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Old 8th August 2006
li Arc's Avatar
li Arc li Arc is offline
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Join Date: Jul 2006                                                
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My Saabs: '85 900 T16 Sedan
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Default ECU Compatibility

Hello all,

My car has been having starting and idling problems for a while. It is a 1985 900T 16V, and I have been slowly restoring it to the best of my ability (which isn't much of it).

Recently I (think I) fixed the high idling problem I had before, which I could not originally fix using the usual methods (checking for vacuum leaks, AIC, AMM, lambda sensor, etc.), but I finally lowered it by a screw on the throttle body that seemed to control idling speed (and no, it is not the one that stops the throttle butterfly valve): since this screw seemed like it has not moved in 20 years, I'm not sure if that was the source of the problem, but at least my high idling problem is gone (comments on this?).

This led me to too-low-idling when the car is cold and difficulty starting (although I have always had difficulty starting, esp. in the cold: this is the major problem with the car I need to nip in the bud). As I understand it, the only elements involved in this particular problem (unable to consistently maintain 800rpm idle) is the AIC, perhaps the NTC, and the ECU. Cold start difficulties may be attributed to a cold start valve, but it doesn't much help when the spot where a cold start valve would normally be is just a non-openning on the throttle body (I also don't know where the thermo-time switch is). I have tried two different AICs that open and close the valve properly when connected to a computer power supply (12V), but that will not react when plugged into the car. I thought it might have been the wiring, so I took a multi-meter to it, and obtained the following (interesting) information that is inconsistent with the information provided on this page:

Saabnet LH Info

LH2.2 pin 3 (throttle position switch) -> AIC pin 1 (connector pin /w no protrusions in the housing)
LH2.2 pin 10 (AIC valve output 6-11V @ idle) -> AIC pin 2 (centre connector pin)
LH2.2 pin 12 (throttle position switch wide open) -> AIC pin 3 (connector pin /w slanted protrusion in the housing)

It looked like there was no discontinuity in the wiring, but, to me, the pins seem connected wrong! I'm not any rate, since the wiring was okay, I assumed that this is how it left the factory, so I wasn't about to argue (comments on this?). This left the ECU. Happy at narrowing down the problem, I set off in search of a working ECU.

After several weeks of visits to the local junk yard, I was unable to find anything useful (two Jetronic systems from 84 900S cars that were both carborated), until today when I arrived to find two 86' 9000 16V's. They both used the 0 280 000 531 LH2.2 systems, whereas (I think) the 85 900T 16V uses a 0 280 000 515 (or 521). So, after a long winded speech, here is my question: will I be able to swap in a 531 LH2.2 ECU in to replace my 515 (or 521) LH2.2 ECU?

Thanks in advance!

li Arc

Last edited by li Arc; 8th August 2006 at 01:35 AM.
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Old 8th August 2006
Jim Mesthene's Avatar
Jim Mesthene Jim Mesthene is offline
Join Date: Feb 2006                                                
Location: Waltham, MA
My Saabs: Wife's 9-3
Posts: 12,538

You can use any 25 pin Turbo ECU. You can use a N/A for testing if you need to.
Within the same type LH (2.2, 2.2.4 etc.), you can always substitute a later model ECU, sometimes just for improved diagnostic capability.
Turbo ECUs may have enrichment circuits; you can use them in N/A cars, but not the other way around.
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Old 22nd August 2006
Jeremy's Avatar
Jeremy Jeremy is offline
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Join Date: Aug 2001                                                
Location: Syracuse NY USA
My Saabs: 89 900 Turbo 3DR
Posts: 154

Does that mean I could use an '88 turbo jetronic ECU on my '89 Turbo?

Edit: I think i just remembered something, the 88 turbo is a LH 2.2 and my 89 turbo is a LH 2.4? or am I remembering wrong?

Last edited by Jeremy; 22nd August 2006 at 01:25 PM. Reason: Remembered something...
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Old 24th August 2006
RM '92 SPG's Avatar
RM '92 SPG RM '92 SPG is offline
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Join Date: Oct 2002                                                
Location: Calgary, Canada
My Saabs: 1992 900T 3dr
Posts: 1,309

Originally Posted by Jeremy
Does that mean I could use an '88 turbo jetronic ECU on my '89 Turbo?

Edit: I think i just remembered something, the 88 turbo is a LH 2.2 and my 89 turbo is a LH 2.4? or am I remembering wrong?

That's right, '89 is the first year of 2.4 if I remember correctly!

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