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    Thread: Diesel to gas Tachometer question

    1. Junior Member
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      05-22-2012 03:04 PM #1
      Has anyone heard of using a radio knob between the W on the diesel alternator to a gas tach to regulate the RPMs and just turning it to have it be correct? (using a laser tachometer)

    2. Member
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      05-23-2012 02:41 AM #2
      Quote Originally Posted by jon8linden View Post
      Has anyone heard of using a radio knob between the W on the diesel alternator to a gas tach to regulate the RPMs and just turning it to have it be correct? (using a laser tachometer)
      radio knob? are you kidding me?!

      potentiometer maybe?

      but no, the idea you presented, would never work..

      weakening the signal (adding resistance to the tach signal) is not going to do anything besides make your tach even LESS accurate..

      so, scratch this idea, because you cant feed a gasser tach with a W signal from a diesel alt and have it read..

      gas and diesel tachs use TOTALLY 100% different signals.. thats why you have to change the circuitry on a gasser tach to use it in a diesel..

      basically, if it were as easy as adding a potentiometer inline to the tach, dont you think thats the way everyone would be doing it, rather than re-wiring there whole tach circuitry?

      oh yea, there is an adjustment potentiometer inside every tach ever produced by VW. you use THAT to adjust the tach to read right.. but that still does not fix the issue that the gasser tach will not read off a diesel signal..

      gasser tach reads 2 pulses per revolution

      diesel tach reads like 35 pulses per revolution..

      NO, YOUR IDEA WILL NOT WORK.

    3. Junior Member baronvonschwanstein's Avatar
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      06-13-2012 08:13 PM #3
      gasser tach reads 2 pulses per revolution

      diesel tach reads like 35 pulses per revolution..

      i was under the impression that:

      a W tach reads voltage, not "pulses"

      a tdi tach reads pulses, (from crank sensor)

      an efi gasser tach reads pulses (also from crank sensor)

      a cis tach uses pulses from hall sender?

      so a 4 cyl efi gasser tach (and maybe cis tach?) should work with a tdi (but the oil buzzer system still uses the W signal?)

      i'm not having any luck finding a b4 idi dash in the states, and trying to wade through this info,
      if the W terminal used pulses, i think i could modify it like one would make a 6cyl tach work with a 4cyl, essentially just making a simple logic circuit increasing or decreasing the frequency by a set percentage , instead i believe i need to buy a signal converter

      also i had heard that you could use a resistor or potentiometer, but that makes no sense
      "does it have vacuum actuated engine mounts?"

    4. Member
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      1977 VW Scirocco 1.8/Weber carbs, 1986 VW Golf Diesel, 1978 Rabbit 1.6
      06-16-2012 03:17 AM #4
      Glegor....on this diesel W thing.....looking at my spare diesel alternator, I am baffled at where it is getting 32 or whatever pulses per revolution? I know it's getting some sort of pulse out of it.....what do you think, based on the rpm of the alternator that has a smaller pulley than the crankshaft pulley? That looks about a 4:1 ratio. Not enough for the 32 pulses or whatever. I tore my spare alternator apart to clean it up, looked at it...and it doesn't make sense to me

      My recently added diesel cluster with tach is reading about 300-400 rpm too high......I'm thinking it's from a different year, probably from a year that had a larger alternator pulley?? (Which makes me wonder what sizes of pulleys were available. Might just make an aluminum one on the lathe, so the tach reads right.....if I can find the correct size to make it.)

