Word Of Warning About Tacho Modifications

[TVH 0]

Hello everyone,

 

After 1,5 years and just about 300km of track abuse, my tacho suddenly died. It was wired from Megasquirt tacho output straight to pin 1 of the tacho IC, and there was a 1K pull-up resistor to regulated +5V on the Megasquirt tacho driver board. My advice is, always use a current limiting resistor in series with the tacho output, don't wire it straight to the IC pin 1.

 

I didn't have another OEM tacho to play around with so I bought a cheapo Trisco tacho and stripped the internals out, it was almost a straight fit to the GTI instrument cluster. And it is more accurate too

[Mikey G 3]

I see quite a few people slating the diode method in favour of the tacho mods (probably due to cost/availability/understanding). But the diode mod is the best and safest way to do it IMO as all it does is modify the signal of multiple coils into a single signal that the tacho can read in the same way it reads the signal from the original single coil setup. Why people insist on modifying the tacho board when they can either fit a tacho designed to read a digital signal or use the diode method is beyond me. I've done a number of diode conversions now and they all worked first time and continue to work as we speak.

[dazEmad 0]

I see quite a few people slating the diode method in favour of the tacho mods (probably due to cost/availability/understanding). But the diode mod is the best and safest way to do it IMO as all it does is modify the signal of multiple coils into a single signal that the tacho can read in the same way it reads the signal from the original single coil setup. Why people insist on modifying the tacho board when they can either fit a tacho designed to read a digital signal or use the diode method is beyond me. I've done a number of diode conversions now and they all worked first time and continue to work as we speak.

 

Can you get these diodes from maplin and is there a diagram showing where to wire in.

[Super Josh 4]

I think that the tacho mod is a far more elegant solution. I think that wiring in a set a diodes is a PITA.

 

Maybe you have been unlucky or maybe everyone else that has modified their clocks has been lucky, who knows?

 

I would rather understand what are the differences between a set of clocks from a later Pug that are designed to run off an ECU tacho signal and the 205 clocks.

 

I know Graham (Pugtorque) favours transplanting the inards from a leter set of clocks, but it would be good to understand the problem.

 

What value of currrent limiting ressitor have you used by the way?

 

 

 

Josh

[welshpug 1,621]

wiring in the 4 diodes is a doddle, about a quid's worth of diodes soldered together in a plastic film canister and some wire tails attached to the wiring loom is far easier than trying to dismantle the dash of a 205

[Super Josh 4]

wiring in the 4 diodes is a doddle, about a quid's worth of diodes soldered together in a plastic film canister and some wire tails attached to the wiring loom is far easier than trying to dismantle the dash of a 205

 

True, True, cue bits of the dash cracking, threads stripping etc. :lol:

Just doesn't seem that elegant to me though.

 

 

 

Josh

[pee vee 1]

can anyone give me a link to the EXACT diodes to use ?

as i tried some i saw on a forum, and it didnt work gonna scrap the crappy emerald tacho driver i think.

[Mikey G 3]

can anyone give me a link to the EXACT diodes to use ?

as i tried some i saw on a forum, and it didnt work gonna scrap the crappy emerald tacho driver i think.

 

1N4004 rectifier diode, 1 per coil.

1N4744 Zener diode

1Watt, 1Kohm resistor.

[gti-si 0]

Also check Spiky's thread about it, plenty of info on that including a diagram

 

Clicky

Edited July 21, 2008 by gti-si

[pug_ham 240 3 Cars]

Getting the clocks out of the car is about the hardest part using my method but even then its not exactly difficult.

 

wiring in the 4 diodes is a doddle, about a quid's worth of diodes soldered together in a plastic film canister and some wire tails attached to the wiring loom is far easier than trying to dismantle the dash of a 205

 

Its hardly a struggle to undo at most a dozen screws to get them out & then two 7mm bolts, two screws on the binnacle after unclipping the front & a bit extra modifiing to the facia then maybe but fitting the digital tacho conversion like I do only takes two hours maximum to do.

 

I've changed how I do them now & the back of the clocks doesn't need any work to get it all to fit.

 

Just ask PhilCTi who I did one for recently.

 

Graham.

[petert 549]

Why people insist on modifying the tacho board when they can either fit a tacho designed to read a digital signal or use the diode method is beyond me.

 

What if you're running GM LS1 type coils which are triggered by the ECU? Or a CDI with multiple coils? Neither method will work with just diodes and a standard 205 tacho.

[B1ack_Mi16 67]

What if you're running GM LS1 type coils which are triggered by the ECU? Or a CDI with multiple coils? Neither method will work with just diodes and a standard 205 tacho.

 

Not sure if it will be very different, but I just run the tacho signal direct of the coil drivers of the Emerald, before the coil.

That's with ignition amplifiers built into the ECU.

 

Or are the LS1 coils very different to regular coils?

[welshpug 1,621]

LS1 coils I guess would be driven in the same way as the GTi6?

 

I.E direct without an additional Amplifier, if they are then a few diodes should work (does for the gti6)

[petert 549]

The LS1 coils have a built in ignitor. So all the ECU has to do is earth out the coil to fire it, thus producing a 12V square wave. Sure, you could add all signals together with diodes, but you've still only got a 12V signal which the std. 205 tacho can't read.

 

I agree that a current limiting resistor, replacing the std. filter capacitor/resistor, is the safest and easiest solution for a 12V input.

[Mikey G 3]

The LS1 coils have a built in ignitor. So all the ECU has to do is earth out the coil to fire it, thus producing a 12V square wave. Sure, you could add all signals together with diodes, but you've still only got a 12V signal which the std. 205 tacho can't read.

 

I agree that a current limiting resistor, replacing the std. filter capacitor/resistor, is the safest and easiest solution for a 12V input.

 

This is my understanding of it.

 

The tacho is reading, a 12v signal. All coils are powered by the same 12v source and are fired by the ECU by grounding to earth. In much the same way the original single GTi coil (cylindrical or manifold mount) is fed 12v and fired by grounding to earth through the ignition amplifier by the reluctor coil. The tacho reads the very same signal. All the diodes are doing is stoping the signal spiking back through to the other coils.

[TVH 0]

What value of currrent limiting ressitor have you used by the way?

 

I don't have a working 205 tacho to experiment with, but I suggest wiring a 10K (or even 50K) pot to find out what is the maximum resistor value that can be used and then replace the pot with a normal resistor.