It's kind of a long story but I'll do my best to keep it brief. My friend and I purchased a 98 z28 with a blown transmission super cheap. I had a very low mileage rebuilt 4l60e and 3000 midwest stall from a 94 Z28 I used to own. In my old car, the transmission shifted nice and crisp with the transgo HD2 shift kit, and the stall flashed quickly and gripped right in (it was the first automatic car I had ever owned, but everything seemed to work great to me).

We swapped the transmission into the LS car using the proper crank spacer and 5.3 flexplate. We checked to make sure the TC was properly seated to the input splines and pump. Clearance was less than 3/16" to the flexplate which was perfect. The transmission bolted to the engine without any issues and wasn't forced on. We then swapped the PWM TCC solenoid to enable TC lockup. Swapped the transmission mount and VSS connector, filled with fluid and fired it up! The first test drive seemed to go well. No codes at all. The shifts were WAY more soft than they used to be, almost non-existent, and the stall would flash to 3000 before the car would literally go anywhere. It was almost like an on/off switch at the stall speed where the old car would still pull a little before the stall speed. So part throttle driving was at 3000 rpm, and the shifts were so soft you couldn't feel them, almost like driving a CVT! WOT pulled very hard, but again you could just BARELY feel the shifts and would often hit the rev limiter. My friend tuned the shifts a little earlier to fix that problem, but said the line pressure was already set to maximum.

So what else do you do with a $1500 bolt on/tune z28? Throw on drag radials and hit the track! The car was stupid fast for the mods running 8.0 in the 1/8th with 1.7 60ft, but again was shifting so soft I bet there's another 10th sec. On my last run trying for 7.9, the ten bolt went KABOOM at the launch, so we towed it home. So I threw in a stock posi I had laying around and went for another test drive.

Well I made it to the end of the driveway with the transmission working perfectly normal. I went to pull out onto the street and nothing. The engine light came on, and the car wouldn't move in any gear. I can't remember the code, but it was common for "Limp Mode". Found out the 3-2 downshift solenoid went bad so I replaced it with a new one. Also, the accumulator was stuck in it's bore for some reason, so I swapped one out from the original tranny (also HD2). Everything buttoned up with a fresh filter and filled with fluid. The car would barely pull itself out of the shop, so no change whatsoever, yet didn't throw any codes this time. Pushed it back into the shop. Pulled the pan again and swapped the rest of the solenoids including the pressure control solenoid and boost valve. This still didn't fix the problem.

Now you have to nearly redline the car to get it to move at all, but it definitely goes into gear. We installed a pressure gauge into the test port to get a better idea of what's going on. We have about 55 psi in park and about 75-90 psi in gear at about 2500rpm. It will go as high as 110psi when revved in gear. Pressure does seem to fluctuate smoothly with rpm, but the numbers seem a little low. I figured a bad pump would cause no pressure, but the fluid still drops on the dipstick when the engine is running vs off. When I pulled the transmission cooler line to pump out fluid, it didn't come out smoothly. It kind of spit and sputtered into the pan. As I don't have much experience with autos, I'm hoping you guys and help me narrow the search for the problem. Thanks!!

 

Here's a video of the car running with the pressure gauge hooked up. I started the car, let it idle in P for a bit and revved, then went through each of the gears and gave a blip of the throttle in each to show that the pressure increases with RPM. The car will move ever so slightly if the car rev's up to around 4K RPMs.

The odd thing is that when I started the car a few months ago it seemed to have acceptable line pressure (it was before I'd purchased the mechanical gauge) only it displayed symptoms of a trans in limp mode without a CEL or throwing a code. It went in to reverse as expected with normal acceleration. OD and D put it in 3rd and when I pulled it down in to 2 and 1 (swapped a T/A center console in to it with 1st gear selection), it was going in to 2nd with expected acceleration for a 2nd gear start. It would turn the cold MT DR's over easily with a light blip of the throttle. I expected it to still just be a matter of diagnosing the issue causing it to be in limp mode. However, when Josh started it up a month or so after that it presented the exact same issue as before and was present earlier this week when I shot this video of the PSI readings.

