* Transmission Internal Problem: This is the most likely culprit. Worn-out internal components (like clutches, bands, or planetary gears) can cause the transmission to behave erratically, slipping into incorrect gears. Low transmission fluid, or fluid contamination, can also contribute to this.
* Shift Linkage/Cable Issue (if applicable): Older vehicles may have shift cables. If these are damaged, stretched, or misadjusted, they might incorrectly signal the transmission to engage the wrong gear. This is less likely on a '99 Suburban, which likely has electronic shift control.
* Transmission Control Module (TCM) Problem: The TCM is the computer that controls the transmission. A faulty TCM can send incorrect signals to the transmission, leading to incorrect gear selections. This could be due to a malfunctioning TCM itself, or a problem with the wiring to the TCM.
* Valve Body Problems: The valve body is part of the transmission and directs the flow of transmission fluid to control gear changes. Worn or damaged valves within the valve body can cause shifting problems.
* Speed Sensor Issues: An inaccurate vehicle speed sensor signal can confuse the TCM, resulting in incorrect gear selection.
* Governor Issues: The governor regulates shift points based on engine speed. If it's malfunctioning, this could affect shifting.
What to Do:
You absolutely should not continue driving the Suburban with this problem. Continuing to drive it with a malfunctioning transmission can cause significant further damage, leading to a much more expensive repair.
Take it to a qualified mechanic or transmission specialist. They'll need to diagnose the specific issue using diagnostic tools and potentially a transmission fluid and filter change. Don't try to fix this yourself unless you have extensive experience working on automatic transmissions – it's a complex system.
