UKSaabs

I'm trying to diagnose the underlying problem with a 2002 9-5 2.0 automatic.

I have done a fair bit of research and previously diagnose that the TCM was faulty, changed that, did re adaption and everything appeared to be fine. Except it isn't after 50 miles or so it throws the error code again. It can be cleared when cold, it comes back when hot.

Now I know there is a theory that the problem is the SLT linear solenoid, and that may well be true, but I have done a fair bit of measuring and testing to be able to be sure of the diagnosis before I remove the valve cover or open my wallet.

The error occurs with the correct TCM and also a 2003 TCM that is "known good"
Transmission fluid temperature sensor - Has the correct resistance / translation to degrees C
Wiring, continuity and light bulb tests done on the wiring to all 3 linear solenoids
Resistance tests on the linear solenoids show all three reading 5.8 ohms. This is just a little higher that the 5-5.7 ohms I have seen quoted, but all three are the same

Now the important new information, at least from my perspective:
PWM confirmed at 300hz
Current draw - virtually zero , maybe 60 ma
Duty cycle on the SLT 55% at idle in P, dropping to 38% at 2000 rpm

Now reading around I have seen a theory that problems with the linear solenoids cause the TCM to go into "high current" mode. I am pretty convinced that this is a myth. Why ?, well these two graphs below give the answer (courtesy of Sonnax)

Before I go any further I am trying to be certain that this is a mechanical problem rather than electrical. I have read and been told that with the development of the valve body the design of the SLT was changed with the electrical connector moving from the top of the SLT to the bottom which was done apparently to reduce contamination.

You can also see from the graphs that low amperage gives high line pressure (the opposite of the prior hypothesis), and also that the duty cycle is above the expected range. It makes sense to me that higher duty cycle would give higher line pressure, but not necessarily current.

I'll admit I haven't got my head around how the different control pressures work inside the box to provide a smooth shift, but do get how those pressures are tweaked by adaption to take account of wear over time. I also now understand that the linear solenoids can be rebuilt and are mechanically adjustable.

Any further thought or suggestions for me. Key to my understanding is answering the chicken and egg question. Does the TCM adjust the amperage to achieve a target SLT pressure, or is it the duty cycle that is adjusted. What is really throwing me is the high duty cycle combined with low current.

I would like to have a better understanding of how the TCM and solenoids work together and how to properly diagnose this, any help or knowledge appreciated

https://www.sonnax.com/tech_resources/1 ... nformation

I'm currently removing the linear solenoids from the valve body, in the process of doing so I checked WIS again and found this useful snippet


Now that's quite interesting. Though I do not know what the correct parameters are for an engine on idle and in Park, what I am seeing is a high duty cycle (55%), but a low current (60 ma)

Those two measures appear to be totally inconsistent, which is what I was guessing before.

Any further thoughts or experience ?

Hi David, we have discussed this before I seem to remember, in my own personal experience I serviced the solenoids first according to the auto-box servicing centre procedure I got from YouTube (it is well-documented) which was referenced in my own auto-box failure thread.

I then tried the car with the old TCM in the car, faults remained including the transmission warning light, all of that cleared with the TCM change. I have not had any faults since then and more than 8000 miles.

Am I not or right in thinking that the third linear solenoid is a pressure control solenoid ?, I notice you don’t mention that anywhere.

It might help if you drew up a schematic which places all of the sov’s in their correct location pressure wise (e.g a hydraulic schematic) to better understand the TCM failure mode but fail (catastrophically), they do.

Lastly, did you clear the previous adaption data with a Tech II first ? (There is a procedure for it).

_________________
MY04 9-3 1.8t Vert
MY02 9-5 Aero Est
MY01 9-5 2.0L LPT
MY98 9-5 SE 2.3 LPT
MY97 Merc SL500 (oops)

This is the link I found on YouTube ...

https://youtu.be/rB5Mw8lGh7A

I have not found anywhere to buy the repair kit mentioned in this link in the UK but I guess it must exist here somewhere, instead I cut some slots in the solenoid body (also documented elsewhere on Youtube) and re-formed them later, they were OK I think (no problems since then anyway).

Good luck

_________________
MY04 9-3 1.8t Vert
MY02 9-5 Aero Est
MY01 9-5 2.0L LPT
MY98 9-5 SE 2.3 LPT
MY97 Merc SL500 (oops)

_________________
MY04 9-3 1.8t Vert
MY02 9-5 Aero Est
MY01 9-5 2.0L LPT
MY98 9-5 SE 2.3 LPT
MY97 Merc SL500 (oops)

Thanks for that, I have had a quick look at the video, which is useful as I was considering inspecting / rebuilding the SLT solenoid, including checking / amending the stroke.

