How To ... Tacho Booster

[midnight_purple_GA70]

it definitely is... is brilliant having my factory tacho working after all these years... now i just have to pull it all apart and reconnect the ground to my shift light that now sits within the dash and set it all ready to go

[adam_k]

once again, resurrecting an old thread, but i'm at witts end trying to get one of these working.
i made a neat little one out of a relay and after receiving flatline off the wire in dash, and nothing off the white wire on the cps, and getting 4000rpm off the red wire on the cps (at idle).... i made another one with the EXACT listed materials only to find it works exactly the same as the one i first made (it doesn't)!
am i doing something incredibly wrong? what is the exact wire i need to tap into on the cps, how is the cps wired internally? (my AEM uses only the red and green wires, yellow is left hanging) ARGH!

[RA35GT]

The tacho wire usually comes from the ignitor.

What engine/car combo do you have?

[andrew_mx83]

Lots of good info in this thread guys!

My Car is an mx83 with 7m-ge running waste spark.
I was hoping to get time over the break to build a booster but now wondering if the local brainiacs have a better solution.

Initially i ran a diode on each coil -ve straight to tacho, no love.
I added a 16v zenner after the 3 coils collect and again no love.
I changed the zenner for a 13v jobbie and now have fairly good (occasionslly jumpy) tacho upto 3200rpm at which point it falls to 0 while the engine continues to rev. If you get the revs under 3200 the needle instantly comes back up.
Under 3200 it is pretty livable but not perfect and reasonably accurate.


My question is, if i continue to drop the voltage on the zenner will it push the cutoff point up or eliminate it altogther? The tach really needs to read to around 8500rpm as there is no limiter and i rev the nuts off it.
Can i go under 12v on the zenner?
I can build a 7m to rev to the moon but im not so cluey with electronics stuff.


Alternately i have 2 different low voltage tach outs i could use with a booster but i feel im fairly close to not needing one...

[dumbass]

U need an oscilloscope to test output
I use one type of booster on most toyotas and microtech a
Then another one for most nissans

Then a really expensive one to get landcruiser tachos to work with new motors

It's all hit and miss with tacho boosters so unless u find someone with exact same setup that has worked it out
And to tell u what to do its all guess work

Sometimes I've spent two hours trying to get tacho to work so how do u tell customer that's 200 bucks to get tacho working
That's y I won't touch tachos if some other wally has wired up the motor or quote them a lot hehehe

[andrew_mx83]

Haha i thought that would be the case....

I cant work out why the zenner diode helps, i know it lets anything over a certain voltage pass through... does the tacho only sense pulses over a certain voltage? Its a simple coil -ve trigger type.

[dumbass]

Most tachos read the top voltage part of each spike
Old tachos need higher voltage to tacho to work
I don't know much about inside the tachos I just know I have 5 or 6 tricks I try
Otherwise I send the tacho to instrument guy

[masterofsinanju]

...

It's all hit and miss with tacho boosters so unless u find someone with exact same setup that has worked it out
And to tell u what to do its all guess work

Sometimes I've spent two hours trying to get tacho to work so how do u tell customer that's 200 bucks to get tacho working
That's y I won't touch tachos if some other wally has wired up the motor or quote them a lot hehehe

You tell them "that's two hundred dollars thanks"
If they bitch "here's one I haven't fixed, why don't you try it yourself?"

If its so easy they shouldn't bring it to you in the first place.

[adam_k]

The tacho wire usually comes from the ignitor.

What engine/car combo do you have?

it's a 7mgte in an ma61 body, with a microtech ecu.
the signal off the ignitor gives me ~500rpm at idle and doesnt rev in proportion to what the what the engine seems to be doing (ie. giving it some beans seems indicates ~2000rpm).
i've never had the tacho working in the car so i thought this was a little low. is this what it would idle normally and smoothly at?

[dumbass]

Try a pullup resistor maybe
I use my own relay setup and it works on all toyotas and most micro techs
So maybe your doing something wrong

[RA35GT]

it's a 7mgte in an ma61 body, with a microtech ecu.
the signal off the ignitor gives me ~500rpm at idle and doesnt rev in proportion to what the what the engine seems to be doing (ie. giving it some beans seems indicates ~2000rpm).
i've never had the tacho working in the car so i thought this was a little low. is this what it would idle normally and smoothly at?

7M-GTE's usually idle around 650rpm when warm.

MA70 dash expects a 12V square wave for the tacho. (12V pullup comes from the dash i believe)
MA61 dash expects the coil negative flyback voltage type signal.
dumbass may be on track with the pullup resistor on the ignitor signal from the 7mgte.

Alternatively, the microtech ecu output is apparently a 5V square wave, which should also trigger the tacho booster.

I have used a tacho booster on a ma61 before and it does work!

[dumbass]

Pull up sometimes works on toyotas
Lucky my relays work on 95% of toyotas

Anything that's single coil from factory needs good 12v pulse
Anything that's multicoil has a weaker signal so it might be around 5v
Anything carby is usually a super high voltage that a coil produces

Just fit an aftermarket tacho
Much less hassles

[adam_k]

Sure enough I ended up finding a 5V output from an un-used ecu pin and it works with the relay to give the signal. I'm finished with wiring for a little while now!

[andrew_mx83]

Ok i got some lower voltage zenners to play with my tacho.

One question -do i need to remove the existing zenner (13v) or can i just connect the lower voltage onesi in parallel? Will the current follow the path of least resistance and ignore the 13v zenner? Id rather not chop up more of new loom than i have to...

[andrew_mx83]

Ok so i gave up rooting around with the zenner diodes and just built a booster today, got tach over 3k rpm again, yay!

I was going to make an external one as per witlz's awesome writeup/photos but wanted to declick the relay so just hid it all inside while the lid was off.
Its a genuine toyota relay from who-knows-what and all the connections are done on the original spade terminals to keep it all neat.



The original (green) resistor was removed altogether once i was sure i didnt need it.



What tacho booster? Its just a relay :-)

Many thanks and +rep for those who helped...

Edit: not that it will intersst many here, but this didnt work with the low voltage 'tach out' signal from my Megajolt. On a whim i hooked it upto IDM on the ford EDIS box (gives out a pulse every spark event) and it worked straight away.