Can you bench test a tach circuit board? Problems.
Thread Starter
Racer
Joined: Jun 2004
Posts: 252
Likes: 0
From: Shawnee KS
Can you bench test a tach circuit board? Problems.
I have a 75 with a non-functioning tach. Here are the steps I've taken:
1) Replaced board - no worky
2) Bypassed tach filter - no worky
3) Checked signal from coil - measures around 14 volts at idle, goes up with RPM
4) Checked signal right at the back of the tach - 14 volts and up
5) Reinstalled old tach - no worky either
I'm stumped now. Can I bench test these boards somehow? I know that Mike recalibrates tachs, but can the mechanism in the tach just go out? I'd like to test my boards before I send them off somewhere? Thanks in advance for the help. I'm determined to get this thing back on the road in time to avoid sweaty-leg weather here in KC!
1) Replaced board - no worky
2) Bypassed tach filter - no worky
3) Checked signal from coil - measures around 14 volts at idle, goes up with RPM
4) Checked signal right at the back of the tach - 14 volts and up
5) Reinstalled old tach - no worky either
I'm stumped now. Can I bench test these boards somehow? I know that Mike recalibrates tachs, but can the mechanism in the tach just go out? I'd like to test my boards before I send them off somewhere? Thanks in advance for the help. I'm determined to get this thing back on the road in time to avoid sweaty-leg weather here in KC!
Team Owner
Joined: Apr 2001
Posts: 54,204
Likes: 180
From: S.W. Ohio. . . . . . NRA Life Member
I don't know how to test just the board without a tach meter movement connected to it. What you can do is check your meter movement.
Remove the circuit board from the meter. Measure resistance between each of the 3 bare meter legs. I don't have my notes in front of me - but it's somewhere around 100 to 200 ohms. If the meter is bad, you'll usually get a complete open between two of the three legs.
you can do a basic bench test of the reassembled tach - by rotating the tach needle to the 3000 rpm position - and then apply 12 volts to 2 of the 3 rear terminals and see if the needle resets itself to zero:
Remove the circuit board from the meter. Measure resistance between each of the 3 bare meter legs. I don't have my notes in front of me - but it's somewhere around 100 to 200 ohms. If the meter is bad, you'll usually get a complete open between two of the three legs.
you can do a basic bench test of the reassembled tach - by rotating the tach needle to the 3000 rpm position - and then apply 12 volts to 2 of the 3 rear terminals and see if the needle resets itself to zero:
Pro
Joined: May 2003
Posts: 601
Likes: 0
From: Nottingham
Sorry if it's a stooopid question, but are you totally sure the ground is good for the tach? For that matter are you also sure the 12V supply is good? (If both of these are ok, the needle should snap back to zero when you turn the key on, like mike described....)
To be really sure about the actual tach signal, you will probably need an oscilloscope to measure the analogue voltage trace - it should oscilate between around 0 and 30V, if mine is anything to go by.
To be really sure about the actual tach signal, you will probably need an oscilloscope to measure the analogue voltage trace - it should oscilate between around 0 and 30V, if mine is anything to go by.
Le Mans Master
Joined: Jun 2002
Posts: 8,339
Likes: 24
From: I can walk to MA
You are right about using an oscilloscope. I can use one at work,
but most people are stuck with a DVM. The good thing is that
most good DVMs these days can measure frequency. A frequency
measurement coupled to an AC voltage measurement should
indicate a good tach signal.
I'm a EE, but never repaired or tested my tach. That will change
soon ... lurking here now to get some tips from experienced folk.
but most people are stuck with a DVM. The good thing is that
most good DVMs these days can measure frequency. A frequency
measurement coupled to an AC voltage measurement should
indicate a good tach signal.
I'm a EE, but never repaired or tested my tach. That will change
soon ... lurking here now to get some tips from experienced folk.
Drifting
Joined: May 2001
Posts: 1,651
Likes: 13
From: Holly MI
I tested my tach with a power supply from something like a cell phone. The tach needs to see a alternating voltage just like the distributor provides. Take a look at just about any power supply and see what frequency it puts out. It should be written on the power supply. When you hook the power supply up to the tach, the tach should read output frequency of power supply in Hz times 15. So, if you have a 60 Hz power supply, the tach should read 900RPM. This will let you know if the tach is working.
Ken
Ken
Drifting
Joined: Nov 2002
Posts: 1,665
Likes: 1
From: Ontario
One clue could be to see if the signal level is different with the board connected compared to it not connected. If the signal drops with the board connected it could be a weak coil, assuming the board is good. Is your replacement board brand new or used?
I seem to remember reading on this forum a long time ago something about not connecting/disconnecting your tach circuit with power on, something about causing damage but I don't remember the specifics...
I seem to remember reading on this forum a long time ago something about not connecting/disconnecting your tach circuit with power on, something about causing damage but I don't remember the specifics...
Team Owner
Joined: Apr 2001
Posts: 54,204
Likes: 180
From: S.W. Ohio. . . . . . NRA Life Member
I tested my tach with a power supply from something like a cell phone. The tach needs to see a alternating voltage just like the distributor provides. Take a look at just about any power supply and see what frequency it puts out. It should be written on the power supply. When you hook the power supply up to the tach, the tach should read output frequency of power supply in Hz times 15. So, if you have a 60 Hz power supply, the tach should read 900RPM. This will let you know if the tach is working.
The tach needs to see a alternating voltage
"Pulsed" IMO is a better term. "Alternating" can get confusing to some ... as in "Alternating Currect - AC" (AC versus DC). But your point and example are a good one.
So, if you have a 60 Hz power supply, the tach should read 900RPM. This will let you know if the tach is working.
correct, but PLEASE - no one should try to calibrate their tachometer using this method. Instead , use this as a basic go no-go test.
For bench calibration - use a square wave generator.
.
Last edited by Mike Mercury; Apr 21, 2005 at 09:07 PM.
Team Owner
Joined: Apr 2001
Posts: 54,204
Likes: 180
From: S.W. Ohio. . . . . . NRA Life Member
and one other suggestion, check the tach voltage with the engine running. Clip your volt meter directly to the tachs negative terminal - and directly to the tachs 12v input terminal. In other words, don't clip the negative lead to just to "any ol' nearby ground".
If your problem is a poor ground to the tach - and you clip your volt meters negative to another ground source (other than the tach ground terminal) - you're not doing a complete check of the ground source at the tachometer.
.
If your problem is a poor ground to the tach - and you clip your volt meters negative to another ground source (other than the tach ground terminal) - you're not doing a complete check of the ground source at the tachometer.
.
Last edited by Mike Mercury; Apr 21, 2005 at 09:08 PM.
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Burning Brakes
Joined: Sep 2004
Posts: 1,167
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From: Ft.Worth Texas
Originally Posted by bb69
I tested my tach with a power supply from something like a cell phone. The tach needs to see a alternating voltage just like the distributor provides. Take a look at just about any power supply and see what frequency it puts out. It should be written on the power supply. When you hook the power supply up to the tach, the tach should read output frequency of power supply in Hz times 15. So, if you have a 60 Hz power supply, the tach should read 900RPM. This will let you know if the tach is working.
Ken
Ken
Drifting
Joined: May 2001
Posts: 1,651
Likes: 13
From: Holly MI
Thanks, but I can't take the credit. My dad thought this one up. He is a practicing EE, and one of the best at coming up with great ideas like this. This also explains why I didn't get some of the language exactly right. I should have said pulsating voltage as mentioned above. However, this is a great way to test the tach. I was having problems getting the electric tach I swapped into my car working and we did this.
Ken
Ken