I have done a fair bit of reading, despite which I have not been able to get my head around how the various solenoids work together to provide a smooth shift. In a sense however, even though that would be very useful, it may not actually help me in diagnosing what the problem is.

Putting it simply, is it an electrical problem, or a mechanical problem.

Previously I thought it was electrical, being the TCM. While the errors cleared, the fault returns after a period of driving.

If mechanical, then the recommendation is to replace the whole valve body (Saab) or replace the solenoids...... provided the valve body is in good condition

First time around I confirmed the line pressure (SLT) was high, confirming P0748.

This time around I have done additional electrical checks and am confused by the high duty cycle and the low current consumption (which is what gives the high SLT pressure apparently)

Before I can go any further I need to remove the electrical connector from the SLT, without damaging the plastic retaining clip.

There are two UK suppliers for the Sonnax solenoids, but it's encouraging that a dismantle / reassembly did the job for you

I modified a bradawl I had in the toolbox, I just bent the last 2-3mm of the point to around 45 degrees, you can then put the point in the back of the connector and turn it to release the 'spigot'.

I would honestly advise trying the the clean-up routine, it is quite easy for someone with moderate mechanical skills so a doddle for yourself I would estimate. I also saw some solenoids advertised (for around £100 for 3) but was not sure about quality and where to go for them so I opted to clean and re-use. Once you get the solenoids apart you will see why this works as a repair procedure.

My thinking is that the mechanical problem causes the electrical problem as you know, I don't know why in light of your findings but my repair strategy did highlight the electrical fault even after the solenoid re-work which fits in with the general thinking. I hope that by just fitting the TCM you have not blown that too as I followed advice to do the SOV's first to avoid that possibility, and S/H TCM's are not that common or cheap on occasion.

As part of my work in the Oil & Gas industry I have had cause to work through numerous hydraulic schematics, hence my suggestion, without that it's all guesswork which is far from ideal. If you do manage to get hold of one, or manage to draw one up from pooling other sources, I would be interested in having a copy

Best of luck

_________________
MY04 9-3 1.8t Vert
MY02 9-5 Aero Est
MY01 9-5 2.0L LPT
MY98 9-5 SE 2.3 LPT
MY97 Merc SL500 (oops)

Yep, there's a good bit of guesswork going on.

There's a partial hydraulic diagram in the Sonnax link I posted. While I read up how K-Jet works and was able to get a really good understanding of that some years ago, I haven't been able to do the same for these auto boxes.

It's the control process that's eluding me.

I get the low amperage = high pressure relationship as it's a normally open solenoid

What I don't get is the function of the PWM modulation. I see that it effectively gives a fraction of the 12v nominal supply, and "guess" that would give a similar fraction of the amperage, but that's probably my logic flaw. Amperage and duty cycle both seem to affect the SLT pressure.

At the moment I can only conclude that the signals to the TCM are telling it that there is insufficient SLT pressure, so it increases the duty cycle / reduces the current to increase the pressure, and in doing so achieves a pressure far beyond what is needed.

I could understand this happening if the temperature sensor was faulty, because that would be a false viscosity signal. But that seems to be working fine. The workings of the TCM appear to be a black box, both in terms of understanding as well as physically due to the potting.

At the moment I believe the proper TCM is still in good working order, as the later 2003 TCM is behaving the same, except for an additional error due to the absence of flappy paddles.

Maybe a quick read on how auto-box control valve bodies work in general would help, I have not gone into this as typically I needed the car for weekly travel when I fixed it so I didn't look any further than the solution.

I can understand that the throttle is one of the inputs (e.g. more throttle travel = more acceleration demanded = later change up of the gear) and speed of the car but other than that I don't have much of a clue, I guess the TCM monitors the linear solenoid travel based on it's output and increases the duty cycle (by changing the pulse width) to achieve the required result.

My limited understanding of the failure mode is that the TCM increases 'something' (my guess would be the pulse width) to get the sticky solenoids working and burns itself out in the process, how I can't guess with the limited information available.

_________________
MY04 9-3 1.8t Vert
MY02 9-5 Aero Est
MY01 9-5 2.0L LPT
MY98 9-5 SE 2.3 LPT
MY97 Merc SL500 (oops)

Have you read this article ?

https://www.sonnax.com/tech_resources/196-aw-55-50-af23-33-diagnosis-and-valve-body-information

_________________
MY04 9-3 1.8t Vert
MY02 9-5 Aero Est
MY01 9-5 2.0L LPT
MY98 9-5 SE 2.3 LPT
MY97 Merc SL500 (oops)

Yes, I have read that thanks - it's actually linked to in my first post above. There's some other really useful links from Sonnax too.

