Shifting problems

BUTCH STREY

MEMBER
1994 GMC SUBURBAN
5.7L
V8
4WD
AUTOMATIC
277,000 MILES

My transmission does not seem to start out in low gear. It feels like it is stuck in third gear. Pulling away from a complete stop feels like trying to drive with the emergency brake on, or trying to start out in third gear with a manual transmission. Then, when you get up to highway speed it does not shift into overdrive. At sixty mph the engine is turning about 2500 rpm, or about what it used to turn at eighty mph. I know these transmissions do not have a vacuum modulator (I am old school, I understand that stuff), but I am lost here.

4 Replies

BUTCH STREY

MEMBER
3 POSTS

Also, it only does it after it has warmed up. Cold, it shifts normally. It does not slip and is full of fluid. I can manually shift into first gear and then to 2be and third but not into overdrive.

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Friday, October 27th, 2017 AT 7:02 PM

JACOBANDNICKOLAS

MECHANIC
109,913 POSTS

I read what you described and to be honest, it sounds like a pressure issue. I believe these vehicles do have a pressure solenoid that can cause this issue when they warm up. Basically, they will shift normally when cold but slip, not work, or go into what is called limp mode. Limp mode is basically designed to get you to a safe location when the problem happens. What I suggest is to scan the computer to determine if there are any codes stored that can help. Since your vehicle has OBD 1 diagnostics, you will need to locate the port under the dash, use a jumper wire or paper clip to jump port A and B. After that, turn the key to the run position (engine off) and the light should flash. Count the number of flashes there are to determine the code. For example, two flashes a short pause and then three flashes indicate a code 23.

Once the code or codes are determined, you can see where the problem is coming from.