Hard bang in rear diff on 1-2 shift

[sailun]

Hi folks, hope someone can help me out here... Everyone has in the past....

Most of the time when the tranny shifts from first to second, it seems to grab too quickly, and really bangs the rear diff. It really makes you cringe, knowing bad things are hapening. I did some searches through the forums today, and I read lots of similar hard shifts, but haven't read of many solutions. I did pull and clean the speed sensors, as one post suggested. They were spotless, and as expected, the problem didn't magically go away. I'll look at trailer wiring grounds next, I've fixed a few problems on this truck with chassis, engine, and ECM grounds.

I drive pretty gently, the 'burb has 188k on it now. Light throttle or heavy throttle doesn't seem to matter too much. I do get a gentle shift once in a while. Problem is only on the 1-2 shift.

Oh yeah, changed to Mobil 1 syn ATF a year ago. Truck received a new rear u-joint about 6 months ago. When truck is up on the lift, and rotate the drive shaft, I can feel the slop/bang in the rear diff.

Solenoid ? Combination of problems ? No SES light showing.

[GMC Hauler]

Transmission temperature sensor possibly...

Wiring.....


When the trans thinks it is hot, it goes into a limp mode for the trans and raises line pressure to make shifts real hard.

Do you have access to a scanner? you could read what the sensor says to determine if it is that.

[chevy1988]

Hi folks, hope someone can help me out here... Everyone has in the past....

Most of the time when the tranny shifts from first to second, it seems to grab too quickly, and really bangs the rear diff. It really makes you cringe, knowing bad things are hapening. I did some searches through the forums today, and I read lots of similar hard shifts, but haven't read of many solutions. I did pull and clean the speed sensors, as one post suggested. They were spotless, and as expected, the problem didn't magically go away. I'll look at trailer wiring grounds next, I've fixed a few problems on this truck with chassis, engine, and ECM grounds.

I drive pretty gently, the 'burb has 188k on it now. Light throttle or heavy throttle doesn't seem to matter too much. I do get a gentle shift once in a while. Problem is only on the 1-2 shift.

Oh yeah, changed to Mobil 1 syn ATF a year ago. Truck received a new rear u-joint about 6 months ago. When truck is up on the lift, and rotate the drive shaft, I can feel the slop/bang in the rear diff.

Solenoid ? Combination of problems ? No SES light showing.

What model gearbox do you have? 700-model seems to be quite sensitive on filters? Even if you changed it when going to another brand of ATF it might be a good idéa to change it. New oil might resolve old dirt in the box and klog up the filter... So I would have a look at that first.

Also I've come over some posts that suggest electrical issues as "GMC Hauler" mentioned and even low voltage can cause gearshift problems so cleaning terminal and battery connections might be a well worth effort.

Good luck

[1629dill]

I just started have the same problem on Sunday. Come off the highway and when I started to move a way from the stop sigh it shifted really hard. Today after sitting overnight it was fine till I got off the highway and it started shifting really hard from first to second. I will be dropping it off at the shop tonight and see what they have to say about it.

[1629dill]

I now know what is wrong with my suburban.. It was the speed sensor units in the transmission. I am so happy that I do not need a new transmission that the $600 that is costing to fix feels cheep.

[1629dill]

After getting the suburban back and driving to work it started shifting hard between 1st and 2nd. I was thinking now what. It was the speed sensor again. This time they found that the wire harness leading to the sensor had been damage where it went into the body and it has shorting out..

[Turbo Al]

Glad you got it fixed. The "SLOP" in the driveshaft at the rearend is probably the limited slip plates/clutches. I had soo much I rebuilt mine right at 170,000 miles. Not a really big job but a little bit tricky. I just put in new clutches and used the old plates.
Al