This is a fun, cheap project car that we picked up from a friend of mine and am hoping to find that it's something silly that we're overlooking with this setup. We don't have a problem with swapping the trans and/or stall if it's determined to be pump failure but would like to mess around and make sure it isn't something that can be easily repaired while the pan is off.

I think we're going to try to tinker around on it this weekend and double check things like the EPC and pressure regulator spring to make sure nothing unusual is going on in those areas. One good thing that's came from all of this is that I've learned a fair amount about the 4L60E thanks to Google and a lot of you guys responses here on the forum!


I'm also attaching a pic of the time slips from the track day in the car. My best time on the left and Josh's on the right. I have about 170 lbs on him and just knew he'd be able to squeeze a 7.9 out of it! I'm still pretty happy with what we were able to get out of a basic bolt-on and stalled '98 'vert Z with 275 MT DR's and chrome ZR1s!

 

We made a little progress today! We pulled the pan with the intention of removing the valve body to inspect it for irregularities. But Rich had heard that the boost valve actually comes in different sizes and thought it would be best to double check that it was installed correctly with the proper springs and is the correct part for our transmission.

Low and behold, we pulled the boost valve and it is the wrong valve! This is the HD2 upgrade we pulled from our '98 donor transmission. We're guessing that when it was rebuilt they used the 2005+ style pump and boost valve. Considering the only difference is that the new valve is about 0.1" shorter than the early valve, we didn't catch it during the install.

After doing some research, that SLIGHT difference may be enough to reduce our total pressure output a massive amount. So our new Sonnax upgraded boost valve for our year is on the way. Hopefully it fixes our problem! We'll let you guys know.

 

Yes, that small boost valve length will make a huge difference.
The HD2 boost valve includes a spacer to make the valve effectively longer which MUST be installed in '94 thru '05 (maybe some '06) transmissions. I have heard that without the spacer the pressure will be so low that the trans will immediately fail due to slipping frictions.
The Sonnax 4L60E-LB1 is the longer boost valve, the 4L60E-LB2 is the shorter boost valve for the '05/06+ pump with ISS.

 

Yep, that's exactly right. And I forgot to mention that the newer HD2 boost valve we swapped in was also missing the aluminum spacer! Good thing we only drove it a few feet like this, so hopefully nothing was damaged. We didn't notice anything abnormal in the trans fluid or any "burnt" odor.

I'm sure the correct boost valve will get us back on the road, but I'm still curious about the super soft shifts we were experiencing before I broke the rear axle. The new part comes in Saturday, so we'll see!

 

FedEx finally dropped the Sonnax stuff off. I did quite a bit of reading to get ideas for what to look for when we pulled the pan again. It seemed ironic that when we compared the two sleeves that the one swapped in to the trans was the shorter one. I had put the 4L60E-LB1 kit in my cart on Amazon the day before in anticipation of having to order it. From what I'd read, there are a limited number of late production early-style pumps made that accepted the later-style pump boost sleeves. Seems like it was just our luck that the trans that was in the '98 had a re-built transmission in it where it appears the builder used one of those bastardized pumps. I never thought twice to check for a .10" difference before swapping.

I know that between us two we have learned a lot about the workings of an auto trans. There's still plenty left to learn though! I'll update the post after I get a chance to put the new boost valve in today.

 

Zipped the trans back up with the new booster and still having a problem with max line pressure. At first I thought the issue was fixed as cold start PSI was reading 75psi where as before (the video above shows) it was around 55psi. The trans sees about 100psi when rev'd to 3K rpm's. It appears that swapping in the correct booster components raised the psi by roughly 15psi across the board.

I guess the next step is to swap out the EPC solenoid since it's easy/cheap to do. Symptoms could point toward a bad EPC but not getting our hopes up too much. Maybe it is the pump after all, not quite gone but on its way out, causing the low line and max pressure?

We'll update the post again once we have time to mess with it again just in case anyone in the future comes across the same issue.

 

did this fix the issue