As I have access to "shop air" as they call it I was thinking about doing that test, but haven't yet decided if that is done with valve body in the car or not

Did you remove the valve body, or just the solenoids ?

Here's another ....

https://vdocuments.mx/t-aw55-50sn-initial-engagements-valve-transmission-digest-aw55-50sn-initial-engagements.html

There is a search facility on the main page for this site which looks useful.

I only removed the linear solenoids, I was too chicken to remove the valve body and am not sure if it is even possible with the engine and gearbox in the car ... in any case I don't have the gear to test stuff like this.

_________________
MY04 9-3 1.8t Vert
MY02 9-5 Aero Est
MY01 9-5 2.0L LPT
MY98 9-5 SE 2.3 LPT
MY97 Merc SL500 (oops)

What you/we need is a 'Control Narrative' or a Process Control Narrative to give it it's full title which would explain how it is all supposed to work.

I'll keep looking ...

Interestingly according to the Volvo forum ... the Volvo auto-box of the same number (it's the same box) also throws up a P0748 fault for these problems so the TCM must be very similar to the Saab.

_________________
MY04 9-3 1.8t Vert
MY02 9-5 Aero Est
MY01 9-5 2.0L LPT
MY98 9-5 SE 2.3 LPT
MY97 Merc SL500 (oops)

This problem is cause by one or more sticking solenoids causing too much current to be drawn which eventually leads to TCM failure. Which is when the gearbox warning light comes on. It is possible to replace the valve body with the box in the car. Although replacing the solenoids should cure it. You have to replace the TCM too.

I think I have also seen that the same error code is P0962 or P0963 in the world of Volvo. Just seems wrong to be looking over that particular fence onto their turf, but they do use the same gearbox and they have a high and a low error code instead of just one...... and the same problems

That link you provided was really quite useful, showing as it does how the amps change as D is engaged, starting off at about 1 amp and dropping down to facilitate the three phases of engagement. It confirms what normal current should be, which is a useful surrogate for SLT pressure. Unfortunately it does not speak about the PWM duty cycle.

Did you see the hydraulic schematic ??

FYI, the PWM signal operates at a fixed 300Hz, I have confirmed this. It's the duty cycle arising from that which varies......... but now I'm into educated guessing again, I don't understand enough about PWM control yet, and it's the first time I used the setting on my meter



I have seen this elsewhere, but having done measurements I don't believe this assertion is correct. Not only because this car is only pulling out 60ma, but also because there is an inverse relationship between current and SLT pressure - see the graph above. This is a major reason for making this post, I'm hoping to get a more definitive diagnosis for the failure mode.

Note I am not disagreeing that the linear solenoids may be at fault (and cleaning / adjustment / replacement may be in order), simply that the "high current" theory is not correct.

I have three TCM's available. Original (bad), replacement correct (still good, I think) and replacement 2003 and access to Tech 2. Along with the meter and pressure gauges to measure pressure I feel like I'm in real danger of having all the gear and no idea I am patient though, both the latter TCM's are giving the same readings

With regard to the PWM, could it not be that the pulse width is adjusted (or extended) until the desired action is achieved, e.g. the control is maintained until the SOV does what is required (engage or disengage a clutch ?). And could it not be that the PW is extended such that marginal components (inherently a bad design) in the TCM overheat ?

Without a control narrative it is almost impossible to understand the failure mode (TCM, not the solenoids) but one thing is for sure, the SOV's do get gunged up and need to be cleaned, there is too much collective wisdom and experience to refute that IMHO. I also think that the procedure I found on the 'Automatic Transmission' link (and he does appear to be a specialist Auto-Transmission repairer) is evidence that it happens very frequently with this gearbox.

About Sonnax :

Did you find the link to Aisin-Warner valve bodies in there, there are four pages of resource for the AW gearbox and a page dedicated to the linear SOV's in there :

https://www.sonnax.com/tech_resources/113-aisin-aw-linear-solenoids

I have not read any of this in any depth but it seems to me that most of the tests would need to be carried out on the entire valve body as most of the test points are on the body itself (makes sense). So you would need a fairly sophisticated test rig to be able to do any meaningful testing I think, anyway good luck, your work will benefit all of us ultimately.

_________________
MY04 9-3 1.8t Vert
MY02 9-5 Aero Est
MY01 9-5 2.0L LPT
MY98 9-5 SE 2.3 LPT
MY97 Merc SL500 (oops)

Thanks for the additional link, yes I had already seen and read that.

With regard to the linear solenoids, I am in no way challenging the collective thought that replacement of these solves the problem. The specific point I am challenging is that a problem with the linear solenoids causes the TCM to go into a high current mode, which causes a failure of the TCM.

