Transmission thump

[baby#yota]

Hi all, looking for some insight on my transmission problem. About 2 months ago I was driving 35-40 mph, went to accelerate and my trans mad a hard thump, then a few minutes later did the same. It felt like it missed the shift. I had the issue one other time a few days later then nothing. The trans doesn’t always shift perfectly smooth but nothing that’s alarming to me. Then this weekend some friend and I went on a road trip up north, very remote. About 10 minutes from the cabin on a dirt road my trans went “thump thump” then caught itself. Yesterday on my excursion to town it did the same thing under light throttle 30mph range, “thump thump thump”. I’m a few hundred miles from home so I’m crossing my fingers I’ll get home fine and address this next week. Any ideas what this might be a symptom of?

I did change out a trans line a while back and lost a few oz of fluid but not much. Not even enough to cover the bottom of a catch pan. Just an anecdote

 

[AuSeeker]

I'm not an expert on transmissions, but it could be a shift solenoid going bad, a corroded/bad electrical connection at the transmission harness on the side of the transmission...or it could be low on fluid, even though you only lost a small amount of fluid changing out the line, you don't know if the fluid level was correct to begin with so you could be low on fluid, the only way to rule out low fluid level is to check the fluid level.

Also you should check for any codes which could point to the problem if it's solenoid/electrical related.

 

[baby#yota]

I'm not an expert on transmissions, but it could be a shift solenoid going bad, a corroded/bad electrical connection at the transmission harness on the side of the transmission...or it could be low on fluid, even though you only lost a small amount of fluid changing out the line, you don't know if the fluid level was correct to begin with so you could be low on fluid, the only way to rule out low fluid level is to check the fluid level.

Also you should check for any codes which could point to the problem if it's solenoid/electrical related.

No codes yet. I have the A750F trans, if that is helpful at all. I have the sealed unit so no dipstick sadly. I’m planning on dropping the pan when I get home. Doing a filter and fluid fill.

 

[SamIn]

Is it shuttering like a vibration then it finally shifts? Because my dads tundra with a750F did that and he had to drive in sequential mode and shift on his own between 35-45 to prevent it and we did a flush and filter change after dropping pan and it shifts like a dream now. We could not get all trans fluid out though since the torque converter holds some so this summer we will do it again to try to get it to have pretty much all new fluid. Hope this helps, thanks.

 

[Montego Murph]

....a corroded/bad electrical connection at the transmission harness on the side of the transmission....

Aside from checking the fluid level (which is very important,) checking this connection is the first thing I would do.

I have learned twice now that this is an area prone to corrosion, and at any sign of transmission issues short of cogs and sprockets tumbling to the ground as you drive, carefully disconnect this connector and spray both sides with something called De-Oxit, let it dry for a long while, put some dilectric grease on the rubber seal, and reconnect it.

This connector is what links the valve body and solenoids to the ECM, so naturally Toyota put it low to the ground in an area which is exposed to all sorts of moisture (compared to the flat tire tool kit which is made of steel bars, wrapped in a special little pad, and stored carefully in a sealed area of it's very own where nothing can bother it.)

Get some De-Oxit, do this, and see if that helps. Be very gentle with those pins though. If they're corroded and broken, you'll have to replace the cable going from the port to the valve body. Not a horrific job -- but probably one of the messiest jobs I've ever done, and I have a barn full of calves.

 

[baby#yota]

Is it shuttering like a vibration then it finally shifts? Because my dads tundra with a750F did that and he had to drive in sequential mode and shift on his own between 35-45 to prevent it and we did a flush and filter change after dropping pan and it shifts like a dream now. We could not get all trans fluid out though since the torque converter holds some so this summer we will do it again to try to get it to have pretty much all new fluid. Hope this helps, thanks.

It felt like the transmission wasn’t going into gear properly, I know very technical of me. Either engaging / disengaging the 4th gear clutch pack or the torque converter locking up and unlocking. It did 3 “thumps”. Then caught itself. I drove home the 300+ miles with no issues. However I did note that when I accelerated up some the mountain roads and got the rpms up the trans didn’t want to upshift again after I let off the gas. Just kept screaming along at 4k then thump back to 5th. Only did this twice on the ride back. Just some out of character behavior. I have an older and higher mileage truck so I’m not whining about not having a perfectly smooth transmission just odd behavior. I will look at checking the fluid level this weekend and cleaning the connections with deox as a first step. I also have some hand written notes by the old owner stating there was a transmission “service” done at my local yota dealer 2 years back. I’m going to call them up and see what exactly that was.

 

[Montego Murph]

The transmission service was likely a fluid exchange or drain & fill.

My money says it's the connectors sending bad info to the valve body and solenoids. Mine was doing the same thing with the TC Lockup misbehaving.

 

[baby#yota]

I’ll take a look this weekend and report back

 

[MikeinNH67]

I've posted this before but assuming you have the 5 speed check out this video.

Bad Toyota Transmission..Can We Fix It For Free? - YouTube

 

[baby#yota]

Update

So today I got around to checking out the connectors on the transmission harness. Like some of you said and stated in that helpful video that may have been my issue. I took it apart, cleaned it with connection cleaner and let it dry before reassembling. In the first minute of driving I got one weird shift but truthfully I was coming to a stop and it could be my driveshaft slip yolk needing grease again. The rest of my 35 minute drive went great. Shifts were smooth, and the trans seemed to be more responsive to kick down and throttle input. This may be placebo but I had zero issues with it. I also did get confirmation from the previous owner that a fluid exchange was done at 175k which is good news for me. Now I’m at 240k so approaching the service “interval” for my “life time fluid”. I have noticed that it only seems to act stupid when it’s below 25°F outside. Tomorrow I have to do quite a bit of driving in a snowstorm so that will be the real test. I will update again.