strokerzedd Posted April 25, 2009 Share Posted April 25, 2009 I posted this in the Chris Rummel wiring sticky but wasn't sure if it would draw any response there. I'm close to firing my car up but still a little confused about wiring an Autometer Tach to an RB26 harness in my '72 240z. This is what I've gleaned from the whole thread from several people: • An Autometer tach wired to the ecm's tach (Pin 7) signal will pop the resistor in the ECU. The Autometer tach needs a negative pulse as the signal, the stock Z gauge needs a positive 12v signal. Don't use a tach adapter. The tach adapter takes the negative signal and converts it to a positive 12v signal and will fry the 52 ohm resistor in the ECU for the tach signal. Summit Racing sells an msd tach translator #8918 to get an Autometer tach working for a diesel truck. Tap this into the white positive wire on the little two wire plug that's by the igniter, it's $49. My tach works perfectly. • The "shunt" is a connector that is used by the dealer to plug a factory style timing light, could probably be tapped for the Autometer tach...It's the big white wire that goes from your coil plug, attached on the rear of the coil cover, through a splice, to a white connector with a little black box plugged on it that doesn't seem to have any purpose... • This sounds like what is needed does anyone know if this would work for the tach? http://store.summitracing.com/partde...2&autoview=sku • This is what I did to get my Autometer tach working: Place 470 ohm resistor between the 12v pin and the Tach signal pin on the back of the Tach itself. This will act as a pull up resistor and provide enough power to drive the Tach. So my question to those of you that have successfully wired an Autometer Tach to the RB26 harness, which hookup is safest/easiest/works and won't blow the ECU resister; and where's the best place to get the signal to run to the tach? I'm using an AEM 1621U ECU. Thanks. Quote Link to comment Share on other sites More sharing options...
240Z_Master Posted April 25, 2009 Share Posted April 25, 2009 This is great information... since I plugged my Autometer Tach and Tach Adapter... and the car went boom which seemed like a huge backfire then shutoff. I quickly unplugged the Tach adapter and now it runs... but now the car doesn't make power when boosting over 3lbs and above about 3.5K RPM. My car falls flat on it's face now when the boost gauge hits 3 lbs... anyone have any ideas? Could the tach adapter done something to the ECU on my RB25? Quote Link to comment Share on other sites More sharing options...
Careless Posted April 26, 2009 Share Posted April 26, 2009 sounds like something went in the ECU. i think at that point where it happened it blew up and the ecu just dumped fuel as whatever DID pop went when the car was higher in the fuel map. that fuel map may have been accessible only by triggering whatever it is that burnt in the ECU. Once it popped... the added fuel from the map may have been dumped and the ECU didn't know how to proceed. The ALU in the ECU probably could not compute the error and just fell flat at that intersection on the Map... ECU's normally have a sensor limp-mode, but not a limp-mode for "where the hell did that diode on the motherboard go" type thing... at least not many of them... The easiest thing would be to check the ECU internals to see if any Diodes have been burnt. Quote Link to comment Share on other sites More sharing options...
strokerzedd Posted April 29, 2009 Author Share Posted April 29, 2009 Anyone? Someone out there has installed an Autometer tach into their RB26 harness and not popped the resister in their ECU. Quote Link to comment Share on other sites More sharing options...
fast-datsun Posted April 29, 2009 Share Posted April 29, 2009 need to add a pull up resistor to the computer tack singnal to make the Autometer work.. Quote Link to comment Share on other sites More sharing options...
chrism Posted April 29, 2009 Share Posted April 29, 2009 Should be the same as the Autometer tach for the RB20DET. 470 to 10k ohm resistor between the signal and 12v supply on the back of the tach. Quote Link to comment Share on other sites More sharing options...
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