Anything not specifically related to the Speeduino hardware. Eg sensors, bluetooth, displays etc
By Experion
#44301
Hello everyone, Im having a bit of trouble with getting my tacho to work on my 1988 Toyota Starlet.
It has a gauge cluster installed out of a 1988 (i think) Toyota Starlet Turbo S (EP71)
The tacho has worked on the stock distributor but ever since installing my Core4 speeduino unit it hasnt.

The stock signal comes from a sensor in the distribitor that makes a signal specificly for the tacho. before installing Everything I measured this with a scope and it seems to be a 12V signal. it was a bit weird shaped though. almost like its 12v for a few miliseconds, then 5v for about 2 times as long, then ground. but i assume a regular 12v square wave would work.

The current setup is a Core 4 like i said before, it has its tacho jumper soldered to 12v and today i measured the signal with my scope, and it gives a nice 6ms long 12v square wave the 6ms being set in tunerstudio.
Its possible that 6ms is too short but I cant set it any higher so I assume I shouldnt have to.

Im not running the distributor anymore since im using wasted spark coils now. those are mounted realy nicely in the location of the stock dizzy so it would suck to have to change that back.

Ive also measured the wire between te ecu and the gauge cluster for continuity and it measured 0.4ohm which is fine I think since it goes through 3 connectors.

when turning on the ignition I can see the tacho needle move a little (like its turning on or something) but after that pretty much nothing.

in a few weeks I think Il be measuring the signal of of another starlet with a similar distributor to compare.

I hope someone can give me some ideas as to what to do.
it would realy suck if I couldnt use it since the tacho clusters are fairly rare and pretty cool imo.

Thanks in advance,

-Gary
By dazq
#44303
Do you have a schematic of the original setup?
Generally most older tacho got their signal from the -ve of the coil, using the back emf pulse to trigger the tacho circuitry.
Later on true square wave trigger usually from the ECU itself took over until bus type clocksets became common.
I am sure a way to get your tacho to work is possible so don't give up just yet :-)
By Chipboy
#44308
As dazza advised (I am Ozzie and thats how it is here) given the age its going to be the collapsing coil that generates a back emf of a few hundred volts to kick over the Tacho,. If there is any IC in there with 494 on it there I am on the money. eg SL474, my Peugeot 205 does and its 87, motronic, yuk.
Last edited by Chipboy on Sun Jul 19, 2020 10:29 am, edited 1 time in total.
By Experion
#44310
dazq wrote: Sat Jul 18, 2020 11:14 pm Do you have a schematic of the original setup?
Generally most older tacho got their signal from the -ve of the coil, using the back emf pulse to trigger the tacho circuitry.
Later on true square wave trigger usually from the ECU itself took over until bus type clocksets became common.
I am sure a way to get your tacho to work is possible so don't give up just yet :-)
Hello Dazq, thanks for the response.
Im having quite a difficult time finding a wiring diagram for the EP71 Turbo S.
But i do think its possible the EMF pulse is what drives the tacho, some people hook these tachos up to one of the sides of points ignition and that seems to work. i would think that would give a similar pulse, although maybe at a diffrent rate.
speeduino seems to give one puls per revolution, the points would give 2 pulses per Revolution.

the stock distributor for 1.3L engines on these car (mine is a 1.0 but it had the 1.3 dizzy) is electronic ignition with an internal coil, there was a connector on there specificly for the tacho output which is not connected most of the time.

like i said, il try to get a scope reading of a Ep81 starlet, this has a very simmilar distributor, the only diffrence being that the coil is being controlled by an ecu as these have EFI, they use the same coil internaly though.

when I measured my own car with the scope I didnt see any weird pulses if I recall correctly, I would assume I would be able to see the emf pulse on a scope
Chipboy wrote:As dazza advised (I am Ozzie and thats how it is here) given the age its going to be the collapsing coil that generates a back emf of a few hundred volts to kick over the Tacho,. If there is any IC in there with 474 on it there I am on the money. eg SL474, my Peugeot 205 does and its 87, motronic, yuk.
Ive never taken the gauge cluster apart far enough to see any IC's the backside pcb is one of those flex pcbs and ive never looked behind that.



Other then measuring the other car with a scope, what should be my next step?

Thanks,

-Gary
By Chipboy
#44311
No worries, a scope will need an attenuating probe, you are looking at several hundred volts potentially on a brief timescale. The chip is a SL494 or TL494, the heart of modern PC switch mode power supplies showing my tacho is an electric motor! Springs hold it back.
By Experion
#44312
is there maybe some IC i could use to make the 12v square wave a higher voltage pulse?
also, the chance that ive damaged anything by running it on a square wave is also fairly low right?

ive been running the car for about 2 months now, at first the tacho needle would sit a 0 constantly.
ever since ive had the cluster apart for an unrelated reason (removing some sort of annoying ping alarm sound when driving faster than 100km/h) the needle sits realy low, like -1000 rpm. the device i removed was some kind of solenoid which if i apply 12v to it pings one note, and if power is removed it pings another note. this was mounted right behind the tacho so maybe it has something to do with it.

-Gary
By LAV1000
#44313
Gary,
I don't know if it specific to you situation, but there are some tricks by using the coil of an automotive relay.
try google on : tacho circuits relay coil.
You might also search on the MegaSquirt forums, your not the only one with this problem.
By Experion
#44314
LAV1000 wrote:Gary,
I don't know if it specific to you situation, but there are some tricks by using the coil of an automotive relay.
try google on : tacho circuits relay coil.
You might also search on the MegaSquirt forums, your not the only one with this problem.
Hello LAV1000,

your suggestion looks very promising, il look into it. do you think it would be possible to test this by connecting a unmodified relay?
or would de mechanical load of the relay slow the coil switching down too much?
is there some way to limit the voltage of the signal to the tacho?

Il be honest im kinda new to electronics in general but i see the term 'shunt resistor' mentioned sometimes, is that something that would be used?

Thanks,

-Gary
By Experion
#44317
Thanks LAV1000 for the link. Ive read it through and I think I understand it fairly well, but Il ask my follow up questions on that thread.

-Gary

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