I would think if something was labeled with a "+" then this would be supply voltage and the one marked "S" would be the sending unit input. If it is labeled properly then it would be getting the wrong signals from the flexible circuit board. The "+" would be getting a signal from the negative side of the ignition coil and the "S" terminal would be getting an ognition 12 volts on it.

To me somethings wrong. What I don't know but somethings not adding up.

Anyway, here's a video I put together of some old pictures when I bench checked the tach for my 74 Nova.

https://www.youtube.com/watch?v=bNVFUTmsnSI&list=UUm48UxKs5y44AgBghahu-sw&index=1

Jim
 

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Look at the link below. It is of the engine wiriing and where the tach wire is in the firewall connector. You're looking for wire number 121, a BRN 18 gauge wire. It's a dashed line which indicates that it's an optional wire. Once you locate the position on the firewall connector, look at the dash wiring in the "best of electrical" sticky for the 1972 wiring. You should be able to follow that. I would say look at the 1974 wiring diagrahms but I think I need to resend the diagrahms since one is missing and one seems corrupted.

Good luck.

http://www.novareference.com/manuals/Wiring/1972/72-Nova-Wiring.pdf

I'll try and get the diagrahms resent. Doesn't look like there's too much difference between the 72's and 74's. Wiring wise.
 

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My car has the in dash tach which is cool but it doesn't work...
I have no idea how they are wired and need some help...

It obviously either is not hooked up or is broken...
What I need to know is can it be checked and how is it wired...

I have had the instrument panel out and have attached pictures of the back of the tach here...

The original engine is long gone and so is most of the original engine wiring
including wires that would have gone to the tach originally...

Any help given would be very appreciated....

[/IMG]


So after looking at my wiring diagrahms,

Your + and "S" terminals are backwards. After you trace out the signal wire you find that the wire #121 comes into the bottom circuit run all the way on the right. This would normally be wire #30, the signal wire, from the gas tank, if the fuel gauge was installed. The circuit run second from the right is 12v and the tach is grounded to the panel.

Hope this helps.

Chris
 

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Discussion Starter #6
Thanx guys....I apologize for not replying sooner but have not had the chance...
Perhaps this will clear up any confusion....my car is an SS clone....

The guy that did it was pretty thorough and hence did the in dash tach along with console that contains, among the others, the fuel gauge...

After reading your posts, I realize now what is probably going on...
He apparently just mounted the tach and never bothered, or couldn't figure out the wiring...

So...I guess now the question is what is the easiest way to wire it....?
Should I try to trace wires and go from there or would I be better off just running new wires to and from it..?

Thank you so much for your help and I am sorry if I cause confusion and unnecessary mind work by not telling this up front...:eek:
 

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Thanx guys....I apologize for not replying sooner but have not had the chance...
Perhaps this will clear up any confusion....my car is an SS clone....

The guy that did it was pretty thorough and hence did the in dash tach along with console that contains, among the others, the fuel gauge...

After reading your posts, I realize now what is probably going on...
He apparently just mounted the tach and never bothered, or couldn't figure out the wiring...

So...I guess now the question is what is the easiest way to wire it....?
Should I try to trace wires and go from there or would I be better off just running new wires to and from it..?

Thank you so much for your help and I am sorry if I cause confusion and unnecessary mind work by not telling this up front...:eek:
No need to apoligize! We're all here to help.
The new wiring diagrams for the 74 Nova will be in the "Best of Electrical" section soon. When I originally sent them in and I forgot to add the engine bay and the front half of the dash harness pages. The optional SS wiring is all the same (I believe) from 68-74. These will all be indicated on the diagrams as dashed lines. Trace your wires out. My guess is your fuel tank sending unit is still going to the back of the dash and not the console. So the next question is, is your gas gauge in the console working?

I too, am in the process of adding the SS wiring to the Wife's 74. It's not an easy chore. Go slow, take your time, your rewards will be great.

Let us know how you makeout.
 

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There may be a couple of things that need to be done.

If it was mine and not knowing what all was going on, I would find out how your tach needs to be wired.

The tach from what I can see in the pictures is not like the backside of the factory Nova tach's I've seen and looks to be aftermarket. Now I have not seen every factory tach out there but it just doesn't look factory so I'm going to assume it's an aftermarket.

If on the tach the terminal marked "+" should have ignition 12 volts applied to it and the "S" terminal should go to the negative side of the ignition coil, then to do this using the existing flexible circuit board, somehow we have to flip these connections. The connections off of the circuit board "could" be used but even if a person flips the connections with the car being converted from an indash fuel gauge that was in that location, the sender wire instead of going to the gas tank sender needs to be rerouted to the negative side of the ignition coil.

