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Speedo issue


damla
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i have link g4+ gtr plug in ecu in my cefiro. the problem im having is that the signal for the speed seems erratic. my speedo is cable driven from the gearbox to the speedo cluster and then goes from the cluster to the ecu, i have yet to trace the exact route. if i create a digital gauge in pclink using  LR wheel speed DI3 i see the speed (its half of what it should be but i can correct that on the dash im using), every few seconds it seems to jump to about double the speed im doing for a fraction of a second, so if im doing 50kmh it shoots up too around 100kmh. im outputting over CAN to my dash and i see the same thing over CAN as i do i pclink. keeping in mind that the factory cluster seems to operate as normal with no fluctuations. have you encountered this before with cable driven speedos? do i need a resistor inline with the signal to the ecu? any help appreciated.

regards Damian

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Assuming you have already tried rising and falling edge on the DI with no improvement then it can only be a problem with the signal from the speedo.  Below are my thoughts of what would be two most likely causes, unfortunately you really need an oscilloscope to work out what the problem is.

  1. In the Nissans with cable driven speedo, the ECU speed signal comes from a simple magnetic reed switch in the back of the speedo, it just has 2 or 4 magnets that wave past the reed switch every cable revolution.  The reeds switches are pretty prone to dying of old age.  They can sometimes get sticky or the contacts get burnt.
  2. Any signal going into a digital input needs to rise above about 1.5V and fall below about 1.2V for it to be detected.  Your signal most likely is 0-5 or 0-12V, but I have seen cases where there was a bad ground somewhere so the signal sometimes didnt drop below 1.2V, causing the odd drop out or spikes.

 

 

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