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Discussion Starter #1
In the last week or so I've been getting a CEL periodically in my 89 900S. It seems to occur when I drive the car from cold; turning it off and back on once the engine is hot seems to satisfy the ECU. When it does come on, it is steady when there is any throttle input, while when off the throttle it flashes four times, pauses, and repeats. Car idles and drives normally.

I pulled the codes and got one - 12112. This is listed as "Oxygen sensor self-compensating circuit problem (incorrect air-fuel ratio at idle)." Oddly, no corrective action is listed in the chart (Bentley p.240-7).

I'm speculating that the problem lies in the sensor's pre-heater, since it only happens when the exhaust is cold. Is there anything else that's a likely factor here, or should I just replace the sensor? I have no idea how long it's been in there, so it may well just be aged out. In the event that I do have to replace the sensor, is the ECU sensitive enough to care about OE vs. generic parts?
 

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4 flashes means an EZK problem.
12112 means nothing in particular, simply that the ECU is trying to adapt to something.
 

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Discussion Starter #3
So what would you recommend as a next step here? I'd rather diagnose it than throw parts at it.
 

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As I recall, you check the knock sensor output and wiring, then you start swapping parts.
The fact that it's temperature sensitive leads me to guess you'll end up needing a Hall Effect transmitter.
Maybe Rodentmaster has some procedure...
 

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Discussion Starter #6
Bear with me, as I'm trying to understand this. First off, when you refer to the EZK, this is the Bosch LH control module, correct? Or is it something else that I should be checking?

I had used Bentley initially, but I checked the FSM and discovered an anomaly - Bentley lists the code as I stated it above, while the FSM gives it as "Lambda adaptation fault (fuel-air mixture when car is being driven)." Frankly the Bentley description makes more sense given the flashing when the throttle is closed, but in any case it just adds to the confusion. The FSM does list checking the oxygen sensor preheating function, along with intake and fuel leaks, and trying a new control module.

The FSM lists the spec for the oxygen sensor heating element as 4 ohms. I checked mine and it's ~18 ohms. My gut suggests this may be the issue, given when the manuals state and what little I know, but am I off-base here? I'm happy to do more testing before I order parts - are there tests for the knock sensor and the Hall Effect sensor?

Problems like this make me long for ignition points and carburetors.
 

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No, the EZK module is behind the left headlight. On turbos, the ICM controlls dwell and timing and the APC controls knock by adjusting boost. On non-turbo 16v cars, the ICM only fires the coil and the EZK module controls dwell, timing, and knock. The EZK module and APC module look virtually identical and are mounted in the same space.

EZK can trigger CEL for its own faults via communication with the Jetronic ECM. As Jim noted, the presence of EZK fault codes is indicated by the CEL flashing four times. I thought there was a way to get those faults, but there may not be... it may just be check the knock sensor, check the crank/cam sensor, then replace the EZK box.
 

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If you have access to the factory manual, throw away the misleading Bentley manual.
There is nothing at all that the Bentley does better than the manual it's stolen from.
An LH fault cannot make the light flash 4 times.
 

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Discussion Starter #9
I'm trying to wrap my head around all this, and I'm not sure I'm succeeding. I have a scan of the FSM and going through it I'm struggling to find any clear explanation of all this. I can't find anything beyond the basic codes except for using a Saab-specific diagnostic tool that I don't have. The EZK (what does this stand for?) is barely mentioned at all as far as I can see, and I don't see anything referring to the ICM (Idle Control Module?).

I'm pretty sure there's only one electronics box behind the left headlight on my car - I'll check in the morning. The FSM says there's another box inside the cabin on the right side kick panel - I'll have to look for that in the morning as well. Aside from the Saab diagnostic tool, I have yet to find a procedure for testing the knock sensor or the Hall sensor. I'll keep looking.

So if I'm understanding what's being said, the CEL flash code I'm getting is essentially meaningless. The four flashes that occur when off the throttle (and only when off the throttle) are indicative of a fault in the EZK itself, or in the sensors that deal with dwell, timing, and knock only, and I should ignore any other sensor inputs in trying to diagnose the problem.

