Using AE86 Tachometer & Aftermarket ECU 12v output?

[Flex]

Andrew,

Here's the schematic of the tacho:



Basically the T2685A is a ASIC from OKI which is designed for running tacho gauges.

From looking at the diagram the tacho should be able to take a +12 signal from the after market ECU, as long it is negative triggered.

Hope this helps

[assassin10000]

I think it's actually positive triggered. From what I understand the coil negative produces a positive EMF spike everytime it fires... that's what produces the pulse the IC reads to convert into needle movement.

People have tried hooking 12v tachometer outputs to the tach directly in the past and it didn't work.


I just got a PM from a guy on a different forum who tried the club4ag solution for the by installing a 15k ohm in place of the R4 30k, he said it didn't work all that well. By which I'm 'assuming' at this point the tach responded/worked in some manner, but wasn't accurate or didn't work normally (twitchy/bounced around, worked only part of the time, etc).


EDIT: Also, part of your diagram looks to be drawn wrong (or my understanding is off). Most likely because of the angle/shadow in the pic.

IC pin 7 does:
IC pin 7 -> tacho out (-)
IC pin 7 -> R5 -> C3 -> R9 OR C4 & ZD


IC pin 9 does:
IC pin 9 -> R9 -> 12v (+) in.


R9 is connected to:
IC pin 9 -> R9 -> 12v (+) in.
tacho out (+) -> R9 -> 12v (+) in.
Gnd -> ZD -> R9 -> 12v (+) in.
Gnd -> C4 -> R9 -> 12v (+) in.


Am I making any sense here? Kinda hard since a bunch of things connect in one spot.


Andrew

[JP]

the vipec will be able to drive it if you do the mod in the PDF. If the accuracy is not spot on you can adjust the vipec tach aux output so it's accurate.

[Flex]

Andrew,

Sorry for the mistake in the schematic, below is the updated one:



I'm not sure what the JP mods does, but looks like it changes the trigger point thresholds.

[assassin10000]

No problem, it's hard to make out in the picture... I grabbed the circuit board I took out and looked at it to confirm the schematic. That's when I saw it was wrong.


I think that's what it does/supposed to do (or so I'm inferring from JP & Anthony @ kaizen). Since I 'think' the GZE ignitor has a low-level output for the tach.

The 'JP' mod, changes 3 items.

C2 = .0047uF (472, instead of 473 which is .047)
R4 = 24k
R8 = 68k

Do you have a datasheet on the T2685A to confirm what the changes do to how the circuit works? Or maybe come up with our own values to replace?

Andrew

[Flex]

Andrew, Sorry I don't have the datasheet for the T2685A. Therefore I can't really say what any changes would do to the circuit's operation.

I'd be interested if these is a demand for modifying tachos out there as I could make replacement boards for any cluster.

Flex

[assassin10000]

Ok, no problem. I suppose there might be, but I'd like to figure out what to change on these instead.

Andrew

[assassin10000]

Moving on to some new (to me) info, and trying to draw some more conclusions on my own. I've found out that the 7mge/7mgte/1jzgte (and probably several other toyota's) use the same circuit board and IC for their tachometers.

This is what I'm thinking, based on the info I've read:
1. The 7mge is the same voltage 'level' of signal as the 4age. As it has a distributor w/ignitor firing a single coil and gets it's signal from the coil negative like the 4age.
2. 7mgte/1jzgte/2jzgte is the same/similar input to the 4agze w/coil packs. These three seem to run the 7mgte 'turbo model' tach's fine, which is supposed to be receiving a 12v signal.

Well, besides the fact it has a different pulse/frequency count because of 6 vs 4 cyl.


Reference pics/links:
http://www.autosoftsys.com/supra/tachcompare/
(note: on the above page R5 mislabelled as 55 ohm, looking closely it appears to be a 56 ohm)
http://i29.photobucket.com/albums/c2...o/P4300056.jpg
http://www.4cefed.com/tech/natachchanges.jpg


Comparing components on the few pictures 'found' in the various threads (see links/pics above). Starting with resistors:

R5 = 56
R6 = 15
R7 = 560
R9 = 47K
Resistors 5-7 & 9 are the same for 4age/4agze/7mge/7mgte/1jzgte. So obviously no changes necessary here...

R1 = 7mge #1 62k & #2 180k, 7mgte 130k and 1jzgte 100k
It appears that R1 varies on both 7mge tachs also. Which I think supports my 'guess' that it varies to adjust accuracy of each tach to their drive motor.

