Rpm fluctuation at highway speeds

[SensRanger]

96 Ranger 4L 4x4 auto 93,000 kms

Trans rebuilt 6,000 kms ago... i am noticing that when cruising at
about 100 km/h (about 60 mph), RPMs are at about 2100, if you let off
the gas they drop to like 1600, seems normal, but then if you just
coast for a couple seconds more, the RPMs dip to like 1000 and back to
about 1500 and back and forth a couple times...you can hear the engine
speed changing...as deceleration continues eventually it levels out
again. Hitting the accelerator brings it back to 2000+ with no
stumbles, slipping or other issues. What could be causing this? Torque
converter issues? I thought the TC unlocking was the initial RPM drop
from 2100 to 1600. I dont know how it can go as low as 1000 at such
high speeds. Sensor issues? Anyone know where to focus my attention? No
check engine lights, very stable idle at about 800, no OD lights...i do
have a 3-1 downshift deceleration 'clank' when coming to a stop, i
think comes from the tranny (new 5r55e VB on the 4r55e) ... any ideas appreciated

thanks

 

[BrooklynBay]

How long ago was that VB replaced? Some common problems with these transmissions are VB gaskets, and loose VB bolts. Check the torque on those bolts, and tighten if necessary with an In.-Lb. torque wrench. Band adjustment might be something to look into. The intermediate band is for 2nd gear, and the front band is for overdrive.

 

[SensRanger]

I went for a drive with the trans tech guy today with the scan tool, it was not the torque converter clutch making the RPMs bounce around... and most of the readings were normal, i think the MAF had some fluctuating readings in time with the RPM drop. So its still a mystery. As for the downshift clunk, it was heard and felt and the scan tool confirmed it was right at the 3-2 shift. But all inputs from the sensors and other readings were normal. I might try cleaning the MAF and replace the range sensor. I noticed the other day I could only start in Neutral not park, and the reverse lights sometimes do not come on... could the range sensor affect/cause the other issues i wonder? Trans tech guy mentioned driveline slop ie. ujoints/rear end...but I just had those checked. He said i could bring it in next week and they would try driving it in 4wd with the rear driveshaft out to test it. Could play in the transfer case chain cause this noise? because that was recently diagnosed, but I was told it was a minor issue and 'might be a problem down the road'...does the tcase chain spin in 2wd mode?

 

[BrooklynBay]

Adjust the DTR sensor, and check if your reverse lights come on when you put it into reverse.