Hello there,
I have an 82 vette that I think has some kinda elec. problem. Just put a new motor in and Im having a problem with the tach. I have noticed that while the tach is bouncing that I have to keep the idle up to keep it running, also when it bounces at some points whether idling or reved that it will just die. Some times when I start it the tach doesnt bounce and it runs perfectly fine. Its only when the tach bounces is that it acts up. Had similar problem before motor swap. Any help would be appreciated.

 

Could it be the tach board?

Will

 

Sounds like your engine might be changing speed rapidly and causing your tach to bounce.

Check for burnt plug wires, sticking/broken egr valve and other conditions that could cause your tach to bounce.

I mean, if it's bouncing between 500-700 rpm or so, then odds are there's something wrong with your ignition or carb. If it's bouncing 500-2000 rpm (something wild), then it's the board/wiring.

 

Do you have an after market ign box and coil? MSD states that if you have a tach filter, it could cause it to bounce and go to zero rpm reading and suggest to by pass the filter.

 

Did you put a crossfire engine back in or switch to something else?

When you say the tach bounces do you mean the engine idle changes?Or does the needle just bounce?

Jim

 

The engine idle never changes. When the tach bounces you have to keep it idled up, but it will just die out high idle or not. When the tach doesnt bounce it idles fine and does not die out. Took out the cross fire motor and put in a 350 crate motor all done up. its got an edelbrock intake and demon carb if that helps any

 

Are you using the same distributor? What did you do with the computer near the battery? You are obviously using the same tach and speedo. With the crossfire everything went through the ECM then through the Vehicle Speed Sensor and to the tach. I don't think you can do this with your current set up. Test it....disable the tach somehow and see if the problem presists. You need to bypass the ECM and VSS. I suggest you look at a wiring diagram for a 1980...pre-computer modle year and wire the tach up for a carb and HEI set up. I suspect you have a problem in this circuit. Try the tach filter as indicated above. You are getting interference somewhere related to the distributor.

Jim

 

Ok.. I'm going out on a limb on this one..

There are three wires that run to the tach.. Power, ground and signal.

The signal wire comes from the distributor.. and it sounds like your running (idle problem) is related to the response the tach is giving you.

So.. with this in mind.. Let's un-plug the tach from the distributor and see if the problem goes away as far as your idle is concerned.

If it clears up....

Next, check the terminal where the tach plugs in to the tach.. I've posted a picture below that will help you locate.

Make sure the terminal is not bent or making contact in some way with the power terminal for the distributor.

If this is ok.. you might check the tach wire for voltage before where it plugs i to the filter.. Possible short inside the car could back feed voltage down the tach wire to the distributor. Are you running a tach filter? In the dash, on the printed circuit board you'll notice the signal wire is sandwiched between the ground on top and the power on the bottom. If the power tab on the printed circuit is loose and hitting the signal tag.. this might be your problem.... (just a stab in the dark)..

I'd check the distributor coil while you are at it.. Just to make sure there is not an issue with this either. Possible voltage interference might be your problem.

I doubt the problem is the tach board.. it might be a problem with the printed circuit on the back of the speedo housing....

So with that in mind.. I've copied a long post I did a while back on the tachometer diagnostics.... the diagrams will help you if you need to go wire searching....

Copied Post.

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.



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.

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