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      06-16-2012 12:17 PM #5
      Quote Originally Posted by rdezsofi View Post
      Glegor....on this diesel W thing.....looking at my spare diesel alternator, I am baffled at where it is getting 32 or whatever pulses per revolution? I know it's getting some sort of pulse out of it.....what do you think, based on the rpm of the alternator that has a smaller pulley than the crankshaft pulley? That looks about a 4:1 ratio. Not enough for the 32 pulses or whatever. I tore my spare alternator apart to clean it up, looked at it...and it doesn't make sense to me

      My recently added diesel cluster with tach is reading about 300-400 rpm too high......I'm thinking it's from a different year, probably from a year that had a larger alternator pulley?? (Which makes me wonder what sizes of pulleys were available. Might just make an aluminum one on the lathe, so the tach reads right.....if I can find the correct size to make it.)
      the alt makes about 8 pulses per revolution. not 1.

      so, 8 pulses per rev, and 4 revs of the alt to 1 rev of the crank

      = 32 pulses from the alt per crank rev..

      if your tach doesnt read right, just adjust it.. theres an adjustment pot on the back of the tach.

      or are you running a non-standard pulley setup?

      ive got a combination of a 1.5 crank pulley and a 1.6 vanagon water pump pulley.. i basically built an underdrive out of oem parts..

      the stock 1.6 crank pulley is over 6"
      the stock 1.5 crank pulley is under 4.75"

    6. Member
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      06-16-2012 12:27 PM #6
      Quote Originally Posted by baronvonschwanstein View Post
      gasser tach reads 2 pulses per revolution

      diesel tach reads like 35 pulses per revolution..

      i was under the impression that:

      a W tach reads voltage, not "pulses"

      a tdi tach reads pulses, (from crank sensor)

      an efi gasser tach reads pulses (also from crank sensor)

      a cis tach uses pulses from hall sender?

      so a 4 cyl efi gasser tach (and maybe cis tach?) should work with a tdi (but the oil buzzer system still uses the W signal?)

      i'm not having any luck finding a b4 idi dash in the states, and trying to wade through this info,
      if the W terminal used pulses, i think i could modify it like one would make a 6cyl tach work with a 4cyl, essentially just making a simple logic circuit increasing or decreasing the frequency by a set percentage , instead i believe i need to buy a signal converter

      also i had heard that you could use a resistor or potentiometer, but that makes no sense
      ALL GASSERS use a signal from the COIL NEGATIVE wire (Carburated, CIS, Digifant, Motronic)

      some gassers/ALL TDIs use a signal from the ECU. i believe mk3 was when they went to ECU controlled gauges.. a TDI uses a ECU signal to drive the tach. there is a magnetic pickup that the ECU gets an engine speed reading from, then it tells the tach where to put its needle. there is NO DIRECT CONNECTION between the engine and tach, like an older car..

      diesel W tachs read directly off the W terminal of the alternator.. the alternator puts out a sine wave (alternating current) and needs the appropriate tach circuitry to read it..

      gasser tachs controlled by the coil negative side use a SQUARE WAVE (on/off) signal to drive the tach.. a SINE wave current will NOT drive a tach that request a SQUARE WAVE signal be sent to it..

      i do have a write up on how to build your own tach setup tho, that is NATIVELY ACCURATE, and needs basically no setup..

      How-To rig up a simple magnetic pickup to drive a regular gasser tach.

    7. Member
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      06-16-2012 01:50 PM #7
      Quote Originally Posted by Glegor View Post
      the alt makes about 8 pulses per revolution. not 1.

      so, 8 pulses per rev, and 4 revs of the alt to 1 rev of the crank

      = 32 pulses from the alt per crank rev..

      if your tach doesnt read right, just adjust it.. theres an adjustment pot on the back of the tach.

      or are you running a non-standard pulley setup?

      ive got a combination of a 1.5 crank pulley and a 1.6 vanagon water pump pulley.. i basically built an underdrive out of oem parts..

      the stock 1.6 crank pulley is over 6"
      the stock 1.5 crank pulley is under 4.75"
      I'll have to take a look at the back of my tach it seems....I didn't notice what you describe when I installed it. Adjustment would be a lot easier!

    8. Member
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      06-16-2012 03:15 PM #8
      Quote Originally Posted by rdezsofi View Post
      I'll have to take a look at the back of my tach it seems....I didn't notice what you describe when I installed it. Adjustment would be a lot easier!
      there is a hole in the BACK side of the tach. you gotta pull the cluster to get to the adjustment, but its there..

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