That seems to be just wrong. The hard shifts are caused by too high an SLT pressure. High SLT pressure is caused by low current, with total electrical failure meaning zero current. (see the two graphics in the first post)

Moving onto the PWM operation of the solenoids, this is what WIS has to say



Next take a look at the Sonnax document as a pdf (just for ease of reference) https://at-manuals.com/wp-content/uploads/2016/manuals/aw_55-50%20Manual.pdf

On page 15 it states the following



I have confirmed with my own meter that the frequency is fixed at 300hz (non changing) but that the duty cycle was 55% in P at idle and 38% in P at ~2000 rpm. These readings are too high.

Re-reading the WIS information for the 4th or 5th time I have now spotted what I suspected. It is the variation in duty cycle (high pulse ratio) that affects the amperage.

So my particular conundrum is that I have a high duty cycle, but I do not have high amps. Consequently, with low amps I have a high system pressure.

There are some more good pictures of an actual rebuild here https://maktrans.net/remont-aw50-55sn, along with the sophisticated test rig alluded to.

Like you I am chickening out of removing the valve body for the moment. At the end of this I am hoping that there will be an increased collective knowledge on how to carry out simple electrical diagnostics to confirm the failure mode without expensive equipment or disassembly.

Stepping back to see the wood for the trees :

1. The sticky solenoids cause the TCM failure - I believe that is a fact, so what we need to understand is what is the feedback mechanism that causes the TCM to self-destruct. I am not advocating that the sticky SOV's cause the TCM to drive into a higher current mode for the SOV's, that was just a rumour I think, perpetuated by lots of people with insufficient research into the actual failure mode.

2. Duty cycle : going back to my electronics theory, frequency (or pulse repetition rate for non-sinusoidal waveforms) can remain the same whilst the width of the pulse can change (via PCM) to change the mark-space ratio, the net result I think is that the driven equipment remains on/energised for longer (via the pulse).

3. Assuming that the WIS is correct in the relationship between duty cycle and current, would it not indicate that your TCM is goosed as it should be producing a higher current at high duty cycle whereas you have recorded otherwise. I would be interested to know how the duty cycle changes with accelerator position, looking at your findings they appear to be indirectly proportional.

I think the Sonnax articles contain the key to the operation but they take a lot of reading and I think are targeted at experienced auto gearbox technicians, so it may take some time to get to the bottom of this, well worth it if you have the time ... however I don't think I read anywhere what the effect would be on the TCM and what the feedback mechanism is from the valve body to change the duty cycle.

How does your fault compare to the Sonnax High and low pressure cases for each SOV (e.g. can you identify which SOV is likely to be the problem from the symptoms ?)

_________________
MY04 9-3 1.8t Vert
MY02 9-5 Aero Est
MY01 9-5 2.0L LPT
MY98 9-5 SE 2.3 LPT
MY97 Merc SL500 (oops)

Thanks for those thoughts, definitely pooling knowledge now

1. I think it's quite likely that SOV issues can cause an TCM failure, but also possible that they can fail by their own account. 100% in agreement that there is insufficient public research into the actual failure mode

2. I don't have the PWM background theory to help me, which is why pooling knowledge is so good. Intrinsically I see more "on time" (pulse width) gives more current simply due to time. As we are on DC there's no waveform to worry about and voltage would also be an analogue for duty cycle ???(I have seen partial 12v delivered)

3. I have been having the same thoughts about the 2003 TCM being goosed, but I very much doubt it. The reason ? The original TCM has failed. I had a replacement, but that showed errors, so got a third one. Eventually, using Tech 2 rather than a generic reader I discovered a problem with a disconnected speed sensor. Refitted that and everything was hunky dory with both the second TCM and the 2003 one. The second one is in the car, the 2003 one came from a reputable supplier and has been sat on the shelf since. This is the one I am using for testing.

The error P0748 does absolutely point at the SLT, the hard engagement into D and R also confirms the diagnosis......... though it is also clear that all three SOV's work together. Big smoking gun, but no bullet found

You're right, a lot of the information is there, but you need the background to understand what is being said, hence this mornings discovery in the WIS article. There are a lot of diverse backgrounds on UKS, so maybe someone with hydraulic, or electrical control knowledge can help the understanding.

At this point I am taking the view that the TCM is pretty dumb, in that what it does is receive a limited number of inputs:

ATF temperature
Input shaft speed
Output shaft speed
Accelerator position
Engine Speed

From that it determines a range of actions on the shift and linear solenoids to engage clutches and shift gears. There is no measurement or feedback on what the actual system pressures are.

That however cannot be the complete picture as how does the adaption work ??

Just got confirmation that my 10 litres of JWS 3309 ATF is on its way