R2 = 12k 7mge/7mgte/1jzgte.
R3 = 510 7mge | 560 7mgte & 1jzgte
Compared to the 4age/4agze tachs, r2 & r3 are different. I'm guessing R2/R3 is also affecting accuracy, after all they are on a different tachometer. R2 stays the same, and R3 close to the same when compared between the different 6 cyl versions. Why that is I don't know, but when comparing what little info I can find on 4age vs 4agze, R3 stays the same and R1/R2 are different, for what appears to me to be a different gauge/sweep. (IF anyone reading this has a 4agze cluster, please take a high res pic and identify the components on it.)

I do know some of toyota's tach's had a variable resistor (or two) on them which would allow accuracy to be adjusted for both range/reading and different pulse counts. So R3 'could' be one thing that is changed to either; 'read' a 6 cyl pulse vs a 4 cyl, and/or another sweep/range adjustment.

R4 = 30k 7mge | 15k 7mgte/1jzgte
This is the first one that I think I will be changing/experimenting with. On the 7mge it's the same as 4age. And the 7mgte/1jzgte motors all change to what appears to be the same value as what I've found so far for the 4agze.

R8 = 33k 7mge | 39k 7mgte/1jzgte
The second to be changed/experimented with. Again, 7mge same as 4age. And the 7mgte/1jzgte is again the same as I saw listed for the 4agze.


Moving onto the capacitors:

C1 = N/A 7mge | 0.1+ 35v, 7mgte/1jzgte.
It wasn't identified positively on the 7mge, but appears identical. Most likely it's the same, so no change here.

C2 = N/A 7mge | .0047uF 7mgte/1jzgte. It's actually labelled "472 50K NCC".
Again, it wasn't identified for the 7mge. But looking at the pic's, physically it appears closer to being the same 473, found on the 4age board than the ones shown for the 7mgte/1jzgte tach's. This is another one I will experiment with.

C3 = 47uF 10V 7mge/7mgte/1jzgte..
Probably has to do with 6 cyl vs 4 cyl signal, as the 4age/4agze which both are 220uF

C4 = 4.7uF 25V 7mge, 33uF 25V 7mgte/1jzgte
Another I will experiment with, as the 7mge once again matches the 4age. And changes for the lower level signal of the 7mgte/1jzgte (which is the same as whats shown for the 4agze).

~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ ~~~~~~~~~~~~~~~~~~~~~~~~

Cliff's: I'm thinking R4/R8 & C2/C4 are what really needs to be changed, now I just need a running car to try it on...

Andrew

[assassin10000]

Small update, wired my motor but haven't started it yet. Still need to sort out an exh system and a few other minor things. But the tach does not (as I thought) respond to the signal generated by the Vipec ECU unmodified.

Andrew

[assassin10000]

Well, I just did the SHMZ/JP mod.




At first it appeared to work fine (and showed zero difference from 10% duty cycle all the way up to 95%) when I turned on a 8k rpm gauge sweep setup for key-on. Which is how I'm able to see if the tach responds without a running motor.

I started to check accuracy with a different RPM value entered for the key-on sweep (2k, 4k, 5k, 6k, etc). I found that anything below a 6k rpm sweep, that the displayed gauge RPM was off by 500rpm. It didn't matter if I had a 5 second sweep time or 1 second, it was off by the same amount. It appears to either be laggy/not responsive enough in lower RPM's with that setup, or it just isn't 100% right.

Anyone else got some input/ideas on this?

Andrew

[uzelac]

What happend in the end, have you managed to run the tacho ?

I'm now in the process of trying to run stock AE86 digital tacho (also have analog 4AGE and 4AC AE86 tachos) with VEMS which (AFAIK) runs 12V square wave output.

I tried solution with two diodes and one Zener connected to two ignition coils with no luck.
Since I run wasted spark it's the same as running with distributor and picking up from him or as connected to all 4 coils which run sequentially.

Now I will try with "gutted" relay, trick which runs on some other AE86 with standalone ECU's as explained here Tacho Wiring - also about 4AGE wiring
I got same instructions on VEMS forum so hope it works OK.

It must be run with stronger output, same ones that run Ignition coils and not regular outputs VEMS has which are limited to 350mA.
Fortunately, 4AGE has just 4 cylinders so i have 4 more avaliable

[assassin10000]

The gze mod works fine. The second thing I tried worked exactly the same.

Once I got the motor running the tach follows what the ECU shows in the tuning software. I think it's how the firmware was written that was causing the oddness.

Andrew