Tach pegs out with key on

[surfdogskier]

Hey guys. My tach is pegs out to the max when I turn on the key and stays there until I turn it off. I have had this issue the entire time I had the car. I finally removed the tach from the dash and check the voltage.

I have not had the black/purple wire connected from the - side of the coil because if it was connected, it would kill my car engine. So what I did today is I ran a single wire from the - coil to the back of the tach directly and it didn't kill the engine so it must be grounding somewhere. So I figured that out.

I have 12v going to it and checked this using the ground connected to it. So it is getting 12 volts. But just having these 2 wires connected, it pegs it to the max. When I unplug this 12v lead, it goes back to 0.

I plan on taking it apart to see if I see any issues. Anything specific I should look for. Or am I better looking for another one?

[Dave at Pelican Parts]

Hunt around the car for an extra non-factory switch. Or possibly a factory-style one repurposed into a kill switch. Grounding the tach wire is an easy way to implement one.

Check for popped fuses. I believe that one in particular (possibly #8 or #9) will give this strange behavior.

--DD

[surfdogskier]

Originally Posted by Dave at Pelican Parts

Hunt around the car for an extra non-factory switch. Or possibly a factory-style one repurposed into a kill switch. Grounding the tach wire is an easy way to implement one.

Check for popped fuses. I believe that one in particular (possibly #8 or #9) will give this strange behavior.

--DD

Hey Dave, Thanks for the info.

This is what I found. I had an old 911 tach laying around and I plugged it up. It did the same thing as my tach when I plug the wires based on the layout I had. Then I switch the 12v and the coil wire leaving the ground as it was. This is different from the diagrams I had. Now my tach registers when I turn the key to on to right around 5K rpm without the engine on. Start it and when I press the accelerator, it will work as normal but my starting position is 5K or so. This is for both tachs.

Can I adjust this to where I can go back down. I notice a center screw on the back of the tach and want to know if this is a adjustment screw before I do anything with it.

I hope this makes sense.

[GregAmy]

Are you trying to use a /6 tach with a /4? spark counts are wrong, can't use that, won't be accurate.

Do you think you fried your tach? Good used ones are around. Or you could go with a modern one, like the Speedhut, with built-in shift lights and such.

Or you could buy a Tach-Adapt to convert the /6 spark count to /4.

[surfdogskier]

Originally Posted by GregAmy

Are you trying to use a /6 tach with a /4? spark counts are wrong, can't use that, won't be accurate.

Do you think you fried your tach? Good used ones are around. Or you could go with a modern one, like the Speedhut, with built-in shift lights and such.

Or you could buy a Tach-Adapt to convert the /6 spark count to /4.

I figure my 911 tach is a 6. The one I have in my car I "assume" is a 4. But they both react the same way. This tach has never worked since I had the car so I don't know much about it. My speedo and gas gauge don't work either. The only thing that does work work on them is the signals lights, high beam, etc. Basically, anything with needles does not work.

[surfdogskier]

Ok, so I got this crazy idea. My tach starts at 5K rpm when I turn on the key. I start the car and it goes up to around 6K. Then it looks to work like it should just the needle is in the wrong spot.

I am thinking, can I just take off the needle and manually place it to where it would work normal on my existing tach? I am really only looking for looks at this point. It just drives me nuts that it pegs every time.

[Dave at Pelican Parts]

Do not do this. The fact that two different tachs have this behavior says that the behavior is in the rest of the circuit, not in the tach. Leave the gauges alone.

Check your fuses. Check that the wires to the coil are correct. Both the green wire from the points (and condensor) and the black-with-purple-stripe wire go to the coil (-) terminal. The black-with-red-stripe wire goes to the coil (+) terminal. The terminal markings are on the cap of the coil, along with terminal numbers that I never remember correctly. That wire gets +12V on it when the key is on; verify that just as a sanity check.

Check that the wires are plugged into the gauge correctly. There should be a power wire, red with a white stripe, that plugs into the "15" terminal of the gauge. That gets +12V when the car is on. Check that. The black/purple tach signal wire goes to the "1" terminal on the gauge. If there is a terminal with a ground symbol (three parallel lines, short and then shorter and then shortest) then it gets a brown wire. Forget the other wires for lights and such. Make sure the wires are plugged in like this.

--DD