View Full Version : how do I bench test a tach?


Steve's 74
01-01-2011, 10:37:34 PM
My tach has been sticking at about 4k. Well today I was rummaging through some old boxes and found another cluster with a tach. So I removed it from the cluster. Before I install it I wanna be sure 1- its for 8cyl and 2- it works smoothly and accurately through the rpm range.
I was thinking maybe first to test it off my current HEI tach terminal. What wires go where? iirc, the body is grounded, then there's these white and black wires on the back of the tach, are they 12v and tach terminal?

77RS
01-01-2011, 11:14:05 PM
Here is a pic that will help with the connections. The connections are the same for all 70-81 Camaro tachs (although the tach circuitry is different on 70-77 vs 78-81)

http://i415.photobucket.com/albums/pp233/77RS/gaugemarksCamarotachback.jpg

77nomad
01-02-2011, 03:14:44 AM
It should read 1800 rpm if hooked up to a 12v battery charger. I read up on this because I wanted see how to calibrate mine. I can't remember the math the guy used but when I looked at his it made sense. I was taking electronics math at the time if that helps. Something to do with household current cycling at 60Hz is all I can remember.

Steve's 74
01-02-2011, 10:34:14 AM
thanks

hogg
01-02-2011, 11:30:33 AM
grd body of tack, 12v to battery +,coil to dis.start car,hold tack in the up right position

77RS
01-02-2011, 11:45:29 AM
It should read 1800 rpm if hooked up to a 12v battery charger. I read up on this because I wanted see how to calibrate mine. I can't remember the math the guy used but when I looked at his it made sense. I was taking electronics math at the time if that helps. Something to do with household current cycling at 60Hz is all I can remember.

Our tachs require a pulsed DC signal to register RPMs. What you mentioned may work on other tachs, but not on 2nd gen GM. Although a battery charger recieves an alternating current (60 Hz), the output is DC (Direct Current non-pulsed). Hogg's instructions are the best way to do a home test of a tach. Otherwise you'll need specialized equipment.
Hope this helps.
Daniel

77nomad
01-02-2011, 10:30:31 PM
I was just repeating what I read. This is a copy and paste from that site. I in no way want to start an argument over this and am posting it because you said it wouldn't work because of the dc not being pulsed. The author states it is. I just want you opinion I guess. I know your background and respect it. I could take mine to school and bench test it but don't really know what freq. to set the generator at. My instuctor is the type that would make me figure it out and not tell me the answers. Sorry for the semi hijack btw I just thought if this works we could all use the information.




How to Check Your Tachometer

I was doubting my tach and was wondering how to test it to see if it is actually accurate. On the alt.autos.rod-n-custom newsgroup from 2001 (used www.dejanews.com to search), I found a discussion that talked about using a 12 volt battery charger to check the tach. So what the heck, I tried it.

The theory goes that a 120 Vac 60 Hz battery charger puts out an imperfect DC voltage. It is actually a pulsating DC voltage that corresponds to

* 1800 rpm for an 8 cylinder,
* 2400 for a 6 cylinder
* 3600 for a 4 cylinder

Well it works! When I hooked it up to my tach's input (+ve to tach +ve and -ve to gnd), it showed exactly 1800 rpm. The technical theory goes like this. The battery charger doesn't put out a DC signal, it is a 120 Hz full rectified waveform. When a battery charger is connected to a battery, the battery acts like a huge capacitor and smoothes out the waveform (simple explanation). I used a frequency counter to verify that the battery charger was putting out 120 Hz.

Here's the math:

120 Hz = 120 cycles/sec

120 cycles/sec * 60 = 7200 cycles/min

There are 8 cylinders, so 7200 / 8 = 900 cycles/min

but it is a 4 stroke engine which only fires on 1 firing stroke (combustion)

It fires once every 2 cycles

So 900 cycles/min *( 4 strokes/1 firing stroke) * (1 firing stroke/2 cycles) = 1800 cycles/min or 1800 rpm