I have a 2000 1.8L Mazda Protege and I just replaced the engine with a 2.0L JDM swap. The new engine is installed and appears to run great, it idles smoothly and revs fine, sounds great.

Oddly, after I replaced the engine, the automatic transmission will not go into gear. Actually it did go into gear a few times when I revved it up to 2500 - 3000 RPM.

I DID NOT pull the transmission during the engine swap. I left all electrical connections to the transmission in place as well. I DID pull the radiator, which has a transmission fluid cooler, and a little fluid was lost. But I have replaced the radiator before and never had a problem.

After revving and getting into gear, I was able to drive the car from the shop to my home (about 1 block distance, 10 mph). Now that it's in my driveway I checked the tranny fluid; with the engine hot and running after shifting thru all gears, it still reads higher than MAX on the tranny dipstick. It is possible that it was overfilled to begin with, or that I just can't read this dipstick. I added about 1/2 qt more fluid, and now it won't go into any gear, even when revved.

I find it hard to believe that the tranny would have failed coincidentally, especially since I did not touch it during the swap. I feel like something else must be going on, but I can't imagine what.

Any ideas? Thanks!

  • Based on the dipstick reading, I would think the tranny is over-filled. I only added more fluid afterwards because of reading about the symptoms, which sounded like "low fluid". I will check the level in the morning and remove enough fluid to bring it between the Min and Max levels. Apr 5, 2017 at 3:30
  • However, if the problem is too much fluid, why would it have driven fine before the engine swap and have a problem afterward? I didn't add any transmission fluid during this swap. Apr 5, 2017 at 3:31
  • I should also clarify that the transmission does not seem to be "slipping". It is simply not engaging at all, like it is in neutral. Before adding fluid, I could get it to engage by revving the engine up a bit, and it would engage suddenly. Since the RPM was a bit high the vehicle would lurch forward when the gear engaged. After engaging I could drive and it seemed like power to the wheels was normal. So the problem seems to be that the transmission will not engage properly. Apr 5, 2017 at 3:35
  • Also, the check engine light is NOT on, and after I got the car home I noticed the "O/D off" (overdrive disengaged) light was blinking. From what I read online this would indicate a detected transmission problem. My guess is that the computer has started flashing this light simply because it can detect that the transmission is not engaging when it should. Apr 5, 2017 at 3:37
  • did you reseat the torque convertor before you bolted it to the flexplate?
    – Ben
    Apr 5, 2017 at 10:59

2 Answers 2


UPDATE: We pulled the engine and trans today, and put the correct flexplate on, which definitely has a different geometry and sits closer to the trans (i.e. not pulling the torque converter out). There was no evidence of spline or seal damage. Put it all back together and ... it runs, drives, and shifts perfectly. So my theory, #1 below, was correct. Also changed real main seal and torque converter seal since it was accessible.

Moral of the story: Always use your original flexplate / ring gear. Make sure the torque converter is seated properly, and don't pull it out if the studs seem too short. :)

So, I talked to 3 friends about my problem, and got their ideas on what may be wrong. One of them actually helped me swap the engine (extra hands), and the other two are mechanics who work in the same shop where I did the work (but weren't directly involved in the project). Based on my description of what I did, it turns out they all have the same suspicion:

  1. I probably pulled the torque converter out a bit too far when bolting it up to the ring gear. The output spline is long (~3/4 in), but the spline that drives the transmission fluid pump is very short (~1/4 in). Pulling the torque converter out too far would cause this pump spline to slip, which means no fluid pressure in the trans and no engagement of gears.

There is some evidence to support this theory: My torque converter has 4 studs that protrude thru bolt holes on the ring gear, and nuts are attached on the back side of the ring gear thru the starter opening. When we first connected the new engine to the trans, things slid together very nicely, but oddly the bolts did not protrude thru the openings in the ring gear, although I could see them flush just inside the holes. I put my fingers around the edge of the ring gear and "wiggled" the torque converter closer, until the bolts began to protrude, then attached nuts and gradually tightened them in sequence. I naively thought that the torque converter had simply been "pushed back" during the assembly of engine to trans; I did't have enough experience to realize that is probably not possible. If this theory is correct, it means the ring gear on the new engine is different than the old ring gear, or some other geometry is out. I know I should have used my original ring gear, but a visual inspection led me to believe they were the same!

Other possibilities:

  1. When bending the wiring harness out of the way, I could have broken some wires that connect the ECU to the trans. I feel this is unlikely, because I was pretty careful not to stress the wiring too much, and there is no evidence of hard or cracking insulation even on the wires that are exposed.

  2. Some sort of contamination could have entered the trans fluid lines between the removal and re-installation of the radiator, which includes a trans fluid cooler, and fouled up the valve body or some such. (I'm pretty sure there isn't a leak, because there is no evidence of coolant contamination or loss of coolant.) But maybe some debris got into one of the hoses when it was disconnected. But I feel this is unlikely, even though I didn't cover (cap) the hoses and connectors, the area of the shop I was using was undisturbed during the three days I was there.

  3. Air that entered the trans cooling lines could have somehow fouled things up. I think this is unlikely, or it would be a problem every time somebody changes a trans cooler / radiator.

  4. The transmission, although high mileage, drove just fine before the swap, and just coincidentally failed while I was working on the engine. How likely is that?

I'm going with #1 for now. Unfortunately this means I need to pull either the engine or the trans again and put my original ring gear on. I will also make a careful comparison (measurements) of them. My friends feel that it is unlikely that I damaged the trans in all this, so I'm hoping they're right.


My name is Robert I work at a transmission shop. I've been doing this kind of work for 20 some odd years. Start your car and note where the transmission fluid is at on the dipstick. Shut the car off wait for about 2 minutes recheck the fluid if there's no changein fluid level you broke the pump. The mechanism that turns the pump is an octagon Rod that runs all the way through your transmission. Now it is accessible from the rear of your transmission but I don't advise you to tackle this on your own ticket to a qualified mechanic at a transmission shop

  • What specific sequence of events could have led to "breaking this rod"? Apr 7, 2017 at 18:03
  • We pulled the engine and trans today, and put the correct flexplate on, which definitely has a different geometry and sits closer to the trans (i.e. not pulling the torque converter out). There was no evidence of spline or seal damage. Put it all back together and ... it runs, drives, and shifts perfectly. (So my theory, below, was correct.) Apr 10, 2017 at 2:45

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