View Full Version : Issues with piggy back Air fuel 'interceptor'. Dam car's ecu safe mode kickin in.

04-20-2009, 12:37 AM
Hi. I wasn't too sure where to put this. But its rendered my car disabled. :(

I have just assembled a jaycar Digital Air fuel controller to intercept my 0 to 5 volt output on my MAP sensor on my 1990 5sge celica (5sfe ecu).

The input and output voltages are all great, no worries, as the signal has to go through the circuit when its turned off, but it just bipasses alot of the chips/processors. But! the ecu kicks into safe mode as soon as the controller is turned on and the signal gets processed, despite having a right voltage output and set to an 'unmodified' signal.

It triggers the 5sfe ecu to give me an engine warning light in regards to the MAP sensor and goes either into this gay gutless safe mode or stalls.

How can i get around this? Is there a way i can get the ecu to just relax and chill and take the signal?


04-20-2009, 01:06 AM
Are you sure you installed all the little "do-dads" on the board the right direction? Looks like a very nice unit (with its 128 load points) try un plugging it and reconnecting the stock map wires back together so see if it salves your problem so at least you know where the problem area is

hope it works out for you
(still vary impressed about the 5sge)

04-21-2009, 03:18 PM
can i give you a wild guess ?

the MAP sensor is a passive variable resistor (not a voltage source like a hotwire mas sensor).

The ecu feeds it voltage and depending on the resistance of the MAP the output voltage varies... right...

Either way, there is a current draw from the ecu's power supply, and the ECU's input of the map sensor

this current draw varies by the resistance of the sensor which means it varies by pressure because there's a current/voltage/resistance relationship.

If the ecu looks at current draw (and ignores voltage):

0 current draw = burnt sensor or open circuit wiring.
Too high a current draw = shorted sensor or short circuit wiring.

(sounds like typical CEL codes right, CEL MAP sensor Open ckt, CEL MAP sensor short CKT)

If you go in and tamper with the circuit... maybe you're voltage looks ok but you're drawing too much or too little current from the ECU making it think your sensor has just failed.

Now without being hands on, it would be hard for me to tell you where to debug it... but i assume you have the knowledge (since you built it and wired it) to go back and back trace your steps and even take some current and impedance measurements ... and figure out what's pissing off the ECU

a good starting point would be to see if it's throwing an open or short CEL... and go from there.

i'll give you a hint from the infamous mr2 fuel cut defense project...

The solution maybe as simple as taking a resistor equal to half the resistance of the MAP sensor and putting it in series with your jaycar box output so that the ECU when it tries to read the 'sensor' has a limited current draw and doesn't think that the sensor has shorted to a direct power source...

; )

04-21-2009, 10:33 PM
thanks guys.

And thankyou particularly for the high detail. I will have to look into that.
Some other people were suggesting i use the same ground reference as the ecu, compared to me just using the chassis.

But you make a great point about the current. I will definitely look into it.

Thanks all!
hopefully it gets me back on the road asap.

04-22-2009, 07:17 PM
you can measure voltage difference between ECU ground and your piggy back ground... if it's negligible then you don't have a grounding problem... however if there is some difference go out and get yourself a grounding kit. ... and do all the main chassis grounds...

04-22-2009, 11:45 PM
Am back from another unsuccessful attempt. lol.

Its gotta be the current issue you talk of. How do you go about measuring exactly what kind of resistance you put inline?

I'm yet to measure current values. I hooked up the multimeter in series with the output terminal and output line of both the map sensor alone and the jaycar gizmo, and i am getting zero amps, even when set to micro amps.

Yet the car still works with the ecu not complaining with the gizmo off.

But i think i need to try it again. that was at the on position but the motor not turned on. So it was at full 5v output, which i assume there probably would be no current left with the variable resistor (being the map) at full resistance.

How did you go about it?
btw, thanks for the help and advice on this. Your my only source. No one else has been any help on any other forums. The ground isn't an issue.

Thanks again,

04-23-2009, 09:05 AM
I connected a potentiometer to the output, with one line to the output, the other 2 to ground, to act as an adjustable resistor parallel to the ecu (so the voltage doesn't drop but the current does). No luck :(

I even got a random 30k ohm resistor hooked in series out of desperation and still the same stuff. I'm starting to give up. I don't know whether to trust the A/F meter i made, and the fuel adjuster is not working, yet the exhaust smells rich sometimes and has black soot.

I'm going to have to take it to a tuning shop. some do jaycar kits, if they can test mine and sort it out and fit, shouldn't cost too much if it ain't completely rooted. And they can compare my a/f ratio meter to their shop one.

Will have to wait for more money then. sux at the moment. i work at a pool chemicals store while i'm studying machanical engineering, and the winter is kicking in over here, so i only get 1 shift a fortnight. thats $115 a fortnight wage.

Will keep ya posted if anything happens.

04-26-2009, 10:23 AM

jaycar DAF working 100%. Marvelous device, makes all the difference. power curve smooth, not jumpy and rough, hugely better throttle response down low, slightly more torque down low. a healthy Air to Fuel ratio makes all the difference.

The issue i was having wasn't the jaycar gadget, it was the wiring of the map sensor. it was connected to Vc instead of PIM. i was mislead after confirming that the map sensor was 5 volt and the pinout diagram of the ecu off a manual i was reading had Vc at 4.5-5 volt and PIM as 13 to 3.9, but it is PIM, i was reading it wrong.

All good now. I just gotta get a jaycar frequency signal controller do dadd for $30 AUD as the tvis controller. I could use a shift light, but this looks cheaper. Its a great gizmo, made for automotive use. It monitors a frequency signal (like a tacho signal, or wateva) and has an adjustable relay that switches on a a particular frequency to let a normal current pass through. can be used for water injection, tvis control and a whole lot more applications etc, anything!

the 5sge N/A side of things is coming to an end/completion. i'm still dreaming of an intercooled sc14 setup, the reliability reputation and electric clutch sounds awesome. Even if i just have it fitted and not turned on till i can afford the intercooler setup, jaycar electronic ignition controller (for the different forced induction ignition advance), boost controller etc.

Would only run a max of 8-10 psi. One day.... one day.