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Old 06-19-2006, 12:03 AM
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Tach wiring help

I've got an old tachometer that I want to test and if it works put it in to my zephyr, the problem is I dont know which wire is which coming off of the tach, the car wiring I can figure out easy enouigh but this tach has 5 mystery wires, 3 short ( red, blue, yellow ) and two long wires ( black, red ) the long red wire grounds to the case inside and the short red one goes to the light, the rest I cant see without tearing in to the thing and risk breaking something.




Anyone?
please?
Scott

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Old 06-19-2006, 01:17 AM
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Doc here,

Red should NOT ground to anything..My guess is that you are reading a low resistance to ground (if read by ohm meter)

Traditionally it should be :

RED (long)=Power from fused , switched source.
BLACK (long) =Hard ground

Who made the tach? It may require a Field calibration card also (the other 3 wires..)

To confirm or deny, measure with an ohm meter between the red and the outer two wires..if you get a low resistance between red and one other, remove the lamp and see if the reading goes away..If so those are your panel lamp wires..The remaining other is Coil - pickup..

If you get high or no resistance, you may need a remote card, Or the bulb is blown or missing..Carefully remove the back to confirm this.

Doc
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Old 06-19-2006, 01:44 AM
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Quote:
Originally Posted by docvette
Red should NOT ground to anything..
My thoughts exactly but...



Quote:
Originally Posted by docvette
Who made the tach?
I dunno, cant find any markings on it other than "precision engineered in japan" at the base of the needle


I've seen gauges by the same manufacturer (exact same casing) in early 70's ford trucks, maybe just coincidence though..

Thanks
Scott
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Old 06-19-2006, 01:58 AM
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Doc here,

I think if you ohm between the red terminal and case ground , you'll find it isolated..Or you could remove the terminal and see if it has an insulating washer under it..However some clueless "Engineer" may have added that wire himself..check to be sure..

The blue and Yellow appear to be going inside the unit which makes me think it may need a calibration card..

You need to carefully pull the two nuts left and right on the back, one of them is the one the red wire is on, and lift the cover..If there isn't a circuit card in there, you only have a remote head unit..and need a calibration card..

If you STILL want to use it (for retro purposes) and that is how it is let me know, I can tell you how to build a functioning tach from it, grafting a new tach circuit..(like a cheepie $19.95 tach)

Let me know!

Doc
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Old 06-19-2006, 03:15 AM
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Heres what i'm dealing with




On the back it allows switching between 4-6-8 cyl operation by moving the brass tab between the contacts (top left of bottom pic), dunno if that helps. Also the long red wire that bolts to the case looks only to function as a ground for the light!? the contact it bolts to doesnt connect to anything else on the circuit board... On the bright side I found The worlds smallest lock washer! If all else fails I do have a cheapie tach to cannibalize that was going in to the zeph if I couldnt get this one going.

Thanks
Scott
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Old 06-19-2006, 04:35 PM
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Doc here,

Try this,

Get a 12 volt lantern Battery, hook the black to the ground, touch the yellow to the + side of the battery, If it jumps that's your power and ground, take all that to the car, and hook your blue to the coil - and fire it up, it should report if correct..

I single out the blue as coil input BECAUSE, it is the only one that goes to a big honking cap/resistor combo on the other side of the card..

The lantern battery should not produce enough current to damage the circuit if it's wrong..just don't use a higher current voltage source.

Doc
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Old 06-19-2006, 07:26 PM
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Your tach looks like a tach I've owned for many years.

My tach has these wires:
red - hot (power source such as fuse block)
black - ground
green - connects to the (-) side of a coil for points style ignition.
yellow - light source to illuminate the tach face.
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Old 06-19-2006, 07:41 PM
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Doc here,

Normally , I would tend to agree...BUT:

If you look at the photos...
  • Red 1: Goes directly into the lamp.
  • Red 2: Goes to the Tach body ground.
  • Blue : Goes to an R/C network (similar to a tach input network)
  • Yellow : Appears to go to a power buss..
  • Black : Appears to go to a ground buss on the circuit card.

NOT Standard Wire color codes Vs Function at all..

AND there is no Green wire at all...which also makes me think the blue is the coil - input..and at first, a remote calibration card might be required..the 3 wires were "goesinta's and goesoutta's " between the two..

A red wire is run to a case ground?? What were they thinking? That sounds like some previous owner "Genius" addition..Engineering the "Dead Short" another milestone..

