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Discussion Starter · #1 ·
Maybe it's because i'm a newbie or because my english is poor.
But I don't understand how LH 2.4 ECU handles fuel-cut during overboost situations since there is no overpressure switch and it doesn't seem to use the boost pressure sensor.

What are the symptoms on (heavy loaded) driving when this occurs? Suddendly lost of power following by a timing position going back to base setting and then will begin to re-advance the timing to retrieve power (and that loop will start again until the load decreases)?

Any help will be appreciated. Thanks.

P.S. Bentley says:
Turbo models have an additional pump safety feature that turns the pump off during overboost situations. On cars with LH 2.2, this feature is handled by an overpressure switch. On 1989 and later cars with LH 2.4, the LH control unit handles this feature.
 

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Maybe it's because i'm a newbie or because my english is poor.
But I don't understand how LH 2.4 ECU handles fuel-cut during overboost situations since there is no overpressure switch and it doesn't seem to use the boost pressure sensor.

What are the symptoms on (heavy loaded) driving when this occurs? Suddendly lost of power following by a timing position going back to base setting and then will begin to re-advance the timing to retrieve power (and that loop will start again until the load decreases)?

Any help will be appreciated. Thanks.

P.S. Bentley says:
Turbo models have an additional pump safety feature that turns the pump off during overboost situations. On cars with LH 2.2, this feature is handled by an overpressure switch. On 1989 and later cars with LH 2.4, the LH control unit handles this feature.
it's in the LH2.4 ECU

the ECU retrieves voltage from the air mass meter to calculate fuel, when the air mass voltage exceeds a pre set point within the ECU it cuts the fuel pump.

for 12psi the voltage I believe is 5v

the only way to erase overboost cut out is to remove the chips from the ECU, flash the eeprom with tunerpro (with the voltage modified) and then re solder the chips to the ECU.

you may find someone who does this, but really it's almost cheaper to just go to trionic 5.5, a way better $ for hp upgrade
 

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this shows a LH2.4 bin in tunerpro, and you adjust the air mass meter limiting constant to 255 (maximum value = disabled)

the equipment to flash the chips is about $200 or so

to buy a chip is about 300 I think from speedparts etc
 

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you mean $3 not $300 for a eprom chip right ?
for a pre programmed chip it's 2-300 (swede dynamics)

the chips themselves are cheap indeed, didn't know they were that cheap.

I was wrong about teh chip burners, looks like they go for between $30 and $200

last chip burner I saw for saabs was around $140 but it was read,erase,write

even with new chips I think you need an erase function, some products only write, do not erase from the little I know about it, if that is the case you need multiple tools.

The soldering is the hardest bit, but with a needle iron and some CHIPQUIK (desoldering flux/wire) it's a piece of cake
 

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Discussion Starter · #6 ·
I just going out of my bed (it's sunday, Montreal time).
Very interesting S900t8v, many thanks. My question was more theorical then practical because i want to understand well before modifying anything.

I thought that the air mass meter (AMM) measures atmospheric air density, what you mean is, under heavy load, the boost increases and the compressed air produced by the turbocharger is felt by the AMM who delivers consequently a higher voltage?

Let say 5 volts for 12 psi, then at 5.1 volts the LH2.4 ECU cuts the fuel pump. What append then on a loaded driving situation? Do you agree with the symptoms described in my first post (suddenly lost of power tralala…)? Have you ever experienced that fuel cut?

I just want to be sure this is my problem (that makes sense because the car has no APC, just a manual boost controller). I feel this suddenly lost of power when, for example, the car is full filled of camping stuff (loaded rooftop carrier, bikes on the rear rack, friends in the car…) and i try to climb a mountain road in the New Hampshire.

Well I think that I start to understand something… you are very helpfull.
 

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The compressed air is not felt by the AMM. It is before the turbocharger and the air passing through it is at atmospheric. It is the volume of air at atmospheric it is measuring, although density is a factor in the equation.
 

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Discussion Starter · #8 ·
That was I thought first peva but then, do you have an idea wich data the LH2.4 use to determine that there is too much boost and the fuel pump has to be shut down?
 

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The fact you can put a resister in line which tricks the air mass meter and allows more boost is confirmation of what I already saod. The risk is leaning out the afr at lower air mass readings. / idle.

People overcome this by 3bar fpr and bigger injectors. This is the shonky way to do it. The real way is to code the chips.

Fuel cut feels like being thrown through windscreen

Where does te gauge get to ?

I would re enable the APC. Your engine has no protection from detonation. You could very easily end up with no engine.

There have been people come and go here who've run mbcs and they destroy their engines.

The air mass meter works by providing a current to the heating element. The more air going by it the more current is required to keep it hot.

When the turbo kicks in it starts causing more air to flow by (vacuum). When the circuitry detects the current drawn exceeds a predetermined value read as a constant set by the ecu. It cuts the fuel pump.

I said voltage but its really the current I think
 

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Discussion Starter · #12 ·
Thanks Ed, your explanation based on current is clear. Yeap, I felt "like being thrown through windscreen" while the gauge was entering in the red zone if I remember correctly. I'm gonna activate the apc system before doing anything on this issue.
 

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Just turn the mbc down first. To under 14 psi
 

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Discussion Starter · #14 · (Edited)
I have already turn down the mbc (maybe not enough), i'll turn it down again tomorrow but i'm afraid that the problem will still be there on heavy load. Can we have the same symptoms with a fuel pump that is unable to provide the asked flow? Or, the pump is a binary device: it works or it doesn't? Since I have another one (from a donnor of the same year), you give me the idea of switching the fuel pump (I'll do it with the help of one of my friends who is a mechanic full time - an Audi fan, and still my friend... -, personnaly i'm not even a partial time mechanic). Many thanks for your inputs and i'll be back in a few days to tell the results.
 

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Unlikely.

Your gauge is going into red.

Yellow / red border = 14 psi
14 psi = fuel cut.

Check your wastegate diaphragm is not torn (air tight) and that the actuator lever moves freely.
 

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Discussion Starter · #16 ·
When I read the post from somebody who wrote 4,718 posts, I feel like a nobody in the middle of the London's O2 Arena on a July 17th 2008, on the stage there is Leonard Cohen. I really appreciate the S900t8v's contribution and i have a lot to do this week on this car to deserve another advice. I'm going to apply what you (S900t8v) said, including the apc stuff, and will be back with results for the benefit of the community.
 

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Yeah good on you! APC is a great system!
 

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Unlikely.

Your gauge is going into red.

Yellow / red border = 14 psi
14 psi = fuel cut.

Check your wastegate diaphragm is not torn (air tight) and that the actuator lever moves freely.
I think Ed's made a mistake here. Yellow/red border is 10 psi on all Saab stock gauges. 14 psi (about 1 BAR) was never a stock boost level) and is about half way through the red. It is correct the stock APC fuel cut is about 14-15 psi (1 BAR), and many people disable it. Base boost (the mechanical setting independent of APC control) is around 5.5-6psi (varies with spec) and is about half-way through the yellow.

If your fuel pump is failing it will lose power at high boost levels but seem OK otherwise. If you suspect this, do a dynamic fuel pressure test, ie whilst driving).
 

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l I made a mistake sorry.



I request 1.2 bar and see 1.1 bar on trionic.that is just over red edge on standard gauge. Maybe there's a line leak or the gauge is bad. Maybe my MAP sensor or ground/12v wires are bad/ interference.

Stand corrected

Fuel cut is definitely 1 bar ;D
 
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