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Discussion Starter · #1 ·
Along with my stage1 APC and EPROM and fuel injectors package, i noticed along with the 'installation notes' that came with the APC...

...which bases over boost protection through the air mass signal. It is possible to manipulate this signal by installing a 270-350 ohm resistor in line with the AMM orange/signal wire.[/b]
With a 92 900T, there is a 'hot-wire' AMM, which to my understanding uses voltage to heat a wire which is cooled by the air rushing past said wire on the way to the turbo. the measurement of how much air is moving through the intake is calculated according to how much voltage is needed in order to keep said wire at a predetermined temperature (checking the manual, this temperature is 100 degrees Celcius over the temperature of the air passing over it).

so, using this modification will affect how much air the LH unit thinks that the car is getting. it would make sense to me that adding a resistor in line with the signal wire would reduce the signal output and thus trick the computer to believe it is getting less air than it really is.

i have tried this modification with both a 270 ohm resistor as well as a 330 ohm resistor (not at the same time!). with the resistor in line, i did notice an increase in how long boost held at about halfway through the red before it dropped back to the top of the yellow. this also affected my idle, causing my engine to drop to around 1900 rpm before dropping to around 1200 then settleing down at around 850 (where i beleive it should be). this all takes a matter of 5-10 seconds and is obviously annoying in stop and go traffic, this also was affecting downshifts.

so here is my idea...

trace that orange wire back into the passenger compartment, splice it with a pair of wires running to the driverside with a potentiometer. this will give me the ability to vary the resistance from the AMM, allowing me to switch between retarding the overboost and being able to see a sane idle speed again.

i had considered using a switch to toggle between the resistor setting and no reistor, but affecting the signal i beleive would cause the LH computer to believe the AMM was unplugged and thus revert to the 'safe setting' or 'limp home mode'

i am looking for some feedback, if you have any thoughts on this, or if you are able to give me a better understanding of the components in question, i am open to any suggestions
 

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Discussion Starter · #2 ·
continuing this thought...
so, using this modification will affect how much air the LH unit thinks that the car is getting. it would make sense to me that adding a resistor in line with the signal wire would reduce the signal output and thus trick the computer to believe it is getting less air than it really is.[/b]
if the CPU believes it is getting less air, this would cause it to compensate by reducing fuel, by reducing the duration that the injectors are open... or would it cause the CPU to pump more fuel and extend said duration? i am a bit confused about exactly what this modification is causing to happen
 

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this does not fool the LH unit into thinking there is less air, but actually making it think more air is passing through, causing more fuel to be puped through. this is in effect causing your idle problems. I may use this for a stop gap measure untill i get my new ecu....
 

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Without looking at the cct diagram, I would have assumed that it was fooling the ECU into thinking there was more air, thus enrichening the mixture to prevent knock.

As you are introducing an increase in impedance, a switch would work fine as a bypass- if you put it in parallel with with the resistor such that to bypass it you turn the switch on. You could get really clever and do this with a microswitch on the accelerator pedal or even, as it's an auto, on the brake pedal.

Whilst this mod may give you more max boost, because it is compromising the a/f ratio, it could also limit max power.
 

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Discussion Starter · #5 ·
this does not fool the LH unit into thinking there is less air, but actually making it think more air is passing through[/b]
thank you for the clarification, Grentarc
i see i was a bit confused about exactly what was happenning with this mod.

thus enrichening the mixture to prevent knock[/b]
i did not know that running lean would cause knock. thx Mark E

i see that this may take some trial and error to figure out exactly what works best (without paying for Dyno time or any other test equipment). my idea is to run parallel wires thru the dash to the driver side, cutting and then attaching these to the signal wire at the LH unit. i will try first with the potentiometer controlling the resistance, also with the resistor inline and using the wires i ran for a bypass... i will let you know how it works out
 
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