RPM surging between 40 and 55 MPH
 

My Blazer: 177k miles, 4WD, 2D
I'm having an issue with RPM where I'm driving along and at the point where my Blazer wants to shift into 4th it does, but if I ease off the throttle a bit trying to maintain the current speed the tach will start jumping up and down. I'm quite sensitive to the motions and vibrations my Blazer makes, and I can tell I feel a surge of power in the drive train whenever it does this.

I found this video of a guy driving his Blazer where mine is doing the exact same thing.

Is this indication of the transmission going out?
I notice it won't do this if I stay in 3rd gear or if I accelerate quickly to 65 and cruise from there.
It's very noticable in D driving through city traffic.

This started happened after I had replaced an axle seal and I needed to remove the center pin and axle retaining clip from the rear diff.
I'm faily confident I put it all back correctly, but is it possible I misaligned something?
Other work I've done that may be related is changing out Diff and Transfer Case fluid.

Some people say it's the TCC solenoid, speed sensor, or MAF sensor.
Others in the comments on that video are claiming it could be a bunch of things.

Following... mine moves +-100rpm while coasting at any speed. And I get a tick tick tick while reversing lol...

Does your surge like the video? Also have you done any maintenance on it since you got it? I did a transmission service around 250k km, its got 345k km on it now...

Probably the TCC working exactly as designed. Between 45 and 55 in 4th (especially 45-50) it doesn't take much acceleration to disengage it. Does tach go up the same amount when you tap the brake pedal when going steady 50?

Still diagnosing the issue, so I'm revisiting this thread I didn't mean to abandon.
It has felt like the surging is slowly getting worse, so I'm trying to set some time aside to work it out when I get a second daily driver.

I have not, but it is way over dur for service. It needs a fluid change. I have a new filter and gasket for the oil pan sitting in my garage. I need to set time aside to do it when I learn more about replacing the filter.

Is the whole vehice suppose to feel like it's lurching forward / braking when the tach does surge? That's what mine is doing, and it's actually more pronouced than what the video.
If I'm maintaining speed or accelerating slowly, it will surge between 40 and 55 MPH. If I accelerate fast to 65, then maintain speed, it doesn't surge.

This surging started happening around the same time I started hearing a grinding noise in the drive train somwhere.
https://blazerforum.com/forum/2nd-ge...-right-102683/

I haven't yet isolated the grinding noise to the diferentials, transmission, transfer case, or the CV axels. All of these things are suspect.

• I removed a rear axel to replace a seal. Maybe I didn't put it back right.
• The transmission has been downshifted from 4 to 3 at around 45 MPH once before (totally by accident).
• The front CV axels leaked grease before. I repacked them, the leaked all the new grease again. So they might be worn out.
• I accedentally changed the wrong fluid with the transfer. Ran it for about 15k miles on AutoTrak II when it should have been Dexron III ATF.

So when the engine surges, the tach reading goes up, you hear an increase in engine rpm and the truck accelerates? IOW, the truck acts exactly the same as if you had depressed the accelerator pedal a small amount as apposed to say the transmission is slipping or downshifting with no additional power or speed?


George

TCC is cycling in and out. Are brakes dragging? Something else robbing power from moving the vehicle forward (ie grinding, etc)?

Yes. It feels like something is dragging like a brake when it does this.

Brakes are not dragging. There is a grinding noise I've been trying to figure out.
https://blazerforum.com/forum/2nd-ge...-right-102683/

I'm having a hard time getting my head wrapped around what is happening here., So the truck surges on its own when you are not asking for a sudden increase in power from the accelerator pedal. The rpm gauge increases, the truck audibly goes up in rpm, the truck accelerates and the speed increases and then it suddenly feels like its braking, making an equally strong reduction in power and forward speed as if you had applied the brakes? What is the change in speed during all of this?


George

Oh no, it's not accelerating when the RPM surges. Actually I take back that it sounds like it's reving when the RPM spikes. It just feels more like it's going in and out of 4th/overdrive constantly and I feel it dragging the truck down when it does.

There is no change in speed.

So its downshifting and engine braking?

George