Tachometers/rev Counters, How/where Do I Get An Accurate One?!

[welshpug 1,622]

Ever since my 205 was put on a rolling road I've realised how inaccurate the rev counter can be in these cars, so is there any way to check the actual speed its idling at?

 

(peak RPM was recorded at well over 6200 IIRC, which a standard 1.9 shouldn't rev to!)

 

I am now running motronic which is great but I have used an adapter to get a signal rather than using the digital signal from the ecu, would fitting a digital rev counter be better?

[Anthony 1,000]

(peak RPM was recorded at well over 6200 IIRC, which a standard 1.9 shouldn't rev to!)

6500rpm is the limiter on 1.9 GTi's (and 6750rpm on 1.6's) as far as I know, although on the analog Jetronic ECU's there's bound to be a degree of tolerance involved.

 

All I usually do is swap rev counters around until I find on that reads approximately correct - a bit of a get-out maybe, but works for me...

[welshpug 1,622]

thing is how do I know how accurate it is?!

[pug_ham 240 3 Cars]

My rev copunter is only approx 50rpm out at idle, i had it checked when I was at Chipwizards when it was beign mapped just in case I'd refit the needle wildly out of position.

thing is how do I know how accurate it is?!

With a garage HT lead signal thingy clipped over an HT lead & half the reading, thats how we did mine to check its calibration.

 

TBH with a proper digital rev counter signal taken directly from the ecu output it shouldn't be that inaccurate though imo. No idea if the signal convertors you've fitted will effect it but imo they shouldn't make much difference.

 

Looking at the difference between the back of the original rev counter & a digital signal one there is a lot more weight to be driven on the oe one which might make the difference in the readings but your car with a ZX MP3.1 ecu should be limited at approx 6.6k iirc.

 

Graham.

[welshpug 1,622]

Thanks Graham, maybe I should have gone to the trouble of sourcing another rev counter, I dont think the inaccuracies have much at all to do with the signal converter as it was a fair way out previously.

 

The main reason I want to set the RPM right is that its been set to 900 rpm hot, but I'm sure its a fair bit lower than that as it sounds a lot slower, more akin to an XUD idling

[ajm-e30 0]

try a stack rev counter from deemon tweeks mate, i had one in my 320 and it was so much better than the standard one

[snillet 1]

Stock item is not bery accurate no, when i set mine to be accurate (remove and put back the indikator needle) at idle it shows quite a lot to little at high rpm. Seee this with the megasquirt software.

[cybernck 400 1 Cars]

^ yeah i've always wondered why my Mi16 seemed to top out only at about 6300-6400 rpm

(although i was sure it was more going by the sound of the engine) and i confirmed i was

right when i compared it to MegaSquirt tacho which said more like 6700-6800 rpm.

[snillet 1]

^ yeah i've always wondered why my Mi16 seemed to top out only at about 6300-6400 rpm

(although i was sure it was more going by the sound of the engine) and i confirmed i was

right when i compared it to MegaSquirt tacho which said more like 6700-6800 rpm.

 

That sounds like the same figures i´ve seen really.

At first i did not have the in dash tacho connected when settin up the MS, but when i connected it and drove around i was concerned that i got a strange "surge" feeling at around 6500rpm just before changing gears.

 

Turned out it was my soft revlimit at 7200 that kicked in , duuh..., no wonder i thought the engine suddenly had become daft after 6000rpm

[cybernck 400 1 Cars]

mine also had a drop in power at about 6500, which is actually over 7000.

 

but surprisingly, my GTI 1.9's rev counter seems better... maybe it's inaccurate at lower end though.

[Grahamrally 0]

You know there is a way of making the standard rev counter as accurate as you need. It involves a bit of time and effort, but it's definately worth while. The original unit is good quality and well weighted.

 

If you take apart your dashboard and remove the rev counter, you will find that on the back of the rev counter is a small resistor which can be changed to alter the sensitivety. All you need is some way of reading the exact RPM off the HT leads or from the ECU and then basically changing the resistor to correct the inaccuracy. I find the easiest way to do this is using a 0-22k linear variable resistor, (69p from maplins), soldering it in where the original resistor was, then adjusting it to suite the right rpm accuracy. Then you can measure the resistance with a multimeter that you have adjusted to with the variable resistor and replace it perminently with that resistance value.

 

This works for all ECU's when you can't get the original rev counter to work with an aftermarket ECU. E.g. megajolt doesn't work with the original rev counter immediately, but you can make it work with this method.

 

If anyone would like, I can post a tutorial on how to do it all?

 

Graham

[brianthemagical 1]

i'd say yes to the tutorial. would it include making it work with MS?

[Grahamrally 0]

It's the same process for any ECU or signal that you're trying to get the rev counter working with. The GTI rev counters have a different resistance to the carb model 205's, I guess because they run an ECU rather thantaking the feed from the coils.

 

Graham

[petert 550]

I've recalibrated mine using a square wave generator for an input. It's accurate at idle and 7000, but is up to 300 out at 5000. I found it impossible to get right throughout the entire range.

[snillet 1]

I've recalibrated mine using a square wave generator for an input. It's accurate at idle and 7000, but is up to 300 out at 5000. I found it impossible to get right throughout the entire range.

 

That is about my experience to, but i am running the tacho from a 205rallye(the swedish verion with XU9J1 engine) in mine, connected to the coil(-) as it is in that car. That is not the same one as in the GTi really.