Stack Tacho - Electronics Help Needed

[Harry Flatters]

I have a StacK ST200 tacho which when connected on its own to the tacho output of my MBE967 work perfectly. If however, I connect up my Omex shift light to use the same 12v square wave tacho output from the ECU, it works fine but the tacho stops working. Just to check that it's not the Omex at fault I have another device which also uses the the tacho output and the same happens - it works but the tacho doesn't. Revert to just the tacho and all is OK.

 

I guess this means that the Omex is absorbing too much of the signal for the tacho to work? Iknow I could connect one or the other to the coil pack but I wanted to try and avoid that if I possible could.

 

Someone suggested connecting a resistor from a permanent 12v across to the tacho input on the ST200 to hold the signal higher. This is where it all goes hazy for me cos I wouldn't have the first clue as the the correct value to use or whether it would even work or cause any damage elsewhere.

 

H E L P................................... *confused*

 

*arrowright*Harry Flatters *arrowright*

AKA Steve Mell of Su77on Se7ens

[Chelspeed]

Talk to STACK direct. They have (or used to have...) a technical support person who should be able to answer this sort of question. They're on 01869 240404

[Chris W]

My guess (intelligent one I hope) of what's happening here is that the Omex is presenting too low a resistance across the tacho input and thereby not allowing enough signal drive to the tacho. ie: it is diverting current away from the tacho in simple terms. The idea of using a resistor to "pull up the signal" will not work IMHO.

 

Best advice is definitely Chelspeed's one - Talk to Stack or Omex.

 

However, failing any joy there, I suggest you connect (or have someone connect) an approx 20K Ohm resistor in series with the input to the Omex and connect the open end of this new resistor to the tacho input. This will prevent the Omex from loading the tacho. This may of course reduce the signal too much for the Omex and you may find you have the opposite situation, viz: tacho works, Omex doesn't. I'm confident however that you will find a value of resistor that will allow both to work.

 

Basically, too low a resistor value will make no difference to your current situation (excuse the pun) and too high a value will allow the tacho to work but prevent the Omex from working. You may need to try a couple of values to get it right but I feel 20K would be a good start. The final value is likely to be in the range 10K to 30K in my view. You won't find exactly 20K (it's not a "preferred value") but 18K or 22K are and either would do.

 

An alternative thought occurs as well..... if you open up the Omex, I bet you will find a resistor wired across the input terminals (no idea what value but my guess would be in the range of 1K to 10K Ohms. Remove this and you may find everything works without adding the additional resistor I suggested above.

 

Hope this helps

 

Chris

 

1.8K SV 140hp see it here

 

 

Edited by - Chris W on 11 Mar 2003 10:02:11

[Harry Flatters]

Chris,

 

Thanks for that. Just so I can be clear as to what should connect to what, would you give me a buzz on 020 8642 0162. A scribbled sketch on a fax would be equally good on 020 8643 2275.

 

Thanks again

 

*arrowright*Harry Flatters *arrowright*

AKA Steve Mell of Su77on Se7ens

[Chelspeed]

Omex are on 01242 681044 and are even more helpful on the phone than Stack.