RA40 tacho & aftermarket ECUs - anyone had trouble?

**thechuckster** · 26-11-2006, 02:38 AM

i'm having a little difficulty getting a reliable tacho display.

My ECU has a 5+ logic level tacho out and I've setup a tacho booster as per the gutted-relay-and-other-stuff circuit. But the tacho display randomly drops out (needles plummets to 0).

i was just wondering whether i could use an old igniter as a tacho booster? they're designed to take a 5+ level input and have generally a dedicated tacho output as well as driving the coil... these would probably do a better job of driving the tacho?

Would they get seriously confused (e.g. smoke & die) if there was no coil connected at all?

I have collected a few from 4cyl camry's as donors but would like some guidance first before blowing up things in the garage (again).

**pilot** · 26-11-2006, 05:32 AM

Hi Charles. I have a similar problem in my Celica (RA63 with 18R-G engine). My Wolf ECU has a 12 v square signal output that doesn´t make the tacho work at all. As I´m running wasted spark in my car, I have 2 options: adapt the square signal from the ECU output to my tacho input or "join" the signal of both coils and send them to my tacho. A friend of mine, who deals with electronics recommended the last option as being better (in the first option, in case you don´t have the tacho connected, the circuit would fry).

**SIKTOY** · 26-11-2006, 01:34 PM

Chuck
What ecu are you using
When I had the EMS I had to conect 12v direct to the tacho wire with a 20k risistor in the line

**thechuckster** · 26-11-2006, 10:34 PM

My ecu is a megasquirt (msefi.com) see the member's ride thread fro more info.

The standard driver circuit is described on this page:
http://megasquirt.sourceforge.net/extra/tachopin.html

But i'm using the circuit as suggested by Wilbo &co:
http://www.toymods.net/forums/showpo...9&postcount=10

i'm pretty sure that i followed the specs for the wiring - but am willing to cast doubt on my own work ;-)

**minorobsession** · 26-11-2006, 10:48 PM

Iam using a Megasquirt in an AE86 setup with the coil and it drops abnormally during gear changes, I had not worried about it try the Megasquirt Extra forum as I think that this is a megasquirt issue.

**ViPeR_NiPPleX** · 26-11-2006, 10:56 PM

got an oscilloscope? if possible, would be worth confirming the output of your adaptor isn't dropping out. Seems odd it's random

**Grant** · 26-11-2006, 11:26 PM

I tried many DIY boosters and all had drop-out or other problems at high revs which is why I ended up getting an electronics engineer to develop one that would work at all voltages and frequencies. Good luck if you can get them to work properly.

**JustCallMeOrlando** · 27-11-2006, 10:55 AM

Originally Posted by pilot

I have a similar problem in my Celica (RA63 with 18R-G engine). My Wolf ECU has a 12 v square signal output that doesn´t make the tacho work at all.

I would be trying to adapt this, as it's exactly what's coming out of my ECU. I did the gutted relay trick, and it's been nothing but faultless since I built it.

**The Witzl** · 28-11-2006, 12:01 AM

but if you are still using a single coil chuckie, couldnt you just hook up mr tacho to the negatory coil terminalisation?

... just a thought

**thechuckster** · 28-11-2006, 12:48 AM

i was:
-worried that the IGBT might take exception to driving the coil and the tacho
-planning to go to waste-spark once things are running good so an ECU output would become a lot more important

i'll borrow a scope and see what my circuit is doing - perhaps it is my own (lack of) skill at circuit building.

anyway - thanks to all!

**minorobsession** · 28-11-2006, 01:05 PM

This is how it works...

Is the tacho high or low voltage?
If the tacho connected to the negative of the coil when the car was std then it is High Voltage.

If the tacho was originally connected to a tach output on an ignitor or equavalent then it is low voltage.

If it is high voltage and you have one ignition coil then connect the tacho to the negative of the ignition coil.

If it is high voltage and you have more than one coil i.e. wasted spark you will need the relay coil to mimic the ignition coils pulse to the tacho, as you have done. If you connect direct to one of the ignition coils it will read only half the rpms etc.

If it is low voltage then connect the tacho using the diagram with the resistors, I am assuming you are using inbuilt ignitors in the Megasquirt, or you can connect it to an external ignitor tach output if you have just one ignitor.

My assumption is an RA40 had points originally and so it is high voltage tacho and I would think that your tacho would not have survived being connected to the high voltage pulse if it were a low voltage tacho.

Obviously check all your connections for intermittent faults but as I said above there is some strange behoiur with the Megasquirt tach output that needs to be raised with the Megasquirt extra forum.

**TurboRA28** · 29-11-2006, 03:32 PM

Hi guys, i'm trying to make the circuit that Wilbo posted.. Problem is my local jaycar cannot find me a BC433 or anything similar as they dont know what it is.

Does anyone know another number or something that jaycar would carry?
Thanks
Joel