Adjusting a factory tach.

[custom10nut]

Do I remember right?
Is there a way to "dial in" the needle on a factory tach?
Mine was Spot On before I disassembled my dash to refurbish the gauges, but now it reads about 400 rpms too low. (My truck will not Idle at 250 on start up)
Is there an adjustment screw on the back? Or did I just get the two wires crisscrossed?

[custom10nut]

Found it.
I thought there was the adjustment screw.
It's kinda top center, above the two hook up tabs.
I attached a pic that explains it.
The thread says that the adjustment needs to be synced with a known source, like a digital tach or even a reliable dial one.

[LongBox]

Good to know, and thanks for posting. It is very tempting to add a tach to my dash cluster, and now I see how to calibrate it.

[custom10nut]

The conversion is easy to do. Just get the tach, the lens and the circuit sheet(board).
Run the two wires to the back and you're in business.
I did mine a long time ago, and have always been satisfied with it.

[Already Gone]

I wonder if that will cure my factory tach reading about 1000 rpms too high at highway speeds cause of my Petronix Distributor. Again 13 years on this site and again just learned something new.

[custom10nut]

It's worth a try. I agree with you and the wealth of knowledge and information on the forum. If I remember right I've been on here since 2007, and now I'm on here every day.

[ohboy321]

This is a very interesting thread. I have been trying to set my idle using my in-dash tach and have not been able to get it below ~800 RPM without stalling. Also, because my speedometer is off by quite a bit, I ran my specs through the summit gear ratio calculator and found that it wasn’t even close when compared to a GPS.

Can I connect the vintage tach/dwell meter pictured to the tach wire of my HEI distributor to verify the in-dash one is calibrated properly?

Thanks

[Coley]

Make sure you test the 'gain' after you set the lower 'zero' point. You might have to make a compromise if it doesn't rev up to upper rpm's correctly.
Coley

[custom10nut]

It's a two person job (unless somebody has another way),
I hooked up my multi-meter, and laid on my back and turned the screw, while he told me what it was reading. It's still about 150 low but I think it may be because of the connections.
I'll keep tinkering with it till I get get it to read correct, which I know it will, because it was right before I took the dash apart.

[davepl]

What you need is a frequency generator that can put out 500 and 5000Hz, then you can dial in the lower and the gain. I just can't think of one we all have kicking around the house... My scope has a test lug, but that's a fixed frequency.

[custom10nut]

I think I figured it out. I had the wrong wire on the COIL terminal, and it reads fine.
Right now I don't have the other wire hooked up. What do that one go to?

[custom10nut]

I found a good pic of the whole back of the tach.
One wire goes to the coil, and the othe to a 12v source.