[Tom Shields]

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

 

[Dogs]

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).