can I use E36TDS 6 cyl ecu engine speed output wire to power an old jag 6 cyl rev counter?

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  1. can I use E36TDS 6 cyl ecu engine speed output wire to power an old jag 6 cyl rev counter? 
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    classic smiths 6 cylinder rev counter, electronic, in a car now powered by a 6 cylinder bmw E36 TDS.

    The bmw engine loom plug, X20, pin 20 is engine speed. I am hoping I can use that wire to power the smiths/ jag rev counter.

    If I understand correctly the original jag signal powering the rev counter comes from/ goes back to the coil, does not `plug into` the rev counter but a small loop in that wire is clamped against the back of the rev counter and induces a signal inside the rev counter through proximity- which is then amplified to operate the rev counter.

    I am wondering if I can use pin 20 from the BMW ecu to dash output `engine speed`, and loop that against the back of the jag rev counter.......but need advice before doing it not being very good at this (although I did manage to strip out the bmw engine, ecu, ews and key and install that in the jag!).

    Q, to try and see if pin 20 can supply a BMW ECU signal to the jag rev counter, should I `earth` the end of the X20 pin 20 SWTD wire to complete a circuit, after looping it against the back of the rev counter in the same way the original jag loom powered the rev counter ? Where does pin 20 wire `go` when it goes into the bmw rev counter ? Does it go to earth or to some other electrical component ? Might I damage the ecu if I run pin 20 to earth vire the jag rev counter?
     
     

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    You're unlikely to damage anything, but it should be outputting some sort of pulse, 3 times per revolution like a 6 cyl petrol would.
     
     

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    thanks - I am a real clutz at this - you say outputting a pulse, but can a pulse travel down a wire if it does not earth or go anywhere ? I am still trying to understand what to do with the cut end of the wire - would it work simply looped on the back of the rev counter, or must it be earther to let the pulse go down the wire ? If I understand you- you suggest connecting the wire to earth after looping it on the back of the rev counter and seeing if that works ?
     
     

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    Ah, right, the rev counter you have is inductive.
    The speedo pulse probably won't be strong enough to trigger it, there might be a slight chance that it'll work, but I'd start thinking of a back up plan.
     
     

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    thanks - and bum! worth a try - will do but.....do i earth the ecu pulse wire in order to try?
     
     

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    Maybe try running it to a bulb. That might do something, but a lot of BMW's stuff is switched negatives, ie, it might want to have the wire from the ECU connected to a bulb, and then to the positive terminal of the battery.
     
     

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    thanks/ dont understand - output from ecu would tend to be positive or am I mistaken - so why to bulb/ live and not to bulb/ earth ?
     
     

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    The outputs are often negatives, they are switched grounds.
    The check engine light is a good example, if the ECU wants to flash the check engine light, it would ground the pin and anything connected to it.
     
     

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    ok will try sunday see wot apns - incidentally.....replica xk150 jag, all original jag chrome red leather, walnut dash, push starter n stuff, got the 4.2 6 cyl 3 carb petrol engine in the garage (auto/ 12-18 to the gallon) but now with remapped 2.5 tds 180 ish hp loadsa torque....and 2 seats/ no roof.....and.....all being well 40+ to the gallon and tax exempt. but must have all clocks working so plan B will be hatched tomorrow if no revs evident ! - thanks for the advice
     
     

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