Let's say a person wanted to flip the connections off of the flexible circuit board to the terminals on the aftermarket indash tach. If it's possible I would look to see if the L shaped bracket/terminals off of the back of the tach could be removed. If so then a person could in one method make up short jumper wires with ring terminals on the ends to go from the left stud on the back of the tach to the right part of the flexible circuit board and then with another jumper wire with ring terminals on it go from the right stud on the back of the tach to the left part of the flexible circuit board. By doing this then the connections from the flexible circuit board are flipped to the connections on the back of the tach.

My concern here is while the studs and nuts on the back of the tach would keep things from shorting out to connect the jumper wire with ring terminals onto the flexible circuit board you would need some type of small and short nut and bolt setup. The flexible circuit board, while designed to be flexible, probably would not like these nuts and bolts hanging off of it and if left to dangle, probably would eventually break the flexible circuit board.

Another option to make up jumper wires to reverse the connections would be to take a length of stranded wire and strip off the insulation and then solder the wire to make it stiff. Now on the one end a ring terminal could be crimped onto the end and then soldered to make a good solid connection on that end. Now you could take this and temporarily bolt it to the left stud on the backside of the tach and then see how far up you would need to put in a 90 degree bend. After bending it and cutting it to length a second ring terminal would then be crimped and soldered onto this. Now the assembly would have two ring terminals on a short section of soldered wire. Basically making this "L" shaped connector like what was removed off of the tach originally but now crossing each other in an X fashion. To insulate it, a short section of shrink tubing could be slid over the assembly and shrunk down leaving just the ends with the ring terminal holes exposed. With this one connector in place a second one would have to be made in a similar fashion but since this will cross over the other one it may have to have a bend or kickout on it to where even if both were insulated there would be no chance of contact.
With these two stiff connectors in place then I would think this would place no undue stress on the flexible circuit board AND if you ever get a factory tach it could easily be converted back to original/stock setup.

One thing that concerns me is possible damage that has occurred to the tach being wired wrong. It may be fine, but may not be so you may want to bench check your tach.

The last thing that needs to be done is to trace down the original gas tank sending unit wire in the harness plug from the instrument panel and then instead of this wire going to the gas tank sender it needs to be rerouted to the negative terminal of the ignition coil. On my 74 instrument panel harness plug I went a few inches away from the connector, cut the brown original gas tank sending unit wire and then on the side of the wire that went to the instrument panel plug I extended this wire to the negative side of the ignition coil. On the wire side that went to the gas tank sending unit I then extended this wire and routed it to the console fuel gauge.

Jim
 

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Discussion Starter #9
No need to apoligize! We're all here to help.
The new wiring diagrams for the 74 Nova will be in the "Best of Electrical" section soon. When I originally sent them in and I forgot to add the engine bay and the front half of the dash harness pages. The optional SS wiring is all the same (I believe) from 68-74. These will all be indicated on the diagrams as dashed lines. Trace your wires out. My guess is your fuel tank sending unit is still going to the back of the dash and not the console. So the next question is, is your gas gauge in the console working?

I too, am in the process of adding the SS wiring to the Wife's 74. It's not an easy chore. Go slow, take your time, your rewards will be great.

Let us know how you makeout.
"So the next question is, is your gas gauge in the console working?"
Yes it is working.....and thank you very much for your help...:D
 

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Discussion Starter #11
There may be a couple of things that need to be done.

If it was mine and not knowing what all was going on, I would find out how your tach needs to be wired.

The tach from what I can see in the pictures is not like the backside of the factory Nova tach's I've seen and looks to be aftermarket. Now I have not seen every factory tach out there but it just doesn't look factory so I'm going to assume it's an aftermarket.

If on the tach the terminal marked "+" should have ignition 12 volts applied to it and the "S" terminal should go to the negative side of the ignition coil, then to do this using the existing flexible circuit board, somehow we have to flip these connections. The connections off of the circuit board "could" be used but even if a person flips the connections with the car being converted from an indash fuel gauge that was in that location, the sender wire instead of going to the gas tank sender needs to be rerouted to the negative side of the ignition coil.

Let's say a person wanted to flip the connections off of the flexible circuit board to the terminals on the aftermarket indash tach. If it's possible I would look to see if the L shaped bracket/terminals off of the back of the tach could be removed. If so then a person could in one method make up short jumper wires with ring terminals on the ends to go from the left stud on the back of the tach to the right part of the flexible circuit board and then with another jumper wire with ring terminals on it go from the right stud on the back of the tach to the left part of the flexible circuit board. By doing this then the connections from the flexible circuit board are flipped to the connections on the back of the tach.

My concern here is while the studs and nuts on the back of the tach would keep things from shorting out to connect the jumper wire with ring terminals onto the flexible circuit board you would need some type of small and short nut and bolt setup. The flexible circuit board, while designed to be flexible, probably would not like these nuts and bolts hanging off of it and if left to dangle, probably would eventually break the flexible circuit board.

