RPM fluctuating, especially on highway

[derekjl]

1999 Tahoe Auto 4WD. Transmission replaced approximately 15,000 miles ago. Lately I've noticed that the RPMs will fluctuate from about 1800 up to 2200 or so for a split second when cruising. This is usually on the highway at at 75mph speed or so. When it happens it seems pretty rhythmic, almost every 5 seconds or so. It will go days or sometimes weeks without happening, then comes back. Where should I start? I would guess torque converter or maybe even the tranny is already going bad? Any help would be appreciated.


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[SunlitComet]

Sounds like the converter clutch turning on and off. Not entirely abnormal. You may have a brake light switch going out or the pcm is sensing something wrong in the tranny. Scan for active and history of codes. Some do not activate the CEL.

 

[derekjl]

Sounds like the converter clutch turning on and off. Not entirely abnormal. You may have a brake light switch going out or the pcm is sensing something wrong in the tranny. Scan for active and history of codes. Some do not activate the CEL.

I will also add that our cruise control doesn't work either. I thought I remember this somehow being tied to that? When I really get on it entering a highway, there is definitely some torque converter shudder happening. Sometimes the CEL will blink a few times then come on solid for a day or two. Not sure that the TC was replaced at the same time as the tranny?


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[SunlitComet]

Yep, sounds like you need to replace that switch. If the CEL is actually blinking at a set rate then you have an active and serious misfire happening. The misfire history will point to what cylinders are having the issues. On solid means there is an active code that has an impact on the vehicle emissions. It could be a bum sensor or loose wire or who knows what. You need to pull the codes to find out.

---------- Post added at 07:02 PM ---------- Previous post was at 07:01 PM ----------

Tranny shops don't typically reuse a converter they put a new or rebuilt one in.

 

[derekjl]

Where is this switch located and can I do it myself?


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[SunlitComet]

Feel up the brake pedal arm. It attaches to the brake push rod. You can do it but it may be hard to access for you. Some people also damage the retainer clip but you can buy those separately in local stores.