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Hard 1-2 shift in Chevy Blazer


I have a 145K on my 2000 Blazer. I had the transmission fluid changed/flushed about 6-7K miles ago.

I know the old adage about about not messing with an older automatic transmission that hasn’t been serviced, but the former owner seemed to be on top of things, and I had seen the ATF temps hit 230* during a long slow crawl in summer traffic, and decided that the fluid may have been compromised.

Anyway, here I am, water under the bridge.

My problem is that on two occasions the transmission has developed a very hard 1-2 shift. The first occasion was after a ~50 mile drive, after stopping at the end of the highway ramp to merge into local traffic the 1-2 shift was terrible and continued to be terrible for the few miles to my destination. My OBD-II device registered ATF temp ~180*, so not overheated. I stopped the vehicle and checked the ATF, level was fine. After a few hours, I got back in the Blazer and it was fine.

In fact, it was fine for nearly 4K miles, including some very long drives.

This past weekend I was headed home from a trip up north, I drove about 150 miles to a nearby small city, drove around in the city a bit, no problems, then back on the highway for another 40~50 miles to get home, similar problem–merging into local traffic from the highway off ramp I get a terrible 1-2 shift, continues to be terrible for the next several miles home. My OBD-II device registered 206*, higher then normal, but not crazy. It dropped back to under 200 during the few bad miles, but continued to be a very bad shift. ATF level is fine.

Today, drove fine on the way into work.

I have no idea what might be wrong, but I’m going drop the pan to change the fluid/filter this weekend.

I figure my worst case is a transmission replacement. All-Data DIY says price for a 4L60E is about $1800 and labor for replacement is about 9 hours. So I’m trying to ballpark my worst case (trans replacement) cost. Is $2500 about right?

I think the Blazer is probably worth it, I love it and it’s got nothing else much wrong with it at this point.

Is it worth it to bring it to a shop at this point where I don’t have a reproducible problem?

Thats classic for this transmission. Did you pull any codes from the computer? P-1870 for example?? More than likely what you have going on, and its very common in this transmission is a worn TCC regulator valve in the valve body. This is a pressure regulator valve which commonly wears out its bore in the valve body which results in ATF leaking by the worn valve and reducing hydraulic pressure in the converter clutch. The converter clutch starts to slip, sets the P-1870 and maxes out line pressure in the transmission. Thats why the harsh 1-2. What I do with these is get a new valve and sleeve kit from a company called Sonnax. I remove both the TCC regulator and isolator valve trains, ream the bores and install the new sleeves and valves. If the converter is still good you’re set. With the number of miles you have, its probably worth it to rebuild it and do the valve job on the trans at the same time.


Thanks for the reply. I should have mentioned a few things–

  1. No codes stored in the computer (at least, none readable by my OBD-II scanner)

  2. I noticed a few months back a droning noise which I’ll explain more about below:

The droning noise only occurs on the highway, 55+MPH, when under slight load–i.e. going up a slight incline or accelerating gently.

When I first noticed it I did a few tests–

  • Don’t adjust throttle, but shift from “D” to “3”, keeping speed same… noise goes away (so it’s not related to speed of tires, differential, drive shaft or engine)

  • Don’t adjust throttle, but slightly push brake pedal so as to cause the torque converter to unlock (I know it’s unlocked when I see a ~150RPM jump, but again keeping same speed… noise goes away (so it’s not specifically related to 4th gear, but more likely the locked-torque converter?)

I don’t know if this fits in the TCC regulator theory you have?

Is this something that I or a mechanic could test for to see if it’s border-line?

Also, is the condition damaging to the transmission? What I mean is, when it’s operating normally (in between incidents) is the transmission being damaged? And when the incidents occur is that damaging the transmission? i.e. can I wait for this to get worse (or at least more frequent) before getting it addressed without running the risk of ruining the transmission?

Thats exactly what I’m talking about. CLASSIC TCC regulator valve problem. Driving it in this condition will just continue to damage the converter clutch and accumulate trash in the fluid, cooler, valve body… I would let a transmission shop check it. They can scan it with a high end scanner and monitor TCC operation (Slip values, etc). All its going to do is just get worse over time so I wouldnt wait TOO long before addressing it. Since it seems to be in its early stages you might have some luck using an additive called Lube-Gard. STAY AWAY from other additives like that gooey LUCAS junk. That stuff will mess it up even worse.


Mind if I ask how you would recommend that I approach a shop? Should I just explain the symptoms as I did here and let them arrive at their conclusions based on working on the car, or should I do all that plus ask them specifically about checking the TCC regulator?

Also, any recommendations regarding what type of a shop to go to? I’d imagine a place that specializes in transmissions is the best choice, but I’m always wary of them since they often don’t end up with repeat business I feel like they’re more likely to go for the gold (in my pocket) with their initial diagnosis.

I don’t have a regular mechanic, I’ve done all the typical maint. myself, used a regional tire chain shop for shocks and the 100K mile service. There is a local guy I brought it to for a wheel stud replacement, but he seems more like the Gomer Pyle type, honest and cheap, but I doubt he’s got the tools or expertise to tackle this properly.

A quick google of transmission repair shops near my home and near my work turn up the usual–no one has more then 5 reviews and they all have a fairly even number of “5-stars they’re wonderful!” and “1-star stay away!” type reviews…

I’m guessing that national chain places like Aamco are still considered hit or miss at best?

Might as well get out the phonebook and darts :-/