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Thread: AFtermarket Tach

  1. #1

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    AFtermarket Tach

    Ok so I installed my aftermarket Tach today.... it looks great, but I dont think it is very accurate. It has the switch on the back for 4,6, or 8cyl. It was way off on 4cyl sounded like my engine was gonna blow and it was only reading about 2.5k RPM, switched it to 8cyl (i have a 4cyl) it seem a little more accurate, but im prety sure my redline isnt 4k RPM, that would just be silly! I lowered the idle all the way to where my truck was just barely running and it said i was at 1k RPM still. Im wondering if maybe I hooked it up to the wrong spot on the coil.... the back of the coil only has the one post, but then right next to the coil you have a white box with 2 more posts, i hooked it up directly to the back of the coil. See pic below.



    Here is the Gauge though:

    Last edited by Ram50Man; 05-02-2011 at 06:17 PM.

  2. #2


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    That thing is nice...small, not all in the way and you can def. See it well. Bright....

  3. #3



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    I'm guessing (its hard to tell from the pic) that post on top of the coil is the positive terminal. It looks like the negative end of the coil is grounded next to that white box you mentioned. You want to hook up your Blue? (usually green) wire to the negative end of the coil, first picture.. bottom left circle.

    Looks like you might have your ground wire hooked up on the right side of that white box. I would suggest moving it to a different location a bit further away. You might get an improper reading having your ground and signal wire that close together.

    Tach looks good though man
    - Jason
    1995 Mighty Max || FQuick | My Build Thread

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    I dont remember which wire it was but I think there might be a test connector you can use as well. In my dsm harness I know where the plug is, and I don't remember where on the oem truck harness. It's been a whole where I had a carb motor in mines. That white box I believe is a resistor. Best bet is to unplug one wire each and see which wire has power to iit on key on and which doesn't.

  5. #5

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    Quote Originally Posted by blindeyed View Post
    I'm guessing (its hard to tell from the pic) that post on top of the coil is the positive terminal. It looks like the negative end of the coil is grounded next to that white box you mentioned. You want to hook up your Blue? (usually green) wire to the negative end of the coil, first picture.. bottom left circle.

    Looks like you might have your ground wire hooked up on the right side of that white box. I would suggest moving it to a different location a bit further away. You might get an improper reading having your ground and signal wire that close together.

    Tach looks good though man
    The blue wire you see in the pick actually is one of the Distributor wires, but i have the Tach hooked up to the same post as that blue one. There is only the one post on the coil itself. then the 2 on the white block. But nothing is labled "+" or "-" And I dont trust the boot colors. Here is another pic with more notes maybe this one will help a bit more....


  6. #6



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    Ok, I gotcha now. My coil is a bit different from yours, so I'm giving it my best guess. Where you have it hooked up right now is on the positive terminal of the coil. You don't want to hook up your signal wire to that. The wire that appears to be "hardwired" looks to be your negative/ground wire for the coil. Follow the wire from the coil to where it mounts next to that white box you described earlier. That location is where you want to attach your signal wire. Try that out and let us know if it worked.
    - Jason
    1995 Mighty Max || FQuick | My Build Thread

  7. #7

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    Was thinking much the same, signal should be coming before the ballast, but I couldn't remember which side! (the white box is your ballast resistor, FYI)

  8. #8

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    Ok I will try it after work. I drove my truck here (to work) and it seems fairly accurate. My buddy was telling me that the 2.0L only has a redline of about 4k RPM's so it may actually be accurate. I hooked up my test light to see which terminal was a negative, but they all light up the test light. So I wa a little confused by that...

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    Using a test light while everything is connected will show all terminals with power. disconnect the wires off the coil and test which is sending power.

  10. #10



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    The 2.0s rev limit is a lot higher then 4k, more like 6k. And they black wire is the wire that needs to be tapped as that's the neg from the coil

  11. #11

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    Quote Originally Posted by DroppedMitsu View Post
    The 2.0s rev limit is a lot higher then 4k, more like 6k. And they black wire is the wire that needs to be tapped as that's the neg from the coil
    uhhhh.... which black wire, every wire around the coil is black minus 1 blue one.

  12. #12



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    Sorry the black and white one hard wired to the coil

  13. #13

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    Quote Originally Posted by DroppedMitsu View Post
    Sorry the black and white one hard wired to the coil
    Ahh ok. you said that and I was like "ahhh there all black, now im REALLY confused!".... lol

    Ok so i have tried the other 2 posts, on the white box, and the tach does nothing when hooked up to them, so im guessing the one on the back of the tach is correct, maybe my tach is just off.

    HAHA im a retard.... there is a "-" symbol under the post on the coil, i just couldnt see it!
    Last edited by Ram50Man; 05-04-2011 at 04:48 PM. Reason: update

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    lol...did you get it all figured out.....

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    Quote Originally Posted by l.k. View Post
    lol...did you get it all figured out.....
    Yeah, i had it right from the start. I think its just the Tach itself is a little off.

  16. #16

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    i know on my 2.4 i use the brownish/orange female connector to hook my tach up.

  17. #17

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    Quote Originally Posted by dray View Post
    i know on my 2.4 i use the brownish/orange female connector to hook my tach up.
    I dont have anything thats Brownish/Orange, i just have bunch of blacks, and the one Blue one, but I got it now, the one post on the back of the coil that you can add wires to is the negative terminal.

  18. #18

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    Is that one of those cheapo tachs from ebay? I had bought one, then while it was on the way I went to the salvage yard and got a 3" autogage tach for $5 so i sold the little one to a friend. The redline on my 2.0l is about 7 1/2 k rpm. I always had my tach connected where you had yours until i replaced my coil with a normal round 12v coil.

  19. #19

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    Yeah it is, one of those "cheapo" tachs from ebay.

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    It could also be that the tach does not have a good ground.
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  21. #21

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    Quote Originally Posted by abeemanator View Post
    I always had my tach connected where you had yours until i replaced my coil with a normal round 12v coil.
    ive been wanting to do this to my truck but how do you wire the round 12v coil?

  22. #22

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    Well, I would have to check it out, I wired it up and I don't quite remember how I did it. I replaced the coil because I didn't know what the ballast resistor did, so I took it off, and after about 20-30 minutes of driving it would overheat and the truck would die. Also where you you live in Idaho?

  23. #23

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    the ballast resistor drops the voltage to a lower level since the oem coil doesnt need a full 12v to run. If I remember right the resistor drops it down around 5 or 6v. I had that problem on mine, the resistor went bad and the coil would overheat after about 15min and shut down. Leave the truck sit for an hour or two and it'd fire right back up. The voltage drop is also why the tach reads funny if its hooked to the resistor instead of the coil negative. I think you can run it off the resistor but only one side will give you a correct reading. As for putting in an aftermarket coil, there shouldnt be any issues just make sure everything negative on the old setup goes negative on the new setup, same with positive's

  24. #24

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    Ballast resistors also reduce RF and EM "noise" from the coil due to the super high voltage output. Not having one can make your radio sound like poo and can interfere with other components/sensors sometimes. Even MSD/other aftermarket coils benefit from a ballast, even if just barely.

  25. #25

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    so keep the ballast resistor with an aftermarket coil?

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