Doc
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Old 06-24-2006, 01:29 AM
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Quote:
Originally Posted by docvette
Get a 12 volt lantern Battery, hook the black to the ground, touch the yellow to the + side of the battery, If it jumps that's your power and ground, take all that to the car, and hook your blue to the coil - and fire it up, it should report if correct..
ok so I tried this and when the yellow and black are connected the needle does flick a small amount, I tested it on my car battery.. I figured if I let the magic smoke out i'd just find another one to use but it didnt escape so I think i'm good. tomorrow i'll connect the blue to my coil and see what happens (i'll set it to 6cyl and put it on my sprint, should be able to somewhat get its accuracy from there)

Thanks for the help!

Scott

Also the red ground wire looks original going from the connector and type of wire used, W T F were they thinking
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Old 06-24-2006, 04:46 PM
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Doc here,

Let us know the outcome

Doc
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Old 06-26-2006, 12:48 AM
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Well it works but it doesnt work properly, I wired up another tach with it to compare readings, set them both to 6cyl and hooked them up to my sprint, After adjustments the closest I get is when the known good one is at 3000 (cars at 6000) the old one is at 2500... i'm going to try it in 4cyl mode and see how close I could get it adjusted, or see if I could loosen the spring pressure a bit while still keeping the needle in the right place.

Scott
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Old 04-07-2013, 07:15 PM
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For anyone reading this all these years later...

Hi everyone (anyone?)!

I stumbled across this thread six years later while looking to find whatever I could for a mystery tach that most certainly is made by the same Japanese company, but appears to be a different model or else produced at some other point on the product's evolution.

Externally, it looks identical except for the 'Precision Engineered in Japan' inscription toward the bottom of the face. It does not appear on mine. Beyond that, everything looks the same on the outside.

Inside, it's a whole 'nother world! Mine has a printed circuit board that is retained in the aluminum chassis by three impressed stamped tabs that are 120 degrees apart. I can't feature any common hand tool that can be used to pry those back out without damage. The case and bezel come off easily enough, but it appears the gauge itself was not intended to be serviceable in any way besides removing the light bulb socket and replacing the little 3W bulb. The gauge in the photgraph appears to be much more spartan on the inside than the one I've got here is! I have no idea what the bulb is, it's little, kinda' looks like the gauge bulbs for my Kawasaki KZ400... I'd plan to hunt around a bit to find a replacement. Fortunately, mine works just fine for now.

I have NO idea why the two red wires power the light. I'm guessing the shorter one may have been intended to be the positive, as my example has a 1/4" spade connector on the short one which probably plugged into a switch or a fuse block like the one on my '73 Chevy C20 that this will soon be mounted on. Being that it's an incandescent bulb, though, I can't imagine it matters a whole lot!

Yes, black is the tach ground. Yellow is +12VDC for the gauge, but not the light. Blue is the pickup lead that goes to the coil. If you have black connected to the battery ground, yellow pressed against the positive terminal, and then brush the tip of the blue wire against the positive terminal as well, the needle will climb erratically. It won't get very high, or at least it didn't for me. It rapidly wandered up and down on the bottom half the the scale as I dragged the wire across the top of the battery post. At no time did it 'peg' at 8K or beyond. I'm hoping that wiring it up to an HEI coil's tach lead won't present any problems...

The mouting base appears to be plated cast zinc and accepts two larger countersunk flat head screws. There is a large round screw with a 'coin slot' milled in it that tightens the stainless band that holds the tach. Loosen that screw a bit, and you can rotate the tach as needed so it looks right (appears to sit level, anyway!) in your specific application. The base is curved just enough that it fits the contour of a 70's GM column just fine. This one is a tilt unit from a '76 Caprice that was a drop-in swap for the fixed truck column and the huge wheel it had. (Incidentally, a nice black two spoke wheel from a circa 1988 Cavalier fit right on it, and the horn button from a late 80's Camaro is right at home on that!) My dash pad is junk and will be until I rebuild the truck, which will be who-knows-when! I didn't want to butcher the original panel to make a newer radio fit where the old dual shaft OEM one once resided, so I just mounted it on top above the 'cubbie' and heater control head using steel pipe hanging tape...looks like something you'd see in a WWII bomber, sort of. Where the pad starts to slope downward right to the side, heading toward the ash tray, is another place where the contour of the mounting base is a good match. The tach will be right next to the radio, again, vintage aircraft style! A couple of self drilling sheet metal screws will hold it down. The numbering, orange needles, and black/chrome color scheme make it look almost like Chevrolet intended for it to be there!

Not a bad deal for something I bought out of a junk box for a buck!
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