Wiring diagram interpretation

[thegreatestben]

build the zener circuit

Not going to be a problem?

The reason I ask is that on the megasquirt site it says not to use that type of circuit unless the tach was originally fed by the coil -ve.

There is other diagrams on there suitable for ignitor/ecu fed tach's.

I know both spencer and mike-e to be knowledgeable, I just want an explanation as to why you think what you think

[Spencer]

Na dont build that forgot it was for the MR2 somehow, the toyota ignitors produce a square wave you dont want to feed that into your tach. Yea there's a couple other circuits in the mega manual, your going to have to play around with those to get it correct. Scope a 4age & see what the signal should look like, I cant remember if its a 12v square or 5v

[Ned]

So from the reading i have done this morning, there are 3 ways a Tach works;

1) 5V square wave

2) 12V square wave

3) High Voltage spikes, somewhere from 18V to 40V or so

Here is how you would make each circuit;

Now, if you need circuit 3 (meaning your old tacho got fired off the '-ve' of the coil) you may find a variable resistor on the back of your tacho that you can adjust to allow for a smaller Voltage signal.

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[thegreatestben]

Ok well I'll get the 1 k resistor and try 5v first, and if that's no good I'll try 12v.

Thanks all

[xsspeed]

I thought all 80's toyota's took tacho off the -ve of the coil?

[thegreatestben]

[Spencer]

I thought all 80's toyota's took tacho off the -ve of the coil?

Nope black wire from the ignitor on most EFI stuff of that era

[Ned]

resistor can be anything from 1K to 10K btw

[forced]

The output from the igniter is a switched earth as already stated.

Those components in the ECU which aren't needed, they're only there to detect a missed or faulty ignition event and then bring up a fault code, they're not there for the tach.

Zener diode circuits are only for tachs driven directly from the coil. That's because the coil output is very high voltage spikes, back EMF and all that. So the zener diode cleans it up and creates a squarewave by clipping the signal, we've already got a squarewave from the igniter.

But a switched earth won't do anything without a pullup resistor, again as stated, 1K to 10 K connected to 12V should do it.

Chances are the reason it won't go is because you killed it by feeding 5V into it, either the tach will be dead or the output from the igniter.

Steve

[xsspeed]

EFI stuff of that era

Could be it - I was thinking of my corolla (carb) as was looking to put in tacho dash to replace non tacho dash.

But also thought this was the case for EFI (ie fxgt/nzgt ae82).

Stand corrected

TGB is that the AW wiring diagram? will check out mine tonight as I assume it must be the same then.

Probably a good thing that I didnt just try to hook up the dash with tacho to the coil -ve then.

Fuck I need to learn this shit better

[Mike-e]

meh, zener setup worked sweet in my GZE setup hence application suggestion

[thegreatestben]

WELL, today I bought the resistors and tried 5v and 12v.

The result?

I got the tach feature of my multimeter to work off've 5v and 12v with the 1k resistors.

However the dash saw nothing. Hmmm.

Dash pulled apart, found the wire I was using in the boot going into the dash cluster.

That pin is labled ign-

In which case I'm gonna try the zener diode.