97 Suburban Trans Question

natep1 · August 31, 2009

Hi everyone

Obviously I'm new (my first post) and I hate it being a question but I'm stumped.

97 Suburban 2500 4wd 5.7/4l80 112k,

Sometimes the transmission will start to search gears or shift for no reason or load on it but, it will still drive and shift normal other times. Weather doesn't affect it one way or the other. (fluid is fine and so are the elec. connectors I can find)

When the trans is acting up, the gauge cluster gets flaky too. I can be cruising at 50, the trans will act stupid and the speedometer with drop 10mph (no loss in actual speed) and if I let off the gas completely the speedometer with correct itself. Some of the other gauges act up too, I can go in more detail if needed.

My questions are

#1 Does the ECM control the trans or is there a separate control unit?

#2 If they are same unit would an ecm issue cause this?

#3 If it's not the ecm where should I start looking?

Any help is much appreciated, this is an annoying issue from all aspects and it screams electrical.

TIA

Nathan

irengineer · September 1, 2009

Hi everyone
Obviously I'm new (my first post) and I hate it being a question but I'm stumped.

97 Suburban 2500 4wd 5.7/4l80 112k,

Sometimes the transmission will start to search gears or shift for no reason or load on it but, it will still drive and shift normal other times. Weather doesn't affect it one way or the other. (fluid is fine and so are the elec. connectors I can find)

When the trans is acting up, the gauge cluster gets flaky too. I can be cruising at 50, the trans will act stupid and the speedometer with drop 10mph (no loss in actual speed) and if I let off the gas completely the speedometer with correct itself. Some of the other gauges act up too, I can go in more detail if needed.

My questions are

#1 Does the ECM control the trans or is there a separate control unit?

#2 If they are same unit would an ecm issue cause this?

#3 If it's not the ecm where should I start looking?

Any help is much appreciated, this is an annoying issue from all aspects and it screams electrical.

TIA

Nathan

If you've got gages on the IP that are acting up at the same time the trans shifts as you've described, there's a good chance you could have a power or ground problem. When the trans loses power the shift solenoids that control which gear you're in, go to an off-off state and that gives you the default gear, which in a 4 Spd trans is typically 3rd gear. You've likely got a 4L80E, I think default gear is 3rd gear but can't remember for sure. If you had an interuption in power while in 4th gear you would likely shift to 3rd gear.