Tachometer question

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rod7515

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I working with a 408 that has an aftermarket ignition box and an Autometer tach. I went out tonight and ran some quick wiring to connect the tach using the green wire to ignition box, red to switch and black to ground.
The first thing to tell you is that this tach has been a 2k since I bought it. So tonight I hooked the wiring together connecting the power supply last. As soon as the power wire was supplied the needle went to 10K and stayed there, immediately unhooked didnt see any smoke, no hot wires so I applied it again. This time the needle moved from the 10k to 0. So I started the car and it appears to work correctly and tracked what I would guess was the correct RPM. I only ran it for about 10 seconds. Turned the car off and the tach goes back to the 10k. No I will say I am not hooked to switch on the power wire, just a direct 12v feed. Once I remove the power feed the needle goes back to 2k but not to 0. Again the 2k is where the needle has been since I bought it. Tomorrow night I plan on hooking my timing light/tach up and verifying the correct RPMs.
But before I do that I wanted to check on the needle position. Also should I hook up the tach wire to the - coil side instead of the ignition box. I am hooked to the correct wire on the box according to the box schematic.
Your thoughts?
Thanks Rod
 
What do you mean "green to ignition box?" WHAT ignition box? WHAT WIRE on the ignition box?

If you have "switched" coil ignition such as points, HEI, ready to run, Mopar factory, etc, you hook the tach green (trigger) to the coil NEG

If you have an MSD box you DO NOT hook to the coil, and you MAY NEED a tach adapter.

Some tachs (some Autometer) do stop where they "were last" that is if the thing was idling at 900 when you shut it off, they may stay there
 
I am using an aftermarket MSD type of box. Heres a pic of the box and also a pic of the wiring diagram for the box.

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DSCN0075.JPG

The green wire from tach to the green wire from box.
Rod
 
My Autometer does that. It's normal for them.

I can check my tach through my ignition box and my tach is 200 RPM high at 1000 and about 100 high 2500. By 3000 is accurate to 8000 which is as high as I checked it.
 
I am using an aftermarket MSD type of box. Heres a pic of the box and also a pic of the wiring diagram for the box.
The green wire from tach to the green wire from box.
Rod

OK. In that case if it doesn't work, no clue
 
Hey Rod,

Just reading your post and have a few thoughts about it.

The first and most obvious to me is that the tach is not calibrated properly. Correct me if I'm wrong guys but the tach is a meter...no signal ,no pointer deflection. If it is already registering 2k with no signal, it is not calibrated to zero. If the tach shows zero when it is not hooked up, and goes to 2k after hooking it up but without starting the engine, it is getting a partial signal from somewhere else in the circuit.
 
I hope to hook up another tach tonight from my timing light to compare. If it's the same I guess I wont worry about it. If it's different maybe I will need another tach. What was really strange was that if I connected and disconnected the hot wire it would go from 0 or 10. As long as its not doing damage to anything I'll use the one that's on it. New ones are $250-$300!
 
Hey Rod,

Check out Autometer's website. They have troubleshooting videos there under the resources tab that may help you. After looking at it I really think you have a calibration problem that may be easily (and cheaply !! ) Fixed.

Resources - Installation Support

Good luck with it!!
 
Hey Rod,

Just reading your post and have a few thoughts about it.

The first and most obvious to me is that the tach is not calibrated properly. Correct me if I'm wrong guys but the tach is a meter...no signal ,no pointer deflection. If it is already registering 2k with no signal, it is not calibrated to zero. If the tach shows zero when it is not hooked up, and goes to 2k after hooking it up but without starting the engine, it is getting a partial signal from somewhere else in the circuit.


All Autometer tachs I've had do that. If you find the instruction sheet I'm sure it talks about that. It's NORMAL for an Autometer tach.
 
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It sounds like the gauge uses a stepper to drive the needle. That is typical for needle gauges in modern cars. They consist of a rotor with magnetic poles, and stator with coils. The coils are driven to step the rotor with a pulse. The electronics initializes the zero position, then steps and micro-steps for desired reading based on RPM. The movement does a fixed value of degrees movement per step, so the movement does not need calibration.
 
All Autometer tacos I've had do that. If you find the instruction sheet I'm sure it talks about that. It's NORMAL for an Autometer tach.
Hey Yellow rose,

That doesn't make sense to me. Rod is saying that his Autometer tach is already reading 2000rpm with no signal from the ignition , how would you know what the idle rpm is? Most engines idle at about 800-1000 rpm . Are you saying that the tach reading 2800-3000 rpm at idle is normal or am I not understanding something here?
 
