Hi folks,

question about pros and cons of a trans cooler. We recently swapped our transmission for a used unit because the original one failed multiple times after a rebuild (2002ltd transmission/torque converter woes).

New transmission shifts fine, but fluid temps get pretty high (around 230 max, rarely below 190F) when highway/backroad driving in California summer weather. All measured using a scangauge (its a 2002 so I can read trans temp directly). I am also on 33s with the original gearing. Thus thinking about reactivating my trans cooler.

I still have a tru cool 4589 lying around that I could put back in. I installed it previously when we still had the original transmission, just before it was going out (not because there were any symptoms, just as precaution). The general opinion seems to be that a trans cooler is a great addition for our rigs. However, I am a bit concerned now, wondering if the failing transmission and the addition of the trans cooler were connected. After all, adding a cooler and more lines should increase resistance and thus make the pump work harder - but my understanding of auto trans is extremely limited. I am somewhat inclined to "never change a running system" as many transmissions seem to make it to 300k+ miles without a cooler. But a cooler trans would make me more confident.

Thus any thoughts appreciated on :
- how hot is too hot for our transmission?
- could adding a transmission cooler be bad for an auto transmission, possibly due to installation errors?

The only likely damage your cooler might cause is if there's still metal bits in it
from the previous failure. I'd probably still buy one of those cheap Harbor
Freight laser temp guns. That will let you check hose routing, hot spots, etc.

Here's a graph showing fluid temps and xmsn life.

I have my aux cooler in line with the in-radiator cooler. This will help warm the ATF to normal temps when it's super cold out.

Otherwise, yeah I'd really flush the cooler and the cooler lines (including the OEM trans cooler lines) and pop it back in.