Tach testing

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JimG
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Tach testing

Post by JimG » Sun Aug 11, 2019 8:14 pm

How can i bench test a tach? I bought one at Carlisle but would like to know that it works before removing the cluster.
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cmouta
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Re: Tach testing

Post by cmouta » Mon Aug 12, 2019 8:14 am

You need a a 12VDC power supply and a Function Generator that can produce square waves with a 10V+ amplitude. I thought I had it shown somewhere but the 3 posts on the back of the tach are 12V power, Signal, and Common. Your signal common and power supply common will both hook up to the same post.

Then use this chart to inject frequencies that correspond to RPM

The pulse frequencies are derived by a four cylinder engine firing twice per revolution so 2/60 * RPM = Frequency input.

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Re: Tach testing

Post by my8950 » Wed Oct 02, 2019 3:22 pm

Function generator would be helpful for a lot of things, but hard to justify huge cost for just a square wave...

JimG
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Re: Tach testing

Post by JimG » Wed Oct 02, 2019 4:47 pm

Does anyone know of someone who could repair and calibrate the Tach?
Jim Gosses
86 XR4TI, 165K, T-5, IC, RR, PBR Fronts, T-bird Rear Discs
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Ed Lijewski
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Re: Tach testing

Post by Ed Lijewski » Wed Oct 02, 2019 7:18 pm

This device which creates a square wave is inexpensive to make. I used one to test a Scorpio speedo/odometer.

viewtopic.php?f=29&t=29946&p=273678&hil ... or#p273678

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whitelx
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Re: Tach testing

Post by whitelx » Thu Dec 12, 2019 7:57 pm

So does the XR tach take 12v or 5v? I have a signal generator that can change duty cycle and frequency, but not voltage. I have an ATX power supply breakout board that will give me 12v, 5v, and 3.3v. I just need to know what I feed the tach with on each of the posts.

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cmouta
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Re: Tach testing

Post by cmouta » Thu Dec 12, 2019 9:15 pm

The tach needs 12V for power and the signal needs to be higher than 10V(I'm assuming it should be 12V square wave but the most I could get out of my Fluke signal generator was 10V and it seemed to work ok, anything less did not register on the tach properly.)

If you turn the tach over I'm 100% sure the highest post is sig and 95% sure the middle is 12V and the bottom post is 0V. You'll tie both your signal generator 0V and power supply 0V to this.

I'm pretty confident on those positions based on where it looks like my probes are in this photo
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1988 Mono Rosso Red Merkur XR4Ti - Sold!

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