please reply if you have heard of this.

I have the 7720 and 7730 grid setup. I have the gray tach wire connected to my green autometer tach wire along with another wire connected to the tach output on my mega 450 delay box. I shift by rpm through the delay box. Last night I started my car and my tach was jumping all over the place but the engine was running fine. I proceed to make the run and my car did'nt shift. Afterwards I shut the engine off and the tach started working properly and the car shifted like it was supposed to. I checked all wiring and it is perfect.

Do ya'll think I have an issue with the grid, delay box or tach?

any help will be appreciated.

How long have you been running the Grid with your present tach and delay box?

I have been running the grid since September of last year. The tach and delay box have been in there 2 years now.

Intermittent electrical problems are hard to diagnose. If it continues to happen I would start with the tach. Use a toggle switch between the Grid Tach out and the Autometer Tach In. When it starts working erratic, toggle the tach input from Grid off and then back on and see if it fixes the problem. If not, make the run with tach input from Grid off and see if delay box will complete the shift.

Are you saying to just cut the gray wire and install a toggle switch in the middle before it connects to the green autometer tach wire and the delay box tach wire?

No. You want to turn off “only” the tach rpm feed, not the delay box rpm feed from Grid. What you want to do is isolate only one device at time of problem, and start with the tach.

Now I understand. Sorry about that. Thanks for your help. I greatly appreciate it.

I really should replace the tach anyway. It's a 3966 playback tach in which the playback never worked. Now that I have the grid I don't need the playback anyway but it obviously had issues before it was installed.

Those are nice tachs. Do you have a remote shift light hooked up to the tach ?? Is the TB hooked up to tach?

I don't have a shift light hooked up and at this time I don't have it connected to the transbrake but I did when I first installed it. Once the playback didn't work I disconnected it from the transbrake. I did all this before installing the grid. I used to use the non programmable digital 7 box.

Just a suggestion. You could assign the shifting to Grid.

I did think about that but I wasn't sure the grid isn't the problem. My thought process had me believing the grid sends the rpm signal to the delay box and the tach and even though the tach was erratic I figured the delay box would still make the shift happen and when it didn't I figured the grid had to be the problem.

I guess you're saying the tach could cause issues with the grid causing it to not send an rpm signal. Is that what you're saying?

I started one of my cars one morning and the tach was jumping all over the place like you described, engine was running fine. I shut it off to think about it, and grabbed the master kill switch and found that it wasn’t all the way on. Pulled it all the way on, fired the engine and the tach acted normally. Could be something simple like that, just a thought.

Ignition boxes use a signal generator to output a 12v square wave so a remote device can use to reference rpm. How that device processes this input probably varies. Determining which device (output or input) device is possibly causing the problem like I wrote earlier is harder. Wiring method could also be a factor.

If the tach is distorting the output signal from Grid, that could possibly cause it not to read properly in your delay box.

How many times has this erratic tach display problem happened?

183n-- that's a good thought but this happened just before going into the water box in a dragster. I feel certain the master switch would have vibrated off during the burnout. Good info though. thanks

mark-- The tach started jumping this weekend. It has worked properly up to this point and so has the shifting of the car.

to answer you question properly. The tach has only acted up 1 time and that was from starting the car to pull in the water box and then during the run. After I shut the engine off and restarted it the tach began working properly again.

If this were me, I would leave the present tach as is, and move the shift activation from delay box to Grid Shift Light Output Yellow wire. This is a Negative output from Grid and needs to activate a relay for the shift device. If you presently use a relay board with a shift device output built in, this is a simple termination.

With Grid activating the shift, this will be available in the Data Acquisition of your run and could be of help when trouble shooting a problem.

If the tach would become erratic during a run it should not effect the Grid Shift Light output.

Thanks. That helps me tremendously. I'll do excatly that.