Adapting 205 Gti Tachometer

[Mad Professor 7]

Hi All.

 

I know over the last few months there has been posts regarding transplanting a tacho from a difent car into the 205, from the posts I have seen the PCBs are bigger, and are not a very tidy install, so if I can help it I do not wish to go down this route.

 

If it is at all possible I would like to adjust the stock 205 GTi tacho to work Aftermarket ECUs

 

I have taken my Tacho apart to gain access to the PCB.

 

 

Does anyone know if and or how it would be possible to add and or change parts on the PCB to make it work with more modern ECU's?

 

Thanks For Your Time.

Edited September 12, 2006 by Mad Professor

[madspikes 41 1 Cars]

Good work! I have just started looking at this myself, as in just pulled the rev counter apart at the weekend! I cant find a good data sheet for that IC, only duff ones see link.

 

data sheet

 

 

That IC is a "PWM (Pulse Width Modulation) control circuit" according the data sheet.

 

See: PWM Explained

 

So the more pulses in a set time frame, the higher the output voltage. eg Higher the rpm, more sparks per second, the higher the output voltage so the higher the needle.

 

I expect, that it might just be a case of tweeking some resistor / capacitor values to allow for the lower input voltage.

 

Is yellow signal, blue ground and red +12v?

 

Mad.

[Mad Professor 7]

I have all ready seen them datasheets.

I don't know if you saw that in the data sheets it shows a 16pin IC, but the IC in the tacho is only 14pin.

 

Yes

 

Yellow = Tacho Input

Red = Power

Blue = Ground.

Edited September 12, 2006 by Mad Professor

[madspikes 41 1 Cars]

I'm going to tweek the first resistor on the tacho input.... See if it goes bang!

[jonah 1]

Looks like the resistors and caps connected to the tacho input and pin 1 of the chip are for signal conditioning, mainly for removing the high voltage spikes you get on the signal from the coil. Assuming that the tacho output from an aftermarket is just a 12V pulse train, all you would need to do is make the input filtering a bit less aggressive. Removing the cap between pin 1 and Ground should do that nicely. Or you could probably just remove both of those resistors and caps and connect the ECU tacho signal direct to pin 1.

[jimbean 0]

i had a similar problem when i fitted a 605 turbo motor to the 205...

 

overcome it by fitting a taco out of a 309 td

 

fitted straight in to my speedo head and works off the pulse at engine speed sensor insteas of coil pulse

[madspikes 41 1 Cars]

Ok, simple really! Well in testing anyway!

 

I cut through the track isolating pin 1 of the IC, then applied a 5v - 0v square wave (also works with a sine and sawtooth) using a sig gen to pin 1. There you go, one working rev counter. I havent got a MS working at the moment to test it, but if the coil packs / ECU's supply a 5v square or such like all should be good.

 

 

 

Mad.

 

 

I hold NO responsibility to any damaged caused trying this out! I have only done some quick tests!

Edited September 12, 2006 by madspikes

[TVH 0]

Ok, simple really! Well in testing anyway!

 

I cut through the track isolating pin 1 of the IC, then applied a 5v - 0v square wave (also works with a sine and sawtooth) using a sig gen to pin 1. There you go, one working rev counter. I havent got a MS working at the moment to test it, but if the coil packs / ECU's supply a 5v square or such like all should be good.

Mad.

 

 

I hold NO responsibility to any damaged caused trying this out! I have only done some quick tests!

 

 

Nice work!

 

MS tacho output is '0V-open circuit' type which may be pulled up to any voltage needed. Here's a schema how the signal from MS tacho out could be pulled up to 4.7V (assumin 14.4V battery voltage when engine is running) with two resistors that should be easy to fit on the 205 tacho PCB.

 

[Mad Professor 7]

MS-1 Extra Tacho Output pin from the CPU is 0v-5v Digital Square Wave.

The current load for output pin on the CPU is quite low.

 

On the MegaSquirt Extra Site there a small driver circuit.

[Mad Professor 7]

madspikes: What did you use to genarate a 5v - 0v square wave?

[pugrallye 0]

whats so intidy about a tacho swop??? mine fitted like a glove (saxo VTS) 4 years ago!!!

[Mad Professor 7]

pugrallye: Whats the size of the PCB?

 

If it's any bigger then, W: 55mm, H: 45mm, It will not fit into my clocks, as I have all ready had to trim 2mm of the bottom of the PCB to make it fit with out fouling my boost guage.

[pug_ham 240 3 Cars]

The BX one i've fitted to my clocks exactly the same size as the standard 205 one but the ones I've used that have come from donor cars as pictured in this topic the pcb is 52 x 52mm.

 

The back of the housing needs cutting to get them to sit in properly because unlike the standard 205 one the pcb sits further into the housing & not directly behind the facia.

 

Pic 1

 

pic 2

 

I have one from a diesel that I want to check on my car also to see if it works.

 

Graham.