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Installed a tach successfully but it is always half of the true rpm

Discussion in 'Subaru Sambar' started by Reese Chambers, Mar 22, 2020.

  1. Reese Chambers

    Reese Chambers New Member

    I installed a tach successfully apart from the fact that its exactly half of the reading it should be. is this normal on these (subaru sambar 93 sc 2wd)
     
  2. Reese Allen

    Reese Allen Member

    What tachometer did you buy?

    How did you wire it up?
     
  3. Reese Chambers

    Reese Chambers New Member

    A basic Bosch tach. I wired it to the negative of the coil (if I'm remembering correctly) I ran a wire to the back.
     
  4. Reese Allen

    Reese Allen Member

    The tach is not interpreting the signal correctly for your application. I have not installed an aftermarket tach before but presumably it is possible to tell it to apply a multiplier to the signal it's getting, e.g. 1/2x, 2x, depending on how many cylinders you have. If you're getting exactly half the actual RPM (and you are confirming it is truly 2-to-1 using an external tachometer of some kind that you trust) then the math is as simple as can be, and is presumably already programmed into the tach and can be enabled if you know the magic word.
     
  5. Reese Allen

    Reese Allen Member

    Cool name, by the way. :) I'm about to turn 35 and I've never actually met another Reese in person. I know a few online, including one genuine Reese Allen who runs a photography business on the east coast.
     
  6. Reese Chambers

    Reese Chambers New Member

    That's what I was thinking. If it's counting the hz or something I might have hooked it up wrong but if it's a voltometer type of deal I might just amplify the signal. I might also just get a custom design on the tach.
     
  7. Reese Chambers

    Reese Chambers New Member

    lol I have only met two other people named Reese. I have actually read your blog online and might actually try the head unit install. It's funny to find someone with the same name and kei truck/van
     
  8. Reese Allen

    Reese Allen Member

    Yeah, this is some seriously unlikely stuff. Do people constantly try to misspell it "Reece" for you too?

    At least your last name sounds like a last name. Mine is a first name, and Reese is more common as a last name. I am always fighting the constant battle of "Yes, my first name is in fact Reese, and Allen is in fact my last name. No, I am not Allen Reese. Do not call me Allen."

    On topic: My understanding at this point is that stock Sambar tachs are NOT voltmeters or anything that simple. They have an IC that counts pulses and then outputs a signal to the coil that turns the needle. I hooked an oscilloscope to my dash connectors yesterday and confirmed that one lead is ground, one is positive battery voltage, and the other is a signal which is something like a square wave whose frequency increases as engine RPM increases. I'd suspect that the gauge you installed probably works similarly. It's probably possible to get it to display the true correct RPM if you know how to tell it what the multiplier should be.
     
  9. Jigs-n-fixtures

    Jigs-n-fixtures Active Member

    All tachs are pulse accumulators. Whether they do it digitally, or analog. The fact that your tach is reading half the actual rpm, indicates you have it set for a six cylinder engine.

    The tach gets a signal every time the coil is triggered, on a 3-cylinder that is 3 signals once every two rpm. On a six-cylinder that is 6 times per every two rpm. So, if your tach is set for 6-cylinders, it is receiving exactly half as many pulses per rpm as it should, and indicates exactly half as many rpm.
     
  10. Reese Chambers

    Reese Chambers New Member

    Oh that helps a bunch I'll open it up and see if there is a switch or something I have a 4cyl but it may be for 8
     
  11. banzairx7

    banzairx7 Member

  12. rkrenicki

    rkrenicki Active Member

    Looks like most of the Bosch tachometers just have a switch on the rear for 4, 6, and 8 cylinders.
     
  13. Jigs-n-fixtures

    Jigs-n-fixtures Active Member

    Reese has a Subie, which has a four cylinder engine. So, hopefully he will be able to just move the switch and solve his problem.

    A lot folks with the 3-cylinders are running the glowshift tachs, because they have the switch to select anything from 1 to 10-cylinders.
     
  14. rkrenicki

    rkrenicki Active Member

    uh.. Yes, I am fully aware of that. I was letting him know that the switch is on the rear.
     
  15. Reese Chambers

    Reese Chambers New Member

    So I found the little switch I'm embarrassed because it was hidden under a little bit of cobweb. That's one of my dumber moments I'll post a video of the tach working after work
     

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