Speed constant but rpm is changing ???

[strykerAKAmack]

Noticed the last few times when out on the Highway
cruising at 65 the rpm is normally 1800 .
It every now than jumps up to 2000-2100 rpm but
speed stays the same at 65 .
Any ideas what it could be ? If I drop the trans into 3rd rpm is up over 2500 at 65 so it cant be down shifting with only a 200 rpm variance .
If back off the throttle and get truck down to about 57 the rpms drop back to normal and can then get back up to 65 @ 1800 .
Any ideas ?
Thanks

[bowtiebutler]

Your torque converter lockup clutch is disengaging. Do you have a check engine light on, or any codes.

Matt

[Triscuit]

Mine does about the same thing. The rpm's will jump up for a few seconds and then drop. The needle is never really consistent while on the freeway. Although I never hear or notice a difference in the motor or trans that would indicate a t.c. lockup failure.

[DennisG01]

RPM should stay rock solid, unless the torque converter is unlocking. It takes a good amount of throttle at 65MPH to unlock it. In other words, light to a "strong" medium throttle shouldn't cause a healthy converter to unlock at 65MPH.

[DmaxMaverick]

The most common cause is the brake lamp switch.

This won't cause a DTC. Almost all other possible causes will.

[strykerAKAmack]

No codes showing up on truck .
I did have to drop the transfer case last year to change a seal .
Had to unhook the wiring harness on case , could that be loose ?
If its the brake switch where would that be located ?
Thanks

[phantom309]

No codes showing up on truck .
I did have to drop the transfer case last year to change a seal .
Had to unhook the wiring harness on case , could that be loose ?
If its the brake switch where would that be located ?
Thanks

brake pedal arm,.

[strykerAKAmack]

Put an order in for one $11 shipped .
Has to be cheapest cost replacement part So far installed on truck
Why an electrical part for brakes would have anything to with a torque converter is beyond me
Thought they worked on pressure alone
thanks

[arveetek]

Put an order in for one $11 shipped .
Has to be cheapest cost replacement part So far installed on truck
Why an electrical part for brakes would have anything to with a torque converter is beyond me
Thought they worked on pressure alone
thanks

When you step on the brake pedal, it unlocks the torque converter. If the switch is malfunctioning or out of adjustment, it will cause the TC to constantly lock and unlock.

Casey

[DmaxMaverick]

The TC lockup is hydraulically powered, but it's electrically controlled (allowed) by the TCC solenoid, which is electrically powered by the ECM. It is a series-collapsible circuit, with the brake pedal switch in the circuit. On the OBD-I models, there is actually an independent TCC power lead at the ALDL. On OBD-II models, it's strictly powered/relayed by the emissions logic board. The internal hydraulic circuit is also collapsible, while certain environmental conditions (output speed, range activation, main pressure, etc.) must be met to complete that circuit.

[strykerAKAmack]

Hi All
Thanks for all the info. Last converter l swapped out was years ago on my 73 dodge charger. No stinking electronics back then :)
Finally warmed up enough to take a look at switch .
WOW difficult position to work on
Anyone do this before without having to take the steering column bracing
apart ?
Any tips appreciated
{ the service manual pic makes it look easy LOL }
thanks

[More Power]

If there is no change in engine sound that would indicate an increase in engine speed when this happens, I'd not necessarily think it's a TC problem.

The tachometer gets a signal from the alternator. A problem with the alternator can cause a problem with the tach.

[strykerAKAmack]

I just remembered .
Ever since I've had the truck Whenever to take you foot off the gas
the rpm's will drop down to 800 or so .
Never gave it a second thought but could be tied in with brake switch ?
I guess that would be the converter unlocking even though I'm not hitting the brakes .
Hopefully will get the switch out soon and see

[DennisG01]

I just remembered .
Ever since I've had the truck Whenever to take you foot off the gas
the rpm's will drop down to 800 or so .
Never gave it a second thought but could be tied in with brake switch ?
I guess that would be the converter unlocking even though I'm not hitting the brakes .
Hopefully will get the switch out soon and see

Correct. That is the way this transmission/engine combo works. And if you LIGHTLY touch the brake, you should then see the RPM's drop to idle.

[strykerAKAmack]

Hi All
Finally got switch swapped out :)
Took truck out for a test run and all seems to be good .
RPMs was rock steady at 65 mph for 10 miles . So looks like that was it .
Thanks to All for the help and info