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2009 - Intermittent bucking problem from rear end as tranny goes thru 1st and 2nd gears?

('09-'13)
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abs light at oil temp traction control light transmission
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3K views 6 replies 5 participants last post by  lars_nordin  
T
#1 ·
I have a 2009 Forester with ~165,000 miles with an intermittent transmission problem (4EAT not CVT). After driving 5-10 minutes, there is a harsh bucking from the rear end - it sounds as if I went over a speed bump. It first happens as I come to a stoplight and the RPM drop to ~1k. As I move forward, the bucking reoccurs when the transmission goes from 1st to 2nd, and then from 2nd to 3rd.
After about 1/2 mile, the ABS and Traction Control lights come on solid and the AT Oil Temp light blinks. After the lights come on, the bucking stops! I can drive for 2-3 miles more and no bucking occurs. The lights stay on. I can't get any codes from my OBD reader, and the CEL never comes on. After 30 minutes, I restart the car with no lights.
This problem has been intermittent (since 2018), but has recently become more common. I had the transmission flushed back in 2018, but this clearly didn't solve the problem. Of course, my Subaru guy couldn't replicate the problem when he test drove it.
Main questions:
1. Why is the bucking coordinated with the gears changing?
2. Why does it have to be warmed up before the bucking occurs?
3. Why is this only an intermiottent problem? and Why is it becoming more common?
4. Why does the bucking cease after the ABS/Traction control/AT Oil temp lights come on?
5. Why is there no CEL or ABS code?

Any suggestions or theories will be welcome - this car is largely used by my kids getting to/from school, and I don't safe having them drive it if the tranny is going to explode.

Thanks,
Tom Shields
 
#2 ·
You’ll need a Diagnostic Scan Tool, to be able to interrogate the TCM (Transmission Control Module). However, reading about the Bluedriver ODBII reader, they suggest that it can communicate with all Control Modules.

Some faults, need to be present for multiple drive cycles (system self check), before a CEL is triggered. However, if a fault was detected by the TCM, it should be stored in the Module.

It could be an intermittent accelerator position sensor error; transmission speed sensor error; or faulty transmission control solenoids. Also possibly something triggering activation of the ABS system.

Temperature, can sometimes be linked to the cause of, or be the exacerbating element for electrical problems. It can cause deterioration of components; expansion and contraction of internal circuitry, or external sensing components; and a change in electrical resistance (and probably inductance, capacitance etc).
 
#3 ·
@Tom Shields,
I have the same issue on my 2009 Forester that just started. Did you ever get this resolved? I thought it was bad gas, but I am into a new tank and have the same issue. Once my engine lights come on, the bucking stops and doesn't come back until I restart the car and clear the lights. I also use this car with kids in it and don't feel safe driving them around anymore.

Thanks for any info you have on this issue. I haven't found anything.
 
L
#6 · (Edited)
I have the same issue as @Tom Shields (and possibly @Grantjg12) but I have a 2010 Forester.
I recently had the shifting issue reoccur but this time with the same lights (Traction-control and ABS lights on and A/T Oil Temp blinking) - which hadn't come on before for all the other times that the car shifted hard. I have a blue tooth OBD2 reader (BlueDriver) and used it while these lights were flashing but it was not able to read anything abnormal. Once I restarted the car the problem went away.

Just like Tom, this issue has intermittently occurred over the last 9 years. Initially, it was once every two years, and later it became once or twice a year. Now it is happening more often. The most recent time, the hard shifting started right after leaving my driveway (it mostly happens when starting to drive). More recently, it is not so hard but it has been a hard shift - as if I hit a small speed bump. Usually stopping and turning off the engine stops the hard shifting. One time I did try shifting into sport mode and found that it would not shift into 4th gear. I have had the transmission fluid changed twice (at a local dealer) - the last time 15k miles ago. Also, I don't see a temperature pattern to these occurrences since they happen in summer or winter.

After this last incident with the dash lights on, I took my Forester to a local dealer again who reported that there were no codes found. (The dealer did offer to perform an AT valve body replacement but the symptoms requiring that fix did not match this issue). I would like to be know if there is a DTC code(s) and what it is(are).

I have a few questions:
  • Do I need to try to capture the code while it is occurring before shutting off the engine?
  • What would be a good reader to use? I'm looking at maybe an Autel MaxiDiag MD808 Pro but I need to research a little more.
 
L
#7 ·
Just an update to my situation:
A few weeks ago, the hard shifting problem continued but didn't go away after turning off the engine as it had in the past. I took it to an independent shop that has worked on Subaru transmissions but they declined to work on it as they were not confident that they could fix it. I took it to an independent Subaru mechanic and after reading my log of this issue said the best solution was to replace the transmission from a wreck of a 2010 Forester they had, for the same cost as the dealer would have charged for the valve body replacement. They gave me a quote for a new transmission, or if they would rebuild my current transmission but I chose their first option of replacing the transmission.
 
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