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Discussion Starter #1
Hello Saab Masters,

I have a 1992 Saab 900 that refuses to run. I previously thought that it was the fuel pump, but by bypassing the relay I have discovered that it works just fine. The relay works (the one that's in the worst place - up next to the glove compartment behind the carpet). But it does not seem to be getting a signal to turn on (we couldn't find one with a meter).

Any ideas what the problem could be?

Thanks a lot!
 

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That relay (which never fails if it's the original part #) gets its pulldown circuit ground from the fuel injection ECU, which provides the ground when it sees a tach (ignition) signal.
 

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Have you checked to see if there is any power at all to the fuel pump when someone else is trying to start the car. If not to rule out that the problem is not the fuel pump, run direct power to your fuel pump and try that. I got my power to the fuel pump from the trunk light and untel I could figure out what the problem was I ran a swetch up to the front so I could turn on the pump manualy until I could fix it. The ECU is probably the problem and not sending the power to the relay. What I did is to take a switched power from the timed intereor light relay that is beside the fuel pump relay and jump it over to the pump relay. Now you will have power to your fuel pump when you turn on the key. The only down fall is that after the car is shut down, the pump will stay running until the lights go off in the car. This is no problem sinse the fuel systum is a loop and the fuel just goes back to the tank. I have had my car hooked up like this for the passed two years, or more and no problems at all, you just hear the pump run a bit after the car shuts off. This is a good fix if you don't want to have to replace the ECU.

Dennis
 

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The purpose of having the ECU respond to a tach signal is to control fire in case of accident.
If a fuel line ruptures in a crash, Federal law mandates that fuel pumps shut down. Manufacturers accomplish that in different ways. SAAB's method is to use the tach signal to detect the broken line (because the engine will shut off with a broken fuel line), and eliminate the ground (not the power) to the fuel pump relay hold-down circuit.
Bypassing this function will result in a potentially very dangerous situation in which a broken fuel line will pump several gallons of gas into a crashed/burning car.
I strongly recommend that you not bypass this mandated safety function. Bypassing safety systems whose functions you don't fully understand can result in burning to death in this case.
 

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Jim Mesthene said:
The purpose of having the ECU respond to a tach signal is to control fire in case of accident.
If a fuel line ruptures in a crash, Federal law mandates that fuel pumps shut down. Manufacturers accomplish that in different ways. SAAB's method is to use the tach signal to detect the broken line (because the engine will shut off with a broken fuel line), and eliminate the ground (not the power) to the fuel pump relay hold-down circuit.
Bypassing this function will result in a potentially very dangerous situation in which a broken fuel line will pump several gallons of gas into a crashed/burning car.
I strongly recommend that you not bypass this mandated safety function. Bypassing safety systems whose functions you don't fully understand can result in burning to death in this case.

Did I just catch heck?:eek: ;oops: I didn't know that this was a mandated safety function. Well instead of just leaving my car on the side of the road, this worked for me until I can now find another ECU.
 

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Have you tried busting the ECU open and looking inside? It's probably something really simple to fix with a 10 cent part and a soldering iron :p

Fuel pump pumps a lot of fuel, almost two litres per minute, that's a big fire :lol: Extremely unlikely to occur but nevertheless.
 

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Discussion Starter #7
Wow. You guys are great. Thanks so much!

I will check out the ECU. I did not know where to look next. I am new at reading wiring diagrams, so that did not seem to help.

Is there a good way to test it to insure it is the problem?

If I open it up, is there somethin in particular I should be looking for?

Thanks for the help.
 

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If you have a bentley handy there are tons of DIY tests you can run to troubleshoot the cause. Contradictory to what Jim has said I tested my system out earlier and found the main relay could be the problem for me.

I think you have lh2.4 correct? (assuming its turbo charged)
If so check for voltage at the pump itself, (with the key on) if no voltage is there check the fuel pump relay (pin 86 IIRC) its in the right footwell next to the ECU. If there is no voltage there check the main relay (same location) pin 87b.

There are a good number of other tests in bentley that might be worth looking at if this doesnt help.

Thanks,
Jeff
 

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Error codes????

How about checking te ECU error codes?

Upon first reading this thread, I thought: "oh good, another crank pulse sensor gone south....", but then, it would seem, according to the later threads, the car runs with the relay/pump energised......:confused:


The error codes just might point to something not definate but still relavent to the fault.


My car spluttered the other day, read the codes,...12241 - air to fuel mixture fault, checked all the hoses and grommets etc. for leaks and such,...... book suggests as a last entry, "check fuel pressure".
Cleaned the fuel cannister and replaced the pump. Car definately runs better. Pump showed no signs of fatigue but seems to be the origin of the fault that the ECU was reporting back.
Pump was approx. 100,000 kms old, which, I tend to believe is the limit of reliable service from a fuel pump, (especially with the sh*t fuel in this country).

Sadly, the code 12111 still remains.................
 

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Unfortunately for my 94 vert the plug to check the error codes is buried in my shifter housing somwhere during the last trans swap and its a pain to get at.

I will test my relay to see if that is my issue.

What is this crank pulse sensor you speak of?
Is that in the distributer and what tells the ecu the engine is running?
 

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Crank Pulse sensor is..........


located on the oil pump, behind the crank pulley or harmonic balancer.
This is, as far as I have experienced, only on N/A engines from 88?/89? on.
All turbo variants use the same signal principle but the sensor is indeed located in the distributor.

As a rule of thumb, no crank pulse at ECU?, No fuel pump relay energised.
As Jim pointed out,.... a vital and integral safety design.
 

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Discussion Starter #15
It was the Crank Pulse sensor after all, in case any of you ever check this thread again. Thanks a lot for the help. I broke down and got the help of a Saab mechanic.

I really appreciated all your thoughts and ideas. Thanks a lot. You guys are really great.
 
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