ST165 - Adjustable Fuel Cut Mod Problem...?

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ST165 - Adjustable Fuel Cut Mod Problem...?

Postby CodingParadox » Sun Sep 26, 2004 4:30 pm

So I tried to do this mod yesterday. I tested the leads on the turbo pressure sensor, and found that the wires didn't quite match the colors in the writeup on alltrac.net, but figured that at least the function of the 3 wires would be identical if I could figure out which was which. So I found out that brown is ground, one of them (I believe the orange with a blue stripe?) had constant +5v, and then the middle one (something with white and dotted with blue) had about +2.4v with the car off (0 vacuum.)

So I figured the middle on should be the PIM, right? Sure. Well, I took a 10k pot and hooked it up in series with that line, and found something very odd. There's higher voltage on the ECU side of the line than on the sensor side, so the voltage on that lead is also coming _from_ the ECU. Also, making sense with that, the more I raise the resistance, the _higher_ my turbo pressure sensor goes.

So I'm semi-baffled with that. But in any event, the next problem is how to properly do this. It would seem to work to put the pot across the +5v lead, because then I'm scaling the input voltage, which should scale the output voltage as well, it's just odd both that this method didn't work and that the writeup explicitly says do it to the variable lead.
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Postby CodingParadox » Mon Sep 27, 2004 2:40 am

No one's done this mod?..

How do you run boost higher than 10psi then?
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Postby BraveUlysses » Tue Sep 28, 2004 3:11 am

http://toymr2.tripod.com/fcd.html

The brown wire is the ground. The blue/orange wire is the other wire you need to splice into to override the fuel cut.
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Postby CodingParadox » Tue Sep 28, 2004 3:15 am

Yeah, that's the wire I stuck the pot across... I didn't do the neat little project box because I'm lazy, but that's what I did. I didn't ground the other end to the brown wire though. I guess I should try doing that.

In theory, when I'm done with this, my boost gauge in the car should show LOWER than actual boost, right? As of right now, it's showing HIGHER than actual boost.
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Postby BraveUlysses » Tue Sep 28, 2004 3:39 am

The stock boost gauge is not to be relied upon for anything other than a "oh shit" light for somebody else driving your car. The gauge is electronically controlled, and slow to boot, so don't rely on it.

The whole point of the potentiometer is to "divert" the voltage that it recieves from the boost pressure sensor, divert a percentage (based on the resistance you tune it to) into the ground so that the ecu (which recieves the rest of the voltage, so to say) sees a voltage less than 4.4v, which is roughly where fuel cut occurs. The whole point is to make sure that the ecu doesn't ever see that voltage, which means the remaining voltage (by changing the pot's resistance (lol) ) must go to ground.

Follow the writeup to a 'T' and you will not regret it.
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Postby CodingParadox » Tue Sep 28, 2004 4:14 am

Ya I know about that, but I figure it's a good sign of if I did the mod right or not. If the mod is correct, the boost gauge should show less than actual boost, right?
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Postby Gary » Tue Sep 28, 2004 1:41 pm

I guess you need to connect that ground wire.
"A pot is often referred to as a voltage splitter. While this is not
incorrect, it is, more correctly, a current splitter. As voltage varies
directly with current, changing the current will cause a proportional
change in voltage. Think of it as diverter valve. It will receive flow
from one direction, and direct that flow to two different outlets according
to how you adjust the valve."

He explains it very well.
Pot is actually a current splitter. If you connect it in series without grounding, the current will be the same. I am not surprised it gives you funny reading/result.
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Postby CodingParadox » Tue Sep 28, 2004 7:14 pm

Well, I was expecting a resistor to do all the work. As described on the page, the voltage goes in on the +5V line and returns on the blue line to the ECU. So, in theory, all you need to do is stick a variable resistor across it and it will do a simple voltage drop. That's what I figured it was doing. By the fact that voltage is higher on the ECU end of things than the sensor, I know that the situation is actually different there, so that would partially explain the need to ground it.
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Postby furpo » Tue Sep 28, 2004 8:39 pm

when i did the mod all i did was pop the lid off the ecu and solder a 4.7V diode between the pim pin and earth. no boxes or cutting wires. 14 psi is now the cut and if you want to go higher just use a bigger diode.

roger
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