Having problems with your 2001 GMC Sierra 2500HD ?

Wont shift ie when running 40 mph tach is 3000/ When you let off though it coasts down as normal. Check engine light has never been on. Took it to auto zone they hooked up code reader and it wouldnt power up.Turns out the cig lighter fuse was blown, replaced that and scanner powered up and showed no codes. He recommended taking it to a local transmission shop for a free 21 point check. Long story short they said theyre scanner showed no power to several solenoids or something like that and they

Posted / edited by AnonymousUser on : 19-03-2018

Answers :

NOTE:The transmission is on the same fuse as the MAF, O2 sensors and at\015\012least one other sensor - if an O2 shorts out, it will blow the fuse and\015\012the trans will go into limp-home mode. Be sure the trans has power and\015\012the fuse is not blown before doing anything else.

Also note that many problems that show up as transmission issues are\015\012actually PCM or electrical, and may be external to the transmission.\015\012Try a different PCM, and check for chafing where the cables come out of\015\012the PCM.

Secondly, put a line pressure gauge ($55 shipped on Ebay, or visit a\015\012local trans shop) on the pressure test port and go for a drive to\015\012verify proper line pressure. Base pressure at idle in P,N,OD should be\015\012around 70psi and around 140psi in R. Max line pressure should be at\015\012least 190psi at WOT in 1st or 2nd, 225psi is optimum. If base pressure\015\012is low, trans or pump is worn out. If max pressure is low, check EPC\015\012solenoid. If reverse is slow to engage, and line pressure does not come\015\012up to at least 130psi in reverse, replace the boost valve with a\015\012Transgo 0.500" boost valve (easy fix in pan).

What is "limp-home mode" ? This is a fault mode in which the\015\012transmission either loses electrical power or the PCM deactivates all\015\012of the electronics. The transmission will default to max line pressure.\015\012The driver will have Reverse and 3rd, with manual 2nd available by\015\012putting the gearshift in 2 or 1 (doesn't matter). 1st, 4th, and TCC\015\012lockup will not be available. It will not hurt the car to drive it like\015\012this for short periods, but use manual 2nd to get the car moving, and\015\012be aware that the trans will generate more heat than normal while\015\012operating in limp-home mode due to increased torque converter slippage\015\012during 2nd gear starts and no lockup in 3rd gear.

Trans shifts into gear harshly, car feels sluggish off the line,\015\012No 1st, 4th or TCC lockup available, Manual 2nd, 3rd and Reverse are\015\012only available gears, CEL is on: Transmission is either in limp-home\015\012mode or has lost electrical power. If there are lots of error codes in\015\012the PCM, check the fuse that powers the transmission, and if\015\012it pops again, look for a short in that circuit like an O2 sensor\015\012harness touching exhaust. Otherwise, check PCM codes for a particular\015\012fault in the transmission causing the PCM to put it in limp-home mode.

Car goes into gear but feels very sluggish like the brakes are dragging,\015\012but it will roll easily (starts out in 4th gear), you manually shift to\015\0122 to get it moving, once it's moving you put it back in OD and the car\015\012shifts 3rd to 4th on its own and locks the converter at the appropriate\015\012time: ShiftB solenoid failed: Easy fix in the pan. Or a wiring problem\015\012from PCM to trans.. or PCM.

Start with the simple things and check the fuse and Wire leads
at the ECM's Transmission control module and the transmission it self. Look for any damages to the wire harness and leads. Try disconnecting the wire harness form the clip on the transmission and cleaning it with brake parts cleaner and reconnect and pull the Fuse to the ECM with the ignition on the on position "engine not running" for 10 sec and place back in and start the engine and go for a test drive and see if that reset the transmission. Good luck and keep me posted, we'll get your transmission running 100% again soon.

Repair Help & Product Troubleshooting for 2001 GMC Sierra 2500HD

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