Postby BCGeo » Wed Apr 18, 2018 11:59 am
Hi JP4,
First time posting but I’ve been lurking for the last few months. Note that this is all based on the information I’ve uncovered between reading the manuals, this forum, the dealership mechanics, and a transmission shop. If anyone can correct my interpretation please do so!
The rumble strip feeling at 40MPH you described is exactly what I just went through. My pathy is currently getting an tranny rebuild since I didn’t ID this in time so hopefully you can get it properly checked before you end up like me. I’ve written out my story below, but I recommend you do a quick check to test if you’re issue is the same as mine.
CHECK:
Find a local road where you experience the rumble at 40MPH with rpms ~1100. Generally, this will happen on a road with a slight incline as the added load is enough to cause slip but not enough to downshift to 4th. You want the truck to stay in 5th gear with about 1/8 of the gas pedal pressed. Once the rumble is occurring, shift into 4th gear while remaining at 40. If the rumble stops then it suggests the issue is in the tranny or downstream (vs engine). In addition, you drive the road again but this time when you feel the rumble press the brake pedal 1/8th or less (enough to have the sensor engage but not to slow down substantially). If the rumble stops this suggests that the issue is the lock up (lock up disengages if the brake is pressed). If both these are correct then go to a good tranny shop and drop the pan. Look for evidence of SMOD and probably look into replacing the torque converter. It’s costly due to labor, but better than rebuilding the whole thing.
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My Backstory:
My pathy had original rad until I bought it at 91k miles. There were no visible signs of rad leaking and no past SMOD, but when I bought it I immediately replaced with in aftermarket. Based on this I went through diagnosis assuming that I had avoided SMOD.
Within several weeks I noticed the rumble when going ~40MPH with RPM ~1100 (ie when in 5th). This led to several visits to the mechanics discussing options, but it was assumed that I could drive the pathy without issue for the time being and even trailer up to 50% rated weight. I put another 2000 miles on it without any noticeable changes, but would use manual mode / OD off to keep the tranny in 4th until above 45MPH. I am moving from east to the west coast for work so started my drive, so hooked up a 2500 lbs trailer and took off. Within a day I had fully blown the tranny and had to spend a week in a town waiting for a rebulild.
The result:
Turns out that there was a tiny bit of rad leak into the tranny. What we suspect happened is that this caused early wear on the torque converter and lock up clutch material. 39 MPH in 5th gear is when the lock up in the torque converter engages. As you speed up, line pressure in the tranny increases and helps to push the clutch harder. Since my clutch material was weak from the rad fluid I was getting lock up slip between 39 MPH (lock up engages) and about 45MPH (where line pressure appears to have been enough to get the clutch to grip). This slip was registered on the TC SLIP SPEED sensor which would tell the tranny to disengage lock up and try again. This repeats, causing the rumble. Were still not sure what caused the immediate failure since the 2k miles I put on did nothing and I was doing the same manual shifting to avoid the rumble, but it is clear that my torque converter ended up failing, causing increased slip, burning the other clutches, and forcing a full rebuild.