Help with (what I think is) a Torque Converter Issue (1 Viewer)

[96TrailSurfer]

I have previously posted about the ATF temperature spiking in my A340F transmission when I let go of the gas. I think I narrowed down the issue a bit, so this is more specific post/question.

In short, the issue arises when I am driving at highway speeds (over 80-85km/h) or uphill for any extended period of time. As long as the transmission is under load, ATF temp is generally fine: 150-200F, measured by a sensor in a t-fitting installed on the flex portion of the outlet pipe.

Once I release the gas, ATF temperature quickly spikes by 50F or more, and if I don't rev up quickly enough, the temp rises high enough to get the ATF light on. Happened to me twice this weekend: (1) I was climbing to a mountain pass and, once I reached the top, I had to slow down because of slower traffic - spiked to ATF temp light within a 30 seconds; and (2) driving at about 100 km/h on flat highway, had to suddenly brake because of traffic, again, had to pull over to cool it down. Otherwise, I managed temps by keeping it in O/D and lower gear to always have a load on the tranny.

I had the tranny looked at by a shop last week and they didn't find any issues. Flow rate checked out fine, as did the ATF fluid and the filter (this was before my recent drive through the mountains).

Here is what I think. As long as the transmission is under load, the Torque Converter (TC) is engaged. It uses ATF as a fluid coupling between the impeller and the turbine. As soon as I let go of the gas, the TC disengages, and the ATF from the TC inside is spilled into the housing. This is generally the hottest ATF in the transmission - hence why the outlet pipe and the stock temp sensor are located right by the TC outlet valve. What seems to be happening is that the ATF inside the TC is way hotter than it should be, and that's what I see am seeing.

This conclusion is reinforced by the fact that, lately, I've felt like the truck has become quite sluggish, as if the transmission is not transferring enough power. Of course, if the TC is not transferring power (kinetic force) from then engine to the transmission, power is lost as heat.

So, has anyone had similar issues with their transmission or the TC? Any ideas? useful links? Can the TC be replaced by itself?

I did overheat my transmission a year ago when I had blocked flow, could it have damaged the TC?


Things that have checked out OK:
- temp sensor - temp reported correctly
- cooling loop - stock rad in series with an external cooler, plus a fan on the cooler. I also just serviced the fan clutch on the rad;
- flow - shop said flows fine, to spec.

 

[gerg]

Did you change your atf after your initial overheat incident? Have you ever changed it? What kind of fluid do you run? As far as I understand raising your ATF fluid to the point of triggering the overheating temp light is something in the order of 300F which will render Dino ATF fluid mostly oxidized and not suitable to be used after, or at best, for a very very short time.

 

[96TrailSurfer]

Did you change your atf after your initial overheat incident? Have you ever changed it? What kind of fluid do you run? As far as I understand raising your ATF fluid to the point of triggering the overheating temp light is something in the order of 300F which will render Dino ATF fluid mostly oxidized and not suitable to be used after, or at best, for a very very short time.

Yes, I flushed and changed fluid after the first incident, and again very recently. I am using synthetic ATF fluid, Dextron III compatible, that has a fairly high breakdown point. I checked the ATF recently, and it's still nice and red (unlike when it overheated a year ago, it was black and smelled like s***)

 

[gerg]

So it happens when your essentially decelerating using tranny resistance if I understand it correctly or when it has little load on it? Have you adjusted your kickdown cable? It sounds like inadequate line pressure, But I'm a super auto tranny noob so forgive me if that is a rookie remark. I have an a440 and a mm on my kick down cable made a world of difference for my shifts. Use to be very sluggish and lacked any power at all when accelerating. Cource now that it's adjusted it shifts tight, but sadly the 3fe didn't magically turn into a more powerful motor.