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Haltech vs comm tacho

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    #16
    Open up the relay and remove the diode.

    The diode allows any voltage above 12 VDC (+ the diode biasing voltage of approx 0.7 VDC) to discharge back to the supply.

    You actually rely on this spike to lift your tacho needle.

    Think of it like water hammer in a hose/house pipe. The coil of wire takes momentum to start flowing current, and conversely has momentum when stopped. Much like a tap being turned off quickly, where the water slams up against the tap handle and causes high pressure which then dissipates backwards against the house check valve etc etc. The current through the coil and in to the ECU aux/DPO suddenly gets shut off by the ECU. The current causes quite a large spike of voltage at the DPO pin - 50 VDC +. The Diode allows that voltage top return to the supply side, but then the voltage doesn't rise above 12.7 VDC and trigger the tacho.

    The resistor to the DPO just limits the current (think water flow) so that when switched off there isn't as much momentum. Without the current limiting resistor of 1k, the voltage would likely exceed the ECU voltage rating. Most ECU's don't need this as they are designed for a wider range of voltage, but if it's drawn on the Haltech, the wouldn't have done that for no reason!
    Jason Broadhurst

    Someone once asked me if they could use my mower. I said "sure, so long as it doesn't leave my yard"

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      #17
      OK I get that, thanks. What's the 13+ v I see then with the older really thing? Do I need any spike for the tacho or one that shows a zero ?

      I couldn't see anything in the other relay so bought one with nothing in it just easier.

      Just found an old saas tacho so will give that a go as well in case my down stream wiring isn't right

      Sent from my HTC One M9 using Tapatalk
      Originally posted by 50RTD
      Dave, have you uploaded the data from the carby?

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        #18
        If it is just a multimeter you will probably just see an average biased by the dpo duty cycle between 0v and alternator charge voltage. You really need a scope or something genuinely high freq with peak hold to make much of the signal. It will still be cycling but the multimeter has some smoothing. If dpo isn't cycling you should just see alternator voltage as there is no real current through the resistor to drop voltage.

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          #19
          I say 12 VDC and 12.7 VDC because a the car battery is typically 12 VDC, and the diode bias is 0.7. In reality the alternator charges the battery at 13.5 odd, so the car voltage when on is actually 13-14 VDC.
          Jason Broadhurst

          Someone once asked me if they could use my mower. I said "sure, so long as it doesn't leave my yard"

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            #20
            Thanks so much. I have another relay with no diode or resistor, that I stripped the points out of. Will try tonight

            Sent from my HTC One M9 using Tapatalk
            Originally posted by 50RTD
            Dave, have you uploaded the data from the carby?

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              #21
              No worries, let us know how you get on.
              Jason Broadhurst

              Someone once asked me if they could use my mower. I said "sure, so long as it doesn't leave my yard"

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                #22
                Still no go.

                Spoke with Vaughan from haltech today at motorex and said we are on right track but oe tacho are tricky to get to work.

                I have a small saas tacho will try that

                Sent from my HTC One M9 using Tapatalk
                Originally posted by 50RTD
                Dave, have you uploaded the data from the carby?

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                  #23
                  It just clicked, the tacho doesn't work because of the stupid inbuilt ignitors and grounding signal used by Barra's

                  Exactly the same as any ba/bf you need a tacho adaptor for any aftermarket tacho to work. Fuck me.

                  Sent from my HTC One M9 using Tapatalk
                  Originally posted by 50RTD
                  Dave, have you uploaded the data from the carby?

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                    #24
                    Originally posted by Dogsballs View Post
                    It just clicked, the tacho doesn't work because of the stupid inbuilt ignitors and grounding signal used by Barra's

                    Exactly the same as any ba/bf you need a tacho adaptor for any aftermarket tacho to work. Fuck me.

                    Sent from my HTC One M9 using Tapatalk

                    not correct.

                    use the DPO, but dont use the haltech pull up, use an external 1k resistor between DPO/tacho signal wire and ignition +
                    Supercharged EBII 4.0

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                      #25
                      I've tried that it doesn't work

                      I just bought an autometer tach adaptor and pulling it off the coil. Like Ford guys do for external tacho.

                      Sent from my HTC One M9 using Tapatalk
                      Originally posted by 50RTD
                      Dave, have you uploaded the data from the carby?

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                        #26
                        last VS I did was DPO1 with 12v pullup in the ecu to the brown wire on the 8 pin plug and it just worked..
                        im a cunt
                        and apparently i dont know shit...

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                          #27
                          Originally posted by 10sec rx7 View Post
                          last VS I did was DPO1 with 12v pullup in the ecu to the brown wire on the 8 pin plug and it just worked..
                          V6 or v8 car?

                          Sent from my HTC One M9 using Tapatalk
                          Originally posted by 50RTD
                          Dave, have you uploaded the data from the carby?

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                            #28
                            V8
                            im a cunt
                            and apparently i dont know shit...

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                              #29
                              A pf'er messaged me these.


                              Luke from haltech sent this.
                              Here’s a diagram for a higher-spec coil negative tach adapter using a relay coil, 2N5550 Transistor, 1kΩ resistor and a 4.7kΩ resistor. Failing this, it might be worth trying a premade unit like the MSD 8920.




                              Sent from my HTC One M9 using Tapatalk
                              Originally posted by 50RTD
                              Dave, have you uploaded the data from the carby?

                              Comment


                                #30
                                the holden v6's were not triggered by coil neg, a 12v square wave will work on these tachos as the signal was from the DFI module.
                                DPO with external 1k pullup resistor works on these, ive done a fair few barra into V series and got them working no probs either with haltechs or std pcm's with my own tach adapters that output a square wave.
                                Supercharged EBII 4.0

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