It sounds like the gauge uses a stepper to drive the needle. That is typical for needle gauges in modern cars. They consist of a rotor with magnetic poles, and stator with coils. The coils are driven to step the rotor with a pulse. The electronics initializes the zero position, then steps and micro-steps for desired reading based on RPM. The movement does a fixed value of degrees movement per step, so the movement does not need calibration.

If this is how the Autometer tach electronics work, I understand that there is no calibration needed, however, as you stated, the electronics re-establish zero whenever 12 volts is applied so the tach should be at zero with the ignition power on without starting the engine. Great description of how the tach works by the way!
 
Just to make sure everyone does understand because sometimes I dont explain things well enough, Before any wiring is connected and tach was in box, needle was at 2k. Once I put 12v to tach without engine running tach jumps to 10K. remove 12v wire and tach went to 0, once started it appeared to function normally. I did not get to go out and ceck it yet tonight. Have to wait until The Voice is over! lol, I will be sure to go out tonight and see what happens again. I did this last night at 10:30 and only ran it for about 10 seconds.
Thanks for everyones help and thoughts on this.
Rod
 
Just to make sure everyone does understand because sometimes I dont explain things well enough, Before any wiring is connected and tach was in box, needle was at 2k. Once I put 12v to tach without engine running tach jumps to 10K. remove 12v wire and tach went to 0, once started it appeared to function normally. I did not get to go out and ceck it yet tonight. Have to wait until The Voice is over! lol, I will be sure to go out tonight and see what happens again. I did this last night at 10:30 and only ran it for about 10 seconds.
Thanks for everyones help and thoughts on this.
Rod


That is totally normal. I'm going to go out and take a picture of my tach in a minute and post it up. I don't remember exactly where the needle is but I know it isn't zero and it has no power to it.
 
image.jpeg
See if this works. So this is my tach. This one is correct at every RPM I tested it at. This is normal with no power to it.

I've had probably 75 Autometer tachs over the years, even a few that worked with magnetos and every one of them did this or similar.


It's perfectly normal for Autometer.
 
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View attachment 1715152697 See if this works. So this is my tach. This one is correct at every RPM I tested it at. This is normal with no power to it.

I've had probably 75 Autometer tach so very the years, even a few that worked with magnetos and every one of them did this or similar.


It's perfectly normal for Autometer.



Well, Yellowrose. I checked out your tach with Autometers website and it does say that the tach leaves zero when not powered up as you said. According to Autometer, the tach has an aircore meter that returns to normal when 12 volts is applied.

Thanks for sharing the pics to explain this .
 

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Well, Yellowrose. I checked out your tach with Autometers website and it does say that the tach leaves zero when not powered up as you said. According to Autometer, the tach has an aircore meter that returns to normal when 12 volts is applied.

Thanks for sharing the pics to explain this .



I tried to find that on their website but couldn't so I just took a picture.

I forgot about the aircore part of it or what it even does.
 
Well, I should have expected this. I hooked up my snap on timing light with tach/advance that has been in drawer for probably 15 years and it doesnt work! So I guess its time to buy a new light. I dont think I need a top of the line light so I guess I go shopping at summit and see what I can buy at a decent price. Noting ever seems to work out! lol
I did start it up and the tach appears to be functioning but when engine is off and power is still there the needle is at 8k. As soon as I begin to crank it over it goes to 0 and when engine fires it looks to be working properly. I'll try to get to dads tomorrow and borrow his timing light so I can compare rpms then.
Thanks Rod
 
Well, I finally got around to hooking up dads tach and my tach is spot on with his tach. So I guess I will accept the goofy needle position when car isnt running. I do want to thank everyone for all your help and especially for the education on how these things operate. Just another example of how this forum helps all!
Thanks Rod
 
Just a quick update update. I found that the tach staying at the 8k when shutting it off is due to the auxilary shift light setting. I looks as tho where ever you set the shift light to come on is where the tach will go when unpowered. I guess I will find out more once or if I ever get this thing on the road!
Rod
 
Just a quick update update. I found that the tach staying at the 8k when shutting it off is due to the auxilary shift light setting. I looks as tho where ever you set the shift light to come on is where the tach will go when unpowered. I guess I will find out more once or if I ever get this thing on the road!
Rod


In the racer forum a member has a thread a member has a start up video and you can see his tach. Most of my tachs have stopped at different RPM's at different times.
 
Rod what exact autometer tach is it? Mine will stay at the last position of rpm the engine is at when you shut it off but as soon as you turn the key on the gauge makes a full sweep then back to zero until the car is started
 
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