Sounds a bit crazy, but does anyone know where the tach filter connections go to? Let me back up by saying that I've got an 81 with a crate engine. The tach only works 1% of the time, and only barely at that!

Anyway, did a bunch of searches and everything seemed to point to a bad tach filter, so I got a replacement. Did all the connections during the install - connected the one end to the distributor clip, the other end went to the dash.

But, lo and behold! The original filter had a wire coming off the distributor side that (after some internet research), I discovered should have a third connector! Mine was just a frayed wire leading to nowhere.

Checked my AIM but found nothing on where that third connector should go. Anyone have an idea??

BTW, the tach still doesn't work and I'm wondering if it has something to do with that missing third connection, or if the board is bad...

 

HMMM, isnt it supposed to have a ground to one of the corner intake bolts? Maybe I am thinking of something else.

 

Try using a jumper wire and bypass the filter then you will know if the tack board is bad. I ran without a filter for 1 year or so with no issues. I do have the filter in place now.

 

Plug the tach wire directly into the dist.

 

Thanks for the response!

81pilot - The frayed wire seems too short to be for a ground to the block. Besides, the filter is already bolted onto the block. Good thought, though. From the photos of original filters on 81s, there is clearly one connector for the firewall side, and two connectors on the distributor side of the filter.

spikebot81 - Well, I just went ahead and installed the new replacement filter and soldered in the old connectors to the new wires. Plugged the white end into the connector from the firewall, and plugged the brown wire end to the distributor. Just taped off the frayed end of wire that would have had the other connector.

Anyways, I fired up the old 81 and still no tach. So I guess that means the board is bad. Still bothers me about that third connector on the filter - wish I had an idea about where it's suppose to plug into on the distributor side of things. Just wondering if it is essential for the tach to work right on the 81s...

Anyway, I downloaded the tech paper on tach repair and board testing from Wilcox. Looks pretty comprehensive and will help greatly with the testing and replacement. Again, thanks to all.

 

Is the 3rd connector a tap for a technician to connect an external tach ?

 

Hmmm....Never thought of that! You know, that actually makes some sense. Thanks!

 

Maybe it went to the long gone computer??

 

Roger... on some cars there was another access point for diagnostics.


This should help you and see the info below!

The wire coming from inside the car to the distributor will be a white wire...
This wire runs from the printed circuit connector to the outside of the firewall and is the wire that supplies the signal to the tach. There is not a third connection to make the tach work.

The filter plugs in to the white wire and then to the distributor as illustrated above.

The tach only requires.. 12 volts, ground and signal. So if you want to know if the tach has power.. just turn the key on without starting the car! All tach's should go to zero when powered up if functioning properly.

I have some more info that may help you so I'm going to copy another thread that I answered questions on below this in another post.

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.

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

 

Hey y'all new 81 vette owner. I have fixed a lot of things by reading articles on here to fix my vette but the tach is still confusing me. I figured tach was bad so I ordered a new tach board from Wilcox after i installed it tach still didn't work. So I started testing wires like everyone explained but still don't work. I'm not good with electric but I hooked a test light up to the distributor wire that said tach and had power tested the battery wire and had power before the filter i had power but after the filter no power. So i plugged distributor directly into tach wire from fire wall and still tach didn't work. I take it the filter is bad and needs replaced but with a new tack board shouldn't it work if i plugged distributor directly into tach wire from fire wall. Don't know if it matters but once i got the vette found out that someone put an 83 camero 305 in the 81 vette. Thanks for any info anyone can give me