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Megajolt Revcounter Probs....
Surrey Dave - 12/8/05 at 07:54 PM

My 'Skoda' rev counter doesn't work with Megajolt, i've tried the diode circuit,and wiring directly to pin 11 on the Edis, but still no joy , has anyone used the Racetech revcounter?

Has anyone tried Josh B's circuit with the transistor?

paulf - 12/8/05 at 08:12 PM

I also had problems getting a ford tacho to work , i tried the edis output and the diode circuit with no sucess. There is a variation of the diode circuit that also uses a resistor across the output to +12v and when i tried that it worked fine.
Paul.

Surrey Dave - 12/8/05 at 09:35 PM

Anyone tried this one? Rescued attachment tachdriverV2.jpg
Rescued attachment tachdriverV2.jpg

GeoffT - 13/8/05 at 08:26 AM

Dave,

I had similar problems with the 'tacho out' line when I tried to use it to drive Megasquirt, i.e. it didn't see any pulses. Eventually discovered that the tacho out signal uses what's known as an "open collector" output, which requires the load to actively pull the signal down to ground between pulses.
I got round this by simply fitting a 1k resistor from the tacho line to ground, then all worked fine. Not saying that this will help you, but being nice and simple to try (for us non-nerds..!), it might be worth a go.

ATB, Geoff.

Surrey Dave - 13/8/05 at 09:44 AM

Geoff,

Did it drive your megasquirt work with no additional circuitry apart from the 1k resistor?

[Edited on 13/8/05 by Surrey Dave]

Surrey Dave - 13/8/05 at 10:59 AM

1) i have tried the transistor and resistor,doesn't work for my rev counter, have tried the diodes , doesn't work for my rev counter!!


2) I have tried connecting the rev counter to one side of the coil pack ,it works but obviously at half speed (better than nothin)

3) then i connected to one side of the coilpack via an in4007 diode , still worked, connect via 2 diodes one one each side of the coilpack in a 'y' connection works briefly at correct speed then fades away to nothing, bit like a capacitor discharging.

I'm guessing that problem could be to do with the load or lack of load the rev counter is pulling on the diodes.

Paulf has mentioned a circuit with 12v and a resistor in conjunction with the diodes, its starting to sound feasible!!!

Its also the same with the zener in the equation.

whats the remedy boffins?

[Edited on 13/8/05 by Surrey Dave]

[Edited on 13/8/05 by Surrey Dave]

Jed - 13/8/05 at 12:56 PM

Have you tried this mod here?

I used the 2 diodes plus a zener and the rev counter died at about 2000rpm. I installed a variable resistor as this meant it was easier to tweak.

GeoffT - 13/8/05 at 01:02 PM

Quote:

Did it drive your megasquirt work with no additional circuitry apart from the 1k resistor?

Yup, but it obviously isn't gonna work for you Looks to me that your tacho is only going to work with the sort of high voltage flyback pulses you'll only get from from the coils. I'm surprised this didn't work, did you use the zener diode in the trunk of the 'Y' arrangement? If so maybe someone will suggest a better way of doing this.
If course, there are those who reckon it serves you right for using Skoda bits in the first place

Having just reread your post you have tried this, that's me out of ideas then..


[Edited on 13/8/05 by GeoffT]

Surrey Dave - 13/8/05 at 02:55 PM

Jed,

did you replace the variable once you had established the values, did you measure them?

Jed - 13/8/05 at 03:36 PM

quote:

did you replace the variable once you had established the values, did you measure them?


No - I left it in place and had taped up the loom before someone else asked what the resistances were.

On my other car using a Racetech rev counter I used a 330R and a 1k resistor (to ground) to knock the voltage down to 75%. This seems to work fine but then the Racetech dials are wierd. You expect a switch to select 4/6/8 cyl operation but it's a potentiometer and I had to calibrate the rev counter against the megajolt runtime display.

paulf - 13/8/05 at 03:44 PM

I have checked mine as i couldnt find the diagram i used, i have the diode circuit with a 15v zener and the output lead is connected to earth[ not +ve as I thought] by a 5k resistor.This works for an old ford tacho originally from an xr3 and would have been coil impulse driven.
Paul

quote:
Originally posted by Surrey Dave
Jed,

did you replace the variable once you had established the values, did you measure them?

Surrey Dave - 13/8/05 at 03:57 PM

Thanks for the trouble guys , i'll have another play tomorrow..............

And report back.

Surrey Dave - 15/8/05 at 10:57 PM

Have tried the diodes circuit again but this time put a 4.7k resistor between the zener output and earth.....................and it works

If anyone can see a problem with this idea can you let me know................

paulf - 16/8/05 at 07:29 PM

That will be fine , mine has been like that for two months with no problems.
Paul.

quote:
Originally posted by Surrey Dave
Have tried the diodes circuit again but this time put a 4.7k resistor between the zener output and earth.....................and it works

If anyone can see a problem with this idea can you let me know................