Another option to make up jumper wires to reverse the connections would be to take a length of stranded wire and strip off the insulation and then solder the wire to make it stiff. Now on the one end a ring terminal could be crimped onto the end and then soldered to make a good solid connection on that end. Now you could take this and temporarily bolt it to the left stud on the backside of the tach and then see how far up you would need to put in a 90 degree bend. After bending it and cutting it to length a second ring terminal would then be crimped and soldered onto this. Now the assembly would have two ring terminals on a short section of soldered wire. Basically making this "L" shaped connector like what was removed off of the tach originally but now crossing each other in an X fashion. To insulate it, a short section of shrink tubing could be slid over the assembly and shrunk down leaving just the ends with the ring terminal holes exposed. With this one connector in place a second one would have to be made in a similar fashion but since this will cross over the other one it may have to have a bend or kickout on it to where even if both were insulated there would be no chance of contact.
With these two stiff connectors in place then I would think this would place no undue stress on the flexible circuit board AND if you ever get a factory tach it could easily be converted back to original/stock setup.

One thing that concerns me is possible damage that has occurred to the tach being wired wrong. It may be fine, but may not be so you may want to bench check your tach.

The last thing that needs to be done is to trace down the original gas tank sending unit wire in the harness plug from the instrument panel and then instead of this wire going to the gas tank sender it needs to be rerouted to the negative terminal of the ignition coil. On my 74 instrument panel harness plug I went a few inches away from the connector, cut the brown original gas tank sending unit wire and then on the side of the wire that went to the instrument panel plug I extended this wire to the negative side of the ignition coil. On the wire side that went to the gas tank sending unit I then extended this wire and routed it to the console fuel gauge.

Jim
well....thanx for the help....
This is pretty extensive info and I want to read it over again later to further ingest the info...
I am concerned as well about possible damage that has occurred to the tach while being wired wrong...

I gotta tell you tho that I don't really give a darn about the flex board and will probably just disconnect it and wire the tach directly with fresh, new wire...
It is interesting tho that it is probably wired backwards...I will have to look more closely at that...
Thanx very much :)
 

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well....thanx for the help....
This is pretty extensive info and I want to read it over again later to further ingest the info...
I am concerned as well about possible damage that has occurred to the tach while being wired wrong...

I gotta tell you tho that I don't really give a darn about the flex board and will probably just disconnect it and wire the tach directly with fresh, new wire...
It is interesting tho that it is probably wired backwards...I will have to look more closely at that...
Thanx very much :)
Hopefully I've made sense. The picture you have just makes me wonder if damage could have occurred or not. There is nothing worse than assuming something may be right like your tach and you get everything else squared away and then it doesn't come to life.

On my 74 it did not have the factory indash tach or the console gauges and the original wiring harness was in real good shape so instead of ordering or tracking down a good used one to support the indash tach and console gauges I decided to modify my harness. In a couple of spots I cut the harness and rerouted wires and with a donor harness I was able to get the correct color and gauge wire through the bulkhead connector to connect to the ignition coil. I also modified my gauge pack and instead of retaining the factory AMP gauge I installed a volt gauge.

If you do decide to abandon the two connections off of the flexible circuit board, make sure they cannot short out against anything and on the power wire for the tach, make sure it is properly fused.

If you do decide to run two new wires from the terminals on the tach, make sure the 1/4" tab in the right corner of your one picture of the instrument panel goes to a good ground. If you try and test the tach with just two new wires on it and the ground not connected it may try and find a ground through some other circuit and this may or may not allow the tach to operate properly.

Jim
 

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Discussion Starter #13
Hopefully I've made sense. The picture you have just makes me wonder if damage could have occurred or not. There is nothing worse than assuming something may be right like your tach and you get everything else squared away and then it doesn't come to life.

On my 74 it did not have the factory indash tach or the console gauges and the original wiring harness was in real good shape so instead of ordering or tracking down a good used one to support the indash tach and console gauges I decided to modify my harness. In a couple of spots I cut the harness and rerouted wires and with a donor harness I was able to get the correct color and gauge wire through the bulkhead connector to connect to the ignition coil. I also modified my gauge pack and instead of retaining the factory AMP gauge I installed a volt gauge.

If you do decide to abandon the two connections off of the flexible circuit board, make sure they cannot short out against anything and on the power wire for the tach, make sure it is properly fused.

If you do decide to run two new wires from the terminals on the tach, make sure the 1/4" tab in the right corner of your one picture of the instrument panel goes to a good ground. If you try and test the tach with just two new wires on it and the ground not connected it may try and find a ground through some other circuit and this may or may not allow the tach to operate properly.

