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Thread: Tach Question

  1. #1

    Default Tach Question

    OK, I searched and this has been "discussed" before for 4cyl and turbo tachs but I didn't quite get an answer to my question.

    Some time ago I got a 140MPH speedo cluster off from ebay (83-86). It had a 7k Tach on it. I disassembled the thing and tossed the cluster housing and temp guage as those were trashed (and painted red)...but saved everything else.

    When I pulled the 7k Tach out, it has that notorious switch for "4/6/8" on it. It is pointing to 4. I was wondering if the switch can be turned to the "8" and if it will work.

    The back of the cluster had 2 circuit board panels...one green and one red.

    The cluster had been obviously apart several times....could the tach face have been changed?

    -Mike

  2. #2

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    ALL tach housings have a hole that is marked "4-6-8" and have a provision for a switch on the circuit board but I have never seen a 7K or 8K RPM tach that physically had a switch in place. I have several clusters (both of either '79-'82 or '86-'86 variety, SVO and non SVO, and w/ 6K, 7K, or 8K RPM tachs), in various states of completion or disassembly, and the only tachs I have ever seen with a switch in place are the 6K RPM tachs.

    Are you sure the 7K RPM tach that you have wasn't taken apart and had a switch thrown in there?

    The 7K and 8K RPM tachs' circuits aren't complete (there aren't any resistors). Even if a switch was put in there, and switched amongst the 4-6-8 circuits, the tach would only read correctly on the one circuit that's complete (with resistors).

    Yes, if each circuit had the correct resistors in place, the switch would read correctly in all positions. Take the tach apart and look at the circuit board.
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  3. #3

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    Quote Originally Posted by FoxChassis
    Are you sure the 7K RPM tach that you have wasn't taken apart and had a switch thrown in there?
    Anything is possible. Can the face be removed?

    If there are two printed circuit boards on the back of the cluster, what does that say about the cluster? Old or new? I thought the new style had only one circuit board.

    One further question, if somebody was playing with guage faces...what if they swapped a 140MPH face onto am 85MPH speedo? Will it work right? How can I tell?

    My first thought is that somebody had an older 4cyl cluster (as evident by 2 circuit boards and the tach with the 4cyl setting) that they wanted to update to look like a v8. Maybe they just swapped the tach and speedo faces over.

    -Mike

  4. #4

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    Yes, the tach face can be changed very easily. Remove the needle the two black screws holding the face to the assembly. But the calibration will be way off, as each of the 6K, 7K, and 8K tachs have a different scale.

    The speedometer face isn't so easy to change. It's riveted to the assembly.

    The two different (greeen and red) colored circuit overlays does sound like an early cluster. I have one of those in my inventory too.
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  5. #5

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    OK, I looked over the speedo. It all looks correct. The rivets are intact and factory.

    The tach gauge face looks like it may have been changed. Near the bottom face screw, there is a small metal tab that is broken off that would fit in a slot in the housing.

    I guess I'll probably keep the face and toss the housing. How can I tell once I take the face off which housing it really is? I suppose if the switch is there and if there is no modification to the internals, its a 6k housing.

    -Mike

  6. #6

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    I believe (it's been a while since I had one apart) the tachometer housings are the same '79-'86. The circuit board inside and the faces are what's different. The 4-6-8- switch just pops out of the circuit board, so they're very easy to R&R.

    If my theory (that only 6K RPM tachs have a switch) is 100% correct, somebody could have put a switch in that 7K RPM tach, thinking it would then be useable in a 4, 6, or 8 cylinder car.
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  7. #7

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    Quote Originally Posted by FoxChassis
    I believe (it's been a while since I had one apart) the tachometer housings are the same '79-'86. The circuit board inside and the faces are what's different. The 4-6-8- switch just pops out of the circuit board, so they're very easy to R&R.

    If my theory (that only 6K RPM tachs have a switch) is 100% correct, somebody could have put a switch in that 7K RPM tach, thinking it would then be useable in a 4, 6, or 8 cylinder car.
    1. Flopping Faces = Obvious and Easy
    2. Adding Switch = Requires knowledge and some Research Skills (even if wrong)

    My gut feeling tells me that since the entire cluster was painted in bright nail polish red, the previous owner was not likely to posess the intelligence needed for option #2.

    I'll take the tach apart but I'm gonna place bets that its a 6k tach with a 7k face slapped on it.

    I'll save the whole thing if the housings are the same and swap in the correct circuit when I can.

    -Mike

  8. #8

    Default

    If it's a 6K RPM tach with a 7K RPM tach face on it, the circuit board will look like this: http://www.geocities.com/mt83rd/6K_tach.html

    If it's a 7K RPM tach with a switch inserted, but the circuit board not modified (no resistors added), the circuit board will look like this: http://www.geocities.com/mt83rd/8K_tach.html
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  9. #9

  10. #10

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    Its got resistors.

    But its also been spliced at one part.

    Maybe somebody was on the ball and modified it correclty?

    -Mike

  11. #11

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    Definately looks like a hack. Only one way to figure out if it'll work....

  12. #12

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    I don't know if I really care.

    The tach in my car works for now and unless this face is any better than what I have (I doubt it) it'll probably just stick around as spare parts.

    -Mike

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