Rev counter problems... Not the usual ones though ;-)

studarude · Post by **studarude** » Sun Aug 02, 2009 10:06 pm

Hi all,

My Megajolt install has been going really well, fired up first time, even the trigger wheel only needed offsetting 1degree to bring up to true..

But... Im having the usual problems with the rev counter, so far, ive tried the Megajolt tacho out, the EDIS tacho output, the PIP line, and even splicing the diode pack into the either side of the coil pack earths with no joy.

The car is a 205 GTI.

Now, with the diode pack in, i get a flicker on initial start-up then it stops.

So.. if i feed the counter off one side of the coil pack, i can get a reading at half value, as you'd expect. So we can rule out any problems the gauge and/or wiring itself.

Now the weird bit.. If i feed off one side of the coilpack for the revcounter, and link in the other feed with a diode (im aware that one of the coilpacks will be firing off the other trigger and hence firing twice as often as it should) then the rev counter works, and gives a good reading.

Any ideas?

studarude · Post by **studarude** » Sun Aug 16, 2009 11:33 am

Anyone? Im rellay struggling to get to grips with this..

NITROPIXIE · Post by **NITROPIXIE** » Sun Aug 16, 2009 6:44 pm

Are your diodes in good codition?? Could be a faulty one.

Post by **brentp** » Sun Aug 16, 2009 8:20 pm

As NITROPIXIE said... also- some have tried different voltage zener diodes with some success- I believe it was discussed here on the board some time ago.

Interfacing tachometers is a perennial problem, unfortunately. We improved the V4 tach out by offering a 12V square wave output, yet it's still doesn't always solve the problem. Have you hit up the 205 GTI boards? Does it require a mod to the tachometer itself? that has been the case with other designs.

Please share your findings if you come up with a solution!

adsport69 · Post by **adsport69** » Mon Aug 17, 2009 3:13 pm

Hi there, I have a 205 GTI fitted with a megajolt, and my Tacho is working fine - you need to get your tacho modified, take a few bits out of it. I had a few issues, i modified mine, now it uses the tacho signal from the megajolt and it works fine now. got a vid on you tube in my shed with it running through VRSSIM.

studarude · Post by **studarude** » Mon Aug 17, 2009 4:26 pm

What modifications did you do to get it work fella? Care to divulge please?

NITROPIXIE · Post by **NITROPIXIE** » Mon Aug 17, 2009 5:36 pm

Been looking on the old 205gti forum and it seems a few people have had a similar problem. Apparently there is a box of tricks which converts the signal from the OEM ecu's. Your best bet would be to check the old 205 forum out and see what other people have done. According to them there is a converter you can buy or there is a cheap and cheerful way someone has done/bodged.

Here is a couple of links to help, but i'm by no means a 205 expert.

http://forum.205gtidrivers.com/index.ph ... 67603&st=0

http://forum.205gtidrivers.com/index.php?showforum=42

adsport69 · Post by **adsport69** » Mon Aug 17, 2009 6:17 pm

I guess i did it the cheap and cheerful way and took out the offending items from my tacho, just took it apart, modified it and hey presto it worked. There seem to be at least a couple of tacho's as the one's i've done have been different - but hey it works job done.

studarude · Post by **studarude** » Tue Aug 18, 2009 12:22 pm

Thanks for the replies guys, ive been browsing the 205forum for a while now with no real joy..

Adsport: What did you do to modify your clock to make it work?

adsport69 · Post by **adsport69** » Tue Aug 18, 2009 1:31 pm

I think I took a couple of components out of the sensing side of the Tacho - do you want me to sort yours for you? You could send me it and I'll send it back. Have you seen the VID i have on youtube of mine - My Tag is the same ADSPORT69.

studarude · Post by **studarude** » Tue Aug 25, 2009 5:25 pm

Ad,

Heres the pics fella...

adsport69 · Post by **adsport69** » Tue Aug 25, 2009 9:33 pm

That one looks like the one in my gti, had another one floating about the boot of my daily drive, just popped out to get it and it looks different to that one.

Do you want to send me it and I'll have a look at it for you? Where has the plug gone?

studarude · Post by **studarude** » Sat Sep 05, 2009 12:23 pm

I *think* im sorted now Ad, but thanks for the kind offer!

Right guys, ive managed, with the help of a mate who is well into electronics, to sort the issue out. After a bit of digging around the internet i managed to find a schematic of the rev counter, and immediately it became fairly obvious that there is quite an aggressive filtering section onboard as you'd expect given that originally the car would have a single fluid coil, so we removed this filtering (essentially 2 capacitors and 2 resistors, bridged the input straight to the chipset, and added a pull-up resistor to increase the voltage of the supply.

Im happy to say that it now works perfectly! And will be more than willing to share the proper modification once i get a second to do a few photos and such.

So thanks for all your input guys, we got there eventually! the car is going in for mapping/carb setup on tuesday so ill hopefully be able to put together a 'success story' thread.

Post by **brentp** » Sat Sep 05, 2009 2:07 pm

Please do share the mods, we would like to add it to our wiki!

Autosport Labs

Rev counter problems... Not the usual ones though ;-)

Rev counter problems... Not the usual ones though ;-)

205 GTI TACHO not working

Call me cheap and cheerful!

Typical Eh