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Discussion Starter · #1 ·
This is as of right now only a theory. But i will try it tomorrow if i get time. As far as i can tell the ECU in LH 2.4 interrupts the ground to the fuel pump relay thus turning off the fuel pump during overboost (if someone can verify please do). So all one needs to do is run another lead to the fuel pump relay that keeps it grounded as long as the engine is running (not disconnecting any other circuit running to the relay). This can be achieved by placing a normally closed relay on the charging light circuit. When the light is out, the fuel pump runs. The only thing that would blast this idea to pieces is if the fuel pump relay is hot when the key is off which i don't believe it is (can anyone verify?). But anyway, what will happen is that the safety of the fuel pump being shut off in if the key is on and the engine is not running will be retained, while whenever the engine is running the fuel pump will run. ALL OTHER CONTROLS TO THE FUEL PUMP REMAIN. This would mean the only downside to this mod would be that if you're not charging while driving down the road, overboost control will be reinstated. Though if you are not charging, having overboost reinstated is the least of your problems. I like the idea of this better than messing with resistors in the AMM as i don't like the idea of messing with how much air the ECU thinks is going into the engine. Let me know what you think....
 

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well, i tried to do this before, but not with the ground, what happens as far as i can see is the 2.4 ecu sends a power signal to the relay wich turns on the fuel pump, not a ground, although i could be wrong. what i did was wire that to a switched 12v source, all that accomplished was making the car not start very well ( i later realized it was because the power source turned off when cranking, durr) but the overboost cut was still there.

my theory about how lh 2.4 does this is that at a set airflow reading, the fuel map simply ends. sounds silly, but when you think about it, there would be no reason for the ecu to know what to do with a car that is flowing enough air to produce 200+ wheel hp.

the other big issue is that at high boost/rpm situations, lh2.4 is known to run MASSIVELY LEAN. really realy bad, on my car at around 18psi and 4800rpms, the narrow band air fuel ratio gauge actually drops off the gauge into the lean :eek:. then it hits fuel cut...really really bad.

best solution if you ask me, ditch LH 2.4 and run megasquirt... check this site for some good info, although not entirely finished as of yet.

http://www.wideopenwest.com/~kgrider/saab/

Paul
 

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That is my understanding of LH 2.4. LH 2.2 has a FPR cut which is triggered from the 'overboost switch'. LH 2.4 does not have this and does just cut all injector pulses when you exceed a certain MAF value. The only way around is a reprogram of LH or a replacement ECU that does not exhibit this 'problem'.:D

MS is one way, and that is what I am trying to get working properly. So far, so good.
 

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Discussion Starter · #4 ·
It's not the 'only' way around this. A simply bypass rewiring would solve this problem. I have determined that the overboost is controlling the fuel pump (if the fuel pump is hotwired there is no overboost control.) I also believe the ECU controls the ground side of the fuel pump relay as this is the only side that goes into the ECU (look at the wiring diagrams), the hot side is hot all the time as long as the key is on. I shall try this today and see what happens.
 

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The ECU cuts both the fuel pump and the injectors, about 10 years ago we wired the fuel pump to not shut off and found the ecu cut the injectors off. We then went to larger injectors and the resistor. There is another way to defet the over boost with out reprograming. That is to use a voltage clamp on the out put signal of the air mass meter so the ecu does not see more than 4.5 volts. When doing so you have to find a way to add the addtional fuel to support the higher boost. Addtional injector(s) is a good way to do that. Or you could create a dual fuel can and injector methanol or ethanaol in your seondary injection system
 

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Discussion Starter · #6 ·
well i just found it to cut injectors as well. i would assume that it cuts it with the signal to the relay though, so by jumping my set up to the fuel injector relay as well, it should bypass the overboost protection.
 

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I'm just not sure about this... 2.4 cuts the fuel because after a certain airflow amount the fuel system can't deliver enough fuel for it to continue running in the proper AFR ratio. I think that making it so that the ECU CAN'T cut the fuel after that point might just make that the fuel continues to run, but there is simply not enough available to keep the AFR's correct.

the solution is not a band-aid like a resistor mod to the AMM, larger injectors, and an RRFPR. The real solution is to go with a stand alone ECU, I.E. MEGASQURT
 

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DeLorean said:
the solution is not a band-aid like a resistor mod to the AMM, larger injectors, and an RRFPR. The real solution is to go with a stand alone ECU, I.E. MEGASQURT
The band aid resistor mod, coupled with the change in injectors is cheap and works. Changing the ECU plus the time required to map and set the unit up is expensive.
 

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Discussion Starter · #10 ·
overboost protection is not put into place due to the fact that the fueling system is incapable of providing fuel for certain AFRs but it is put into place in order to prevent damage to the engine in the result of the wastegate failing. if there is no overboost protection a driver could very easily bomb their engine if their wastegate fails. i will agree that the stock fueling set up is not the best for running high levels of boost but the stock set up with a fuel chip and a 3.0 bar FPR is capable of providing fuel for a higher boost levels. on another note i am considering running the standard resistor mod on the following logic: from what i have read, it seems the ECU determines fueling based on a standard map and with input from the O2 sensor therefore if i run with the the resistor the mixture will be automatically adjusted to compensate due to the input from the O2 sensor. at full throttle the AMM signal is ignored and therefore the resistor has no effect. Is there anything i am missing?
 

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Kirkpatrick said:
from what i have read, it seems the ECU determines fueling based on a standard map and with input from the O2 sensor therefore if i run with the the resistor the mixture will be automatically adjusted to compensate due to the input from the O2 sensor. at full throttle the AMM signal is ignored and therefore the resistor has no effect. Is there anything i am missing?
I believe you have it backwards. At full throttle, the O2 sensor is ignored and only the AMM is used.
 

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Discussion Starter · #12 ·
perhaps it is unclear that beginning of my post is talking about part throttle. But i do believe that when the AMM signal is ignored at full throttle, the O2 sensor is ignored as well...not entirely sure on this though.
 
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