I wish the manual were clearer about these systems and how they interact. It's very frustrating.

EDIT: I have sections 2.1, 2.3, and 2.4 of the FSM - is 2.2 the part that deals with the ignition system? I don't recall finding a PDF of that available when I was looking, but I'll search again and see if I can track it down.
 

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Jim doesn't like the Bentley, but in near 30 years of wrenching on Saabs I have referred to the FSM exactly never and the Bentley more times than I can count. Including just now, to verify four flashes of the CEL indicates either a fault in the knock sensor or the EZK control unit. So check the knock sensor and it's wiring, then replace the EZK box.
 

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Discussion Starter #12
OK, that explains a lot - I was looking in the wrong place. I expected all the engine electrical controls would be grouped together, so I never even thought to check other sections. The BMW manuals I normally deal with are organized differently.

I will do the tests listed for the knock sensor and wiring and see where that leaves me. In the event the sensor checks out OK and I need the EZK box, is there a source for these new or rebuilt, or is it just a matter of finding a used unit and hoping for the best?
 

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Faults in EZK units are not very descriptive other than what is mentioned in the Bently manual.
Jvanabra and Jim have covered it well.......behind LH headlight, does ignition dwell, timing, knock detection and waveshape duty of the coil driver pulse.
Hall sensor - behind oil pump (crank pulse trigger) generaly does not give any reliable warning or telltale signs of failure other than an ocasional engine stumbling or just outright fail.
Highly unlikely that LH code 12112 has any direct tie in with the operation of the ignition module.
12112 is likely to be based on vacuum hoses, air intake path and possibly the Air mass meter.
 

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....And that's what you won't find anywhere in any Bentley.
EZK boxes very rarely fail, any used one should do. Same with knock sensors, both are as reliable as an oar. Thus my unsubstantiated, wild guess that you'll end up replacing the Hall Effect transmitter. I stopped testing them, bad ones tested good way too often; especially the temperature sensitive ones.
 

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Discussion Starter #15
Confirmed the knock sensor and its wiring are good, so now it's either the EZK box, the Hall sensor, or the wiring. I checked the wiring from the Hall Effect connector to the EZK box and it's good, and I checked the sensor wiring itself when I had the front of the engine apart last year to do the crank seal (obviously it could have failed between now and then, but it was checked recently). Chances are, as Jim indicated, the Hall sensor is on its way out.

The question now (aside from why didn't I just replace it when I had everything apart last year) is whether to stick with the original arrangement or convert to a turbo distributor with the Hall Effect sensor in a more convenient place. Recommendations?
 

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....And that's what you won't find anywhere in any Bentley.
What won't you find? Everything in Rodentmaster's post is in Bentley... Function and location of the box, fault code meaning and remedy. All of it.

There is very little missing, and little practical value to what is. Like you won't find the timing curve for the ICM in Bentley, but when does one ever need that info?

The sole thing that the Bentley lacks, and the only FSM section I own, is the transmission section. Bentley won't tell you how to rebuild a gearbox. So I have that part of the FSM, for the day I gotta do that.
 

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At the dealer we had it all, updated every month.
It's not just me. Used cars came with a Bentley in the trunk often as not. I don't know of any instance of one of the guys reading, keeping, or using one....Straight in the trash.
It's better than nothing, but you're not stuck with nothing. The real manual is in the stickies.
 

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Discussion Starter #18
Related follow-up question: assuming it is the Hall sensor, what's the typical failure mode of these? Should I expect it to start misfiring or running roughly before it craps out completely, or do they normally die without warning? Put another way, how reluctant should I be to drive the car?
 

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Remember, it's just an unsubstantiated guess.
Typically, they die when they get hot and work again when cold....Or they just croak.
 

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Discussion Starter #20
That's interesting, given that my car seems to be throwing the light when it's cold, but not when it's at operating temperature. I guess I'll drive it some more - probably not going too far from home - and see what happens.
 
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