rpm problems...

so I was driving down the freeway and on flat ground at about 50mph my engine runs runs at just above 2k...two days ago it randomly starts surging up with out actually going any faster....doesn't do it all time and so far it hasn't done it under 45mph...HELP!

Specs: 1990 xj auto, I6..

Sounds like the torque converter is unlocking.

Try this:

Unplug the TCU, under the dash passenger's side. Drive shifting manually. You will have 1st in 1-2; 3rd in 3; 4th in D; reverse, neutral, and park. If you get exactly that then the problem is electrical. If not, you have a mechanical/hydraulic issue.

If the problem is electrical:

Check/adjust the TPS; check the 10 amp inline fuse to the TCU--yellow wire; check the brake light switch adjustment--it also unlocks the torque converter; very less likely is the NSS--it also has other jobs, one of which is a manual gear selection indicator to the TCU; test the solenoids for correct resistance; test the wiring harness between the trans and TCU and the TCU to the TPS for continuity.

Articles:

NSS http://www.jpmagazine.com/techartic...eep_cherokee_neutral_safety_switch/index.html

AW4 http://www.transonline.com/transdigest/magazines/1997-10/Shift Pointers/index.html

TPS and other Renix sensors http://www.transonline.com/transdigest/magazines/1997-10/Shift Pointers/index.html

Good luck.

Sounds like what mine does. I replaced the TPS with no luck (though it could be that on yours), other candidates are the brake switch, TCU, and solenoids as Joe said.

Try pressing the pedal slightly more and see if the problem goes away - mine only intermittently unlocks at cruising speed, if I am constantly slightly accelerating, or decelerating, it behaves normally.

My son's '96 recently had a torque converter not locking problem. Went on for a while. I had read on here that the brake light switch could be the culprit. He had recently changed boosters. Adjusted the switch, now it works fine!Brake lights worked normal, before and after. Go figure.
Two grand RPM at 50 MPH is excessive, with stock gearing. All four of ours will do 2k at 65MPH. It does sound like the TC is not locking. Not good for long-term life for the tranny, as a locked-up TC will generate virtually no heat, and one that is slipping is a contributor.
The surging could be unrelated, as in a faulty TPS, OR a faulty TPS could be preventing the TC from locking. There are a few recent threads on checking the TPS, a good place to start.

i think i i got it fixed...i drove it 30+ miles on the freeway today and did not have the issue at all (so far it normally would have ran the rpm's up many times over the course of 30 miles..)..i replaced a bunch of vaccum lines that were old and cleaned out the throttle body...when i started to look at the tps i found a small rock under the "arm" that moves with the throttle...the bottom bolt was also loose so it was moving the tps back and forth...cleaned and tightened everything and seems o.k...

thanks for the help guys it gave me places to look...:thumbup:

And you adjusted it correctly? It is adjustable on the Renix.

See here: http://www.lunghd.com/Tech_Articles/Engine/Basic_Sensors_Diagnostics.htm

This is also a MUST BOOKMARK sight for Renix owners.

Yes I fiddled with it for awhile until I got it right then tightened everything down...thanks for the link...

Athompson said:

Yes I fiddled with it for awhile until I got it right then tightened everything down...thanks for the link...

Click to expand...

It ain't right unless you set it with a voltmeter.

cruiser54 said:

It ain't right unless you set it with a voltmeter.

Click to expand...

EXACTLY! AT you need output voltage to be 83 percent of reference voltage; MT you need output voltage to be 17 percent of reference voltage.

The TPS is not hand grenades, horseshoes, or naval gunfire support.