At 2,500 to 3,000 RPMs the transmission slips and will not engage second gear unless I leave off the throttle?

VAL SCHAN

MEMBER
1995 CHEVROLET SILVERADO
5.7L
V8
4WD
AUTOMATIC
100 MILES

Just had the transmission rebuilt by a certified tech. The transmission works fine when driving it easy, shifts through all the gears. When accelerating fast and reaching 2,500 to 3,000 RPMs the transmission slips and won't engage second gear unless I leave off the throttle. Same for third gear. The tech who rebuilt the trans said it is an engine problem. Check the sensors. I have replaced the output shaft speed sensor on the transfer case, replaced the throttle position sensor and manifold absolute pressure sensor still have the same problem. The transmission is a l-460-e.

Val Schan

5 Replies

KEN L

MASTER CERTIFIED MECHANIC
48,363 POSTS

Yep, if the catalytic converter is partially plugged it will cause low vacuum which will cause the transmission to shift like you have described. Here is a guide to help with the testing of the converter:

https://www.2carpros.com/articles/how-to-test-a-catalytic-converter

This guide may help as well:

https://www.2carpros.com/articles/automatic-transmission-problems

Please go over these guides and report back.

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Monday, January 2nd, 2023 AT 1:12 PM

VAL SCHAN

MEMBER
3 POSTS

Thank you for your timely reply and suggestions. Although the catalytic converter was replaced a little over a year ago, I will get an infrared temperature meter and vacuum gauge and do the recommended tests.
Would a faulty oxygen sensor be causing the shifting problem? If I am driving on the interstate hwy for several miles running about 2,500 RPMs the service engine light comes on. As soon as the engine is turned off and restarted the service engine light is off and stays off when driving locally.
How does the transmission know what the vacuum pressure is in the engine?

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Monday, January 2nd, 2023 AT 7:05 PM

KEN L

MASTER CERTIFIED MECHANIC
48,363 POSTS

I don't think an oxygen sensor will cause the problems but the check engine light coming on is a clue lets run the codes to see what comes up.

Here is a guide to help:

https://www.2carpros.com/articles/checking-a-service-engine-soon-or-check-engine-light-on-or-flashing

Please go over this guide and report back.

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Tuesday, January 3rd, 2023 AT 11:34 AM

VAL SCHAN

MEMBER
3 POSTS

I have ordered a code reader and vacuum meter. Will check things out when they arrive on Saturday. My other question was how and why is the transmission shifting affected by engine vacuum? There are not any vacuum lines connected to the transmission?

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Wednesday, January 4th, 2023 AT 8:34 AM

KEN L