I'll attempt to keep this short but concise.

Bought the car (04 2.5 auto) with a bad transmission. Slipped from a stop and back wheels hopped during tight turns. CEL code P0866 (TCM Communicatio Circuit High)

So I picked up a used tranny (133k) from wrecking yard and put it in. Exact same symptoms and exact same CEL. P0866. I pulled the tranny back out and got my money ($550!) back. Yard said its pretty uncommon to have a subi transmission come back. I told them the story and they said the differential might be mismatched. Perhaps someone changed the diff out and that's what's blowing trannys.

Ok, so I bought a 3rd transmission from a reputable JDM engine/transmission replacement company, and came with differential. I installed the "new" tranny and differential yesterday... AND ITS DOING THE SAME THING. Slips from stop and hops during turns. The tranny hard lines on the jdm tranny were longer, and the studs on the diff were longer, but other than that it bolted right up.

With the 3rd tranny, im getting a new code P0502 (Vehicle Speed Sensor A Circuit Low). I did pinch the wires on the harness putting 3rd trans in, but I caught it and made repairs.

Current symptoms/probs
-No speedo
-Trip odo isn't accumulating
-P0866, P0502
-Appears to be operating in only 2nd or 3rd gear
-Torque bind

-Occasional squeal from diff/torque converter area.

-High idle 

I really need some input sadface

Added pics of some of my recent projects.

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Edited September 30, 20214 yr by Rockstaar1226

Actionable steps:

Check for loose or broken transmission wiring. Nearest I can figure, most of the transmission wiring has to be replaced if it, or the sensor, is damaged. At least I have actionable steps with P0502.

My larger concern is P0866 because it apparently something is wrong that isn't the transmission that causes transmission to not operate properly. I replaced the TCM, knowing they don't really go bad. No change. What would cause a "high communication circuit"? Poor grounding, maybe. Where is its ground?

I took a closer look at my wiring repair and am satisfied. 

I thought maybe the pan was dented, so I pulled off the pan and hit whatever dents may have been there (nothing was overly obvious) with my Tonya Harding, and reinstalled. All the wiring, solenoids, connectors looked fine. Reinstalled.

Went for test drive. Code P0866 came within minutes. P0502 didn't come on, but the speedo isn't working so it will. 

I guess my only actionable step now is to replace the transmission harness/sensors.

Is this really that unique of a problem and no one can help!?

tz1a4zkaaa-wc <-- transmission code

Edited September 29, 20214 yr by Rockstaar1226

I have 5 connectors under the dash being unused.

2 are green ecu whatever 

1 white is for testing for... something 

What are the other 2??

Picture

Tell us about your tires. What brand/model and size are they?

wheel sensor issue???

Edited September 30, 20214 yr by 1 Lucky Texan

Thanks for your reply. 

I dont know much about the tires other.than they aren't bald and they match. I'll take a look later today.

Is there a sensor at each wheel? I thought there was 2, both on the transmission. 

195/65 15 is the tire size, they are made by "Westlane". Sizes match, brand/type match, lots of tread left.

Video

Okie I made a video of the symptoms. Near the end, it doesn't shift, I let off.

I haven't changed the trans harness yet. 

I dont think the trans side of the harness has anything to do with the p0866 because it's been consistent through 3 transmissions. Perhaps whatever is causing the the code is causing speedo to not work.

I could really use some help compiling some actionable steps.

Edited September 30, 20214 yr by Rockstaar1226

Do you have a good Subaru/Transmission shop near by?  

Might be worth having them take a look.  At least you could get an idea of what's up.

A friend had a 2013 Legacy that was giving him some issues.  Shop said a Speed Sensor.  I found a good PDF and he went to the yard and pulled the sensor.   Swapped it out and all good so far.

Update

Checked for continuity on the ground wire between the ECU and TCM. It was fine. Assumed I needed a new ECU. Got one, put it in, no change.

However, if i manually ground that wire, I get my speedo and trip odo back, and it seems to start in 2 instead of 3. But grounding that wire also makes the AT temp light flash and P0865 (tcm com circuit to low)

Something must be wrong in the grounding system. I do not believe this is a "transmission" problem, its a wiring problem. 

Hopefully this thread will help some poor soul in the future searching.

I'm pretty close to finding a professional to pay. I sorta figured some of the members here would have that kind of knowledge base  

Edited October 1, 20214 yr by Rockstaar1226

Loadpro if you are reading voltage your meter can lie to you unless the circuit has a load.  Loadpro makes it easy to check the wiring, Opens, Shorts and High resistance.

 

https://www.aeswave.com/LOADpro-Dynamic-Test-Lead-p8955.html

Edited October 3, 20214 yr by lmdew