Jim
I actually planned on cutting the flex board tabs like you said and my 1/4 inch tab is nicely grounded...
In a few weeks I will be doing my 4 speed conversion and I plan on doing this while I am under the dash doing the pedal swap....thanx again for your help :)
 

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Discussion Starter #14
OK...the day has come...I have started the ball rolling removing the engine...
Before disconnecting stuff (battery etc) I tried wiring the tach...

One this I forgot to mention before was that I have an MSD 6A box...
[/IMG]
Seeing as this was where the fuel gauge was, the flex board connections are not correct so I decided to just cut those two pieces off and hard wire it directly...
I proceeded to wire as described earlier in the thread and ran two test wires...
on the tach the terminal marked "+" to ignition 12 volts and the "S" terminal to the negative side of the ignition coil...
Nothing...
Then I remembered that the MSD has a tach blade connector so I moved the wire from the coil to the MSD box tach connection....Nothing...

Then I figure that either the tach is burnt or the wires are wrong so I thought what the heck and switched the wires and walah! I got some action :)
I hit the key and it went right to 500-600 rpms...which was great...
Then when I revved the engine a little it went up and wouldn't come back down...
Turned it off and tried again and it works but erratic...????
I am told that sometimes these MSD boxes need adapters or filters to work properly...
Any ides or thoughts on this...?
 

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Piece of cake: s-Tach Neg, +-Pos.12V engine running, center nut- ground
 

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OK...the day has come...I have started the ball rolling removing the engine...
Before disconnecting stuff (battery etc) I tried wiring the tach...

One this I forgot to mention before was that I have an MSD 6A box...
[/IMG]
Seeing as this was where the fuel gauge was, the flex board connections are not correct so I decided to just cut those two pieces off and hard wire it directly...
I proceeded to wire as described earlier in the thread and ran two test wires...
on the tach the terminal marked "+" to ignition 12 volts and the "S" terminal to the negative side of the ignition coil...
Nothing...
Then I remembered that the MSD has a tach blade connector so I moved the wire from the coil to the MSD box tach connection....Nothing...

Then I figure that either the tach is burnt or the wires are wrong so I thought what the heck and switched the wires and walah! I got some action :)
I hit the key and it went right to 500-600 rpms...which was great...
Then when I revved the engine a little it went up and wouldn't come back down...
Turned it off and tried again and it works but erratic...????
I am told that sometimes these MSD boxes need adapters or filters to work properly...
Any ides or thoughts on this...?
MSD boxes sometimes need an adapter.

http://www.msdignition.com/products/accessories/tach_adapters/
 

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Discussion Starter #17
Thank you both for your help...
I have some follow up questions...

1..."center screw is ground"...I see a center screw but it is tiny and almost flush with the back of the tach...it looks to be brass...
[/IMG]
Do I have to run a ground wire from it to ground...?
Would not being grounded cause it to work erratically...?
The screw almost looks like one that is not to be messed with...looks more like a set screw...

2...If I accidentally get the s and the + wires on opposite...will it hurt the tach.?

3...With the set up I have can't I just run a wire to the coil instead of the terminal on the MSD box...?

Thanx again for your help :)
 

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Thank you both for your help...
I have some follow up questions...

1..."center screw is ground"...I see a center screw but it is tiny and almost flush with the back of the tach...it looks to be brass...
[/IMG]
Do I have to run a ground wire from it to ground...?
Would not being grounded cause it to work erratically...?
The screw almost looks like one that is not to be messed with...looks more like a set screw...

2...If I accidentally get the s and the + wires on opposite...will it hurt the tach.?

3...With the set up I have can't I just run a wire to the coil instead of the terminal on the MSD box...?

Thanx again for your help :)
Most gauges ground connection is in it's housing and the housing needs to be grounded. With a factory tach the metal housing of the tach is bolted to the speedometer cluster and then there is a ground wire coming off of the speedometer cluster that is connected to a spot behind the instrument panel. If you think there is a poor ground through the speedometer cluster to a spot behind the dash a person could make up a jumper wire with ring terminals on the ends to where one ring terminal would be under a mounting screw of the tach housing and then the other end to a clean metal structure behind the dash. Too many grounds will not hurt anything.

NOW that we know there is an MSD box in the mix from what little I know is you CANNOT use the negative terminal on the coil for a signal to the tach.
From what I remember the MSD systems utilizes a higher voltage to the coil than the original 12 volts and ground. I could be wrong but there should be a tach terminal or wire on or off of the MSD box to connect a tach to for the proper signal.

It's hard telling how well gauges are protected against reverse voltage, improper voltages, and so on. Sometimes a fuse blows and once things are corrected the device works again and even if no smoke was let out of the guts of the device it still might have had damage done to it's circuitry. With two connections a person has a 50/50 chance of getting it right and the more connections the higher the odds of guessing the correct way is should be wired. If I had a customer in your situation I would see if there would be a way to bench check the gauge to eliminate variables that may be existing in the vehicle.

Jim
 
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