440T4/4T60 Drops into Neutral at Stops

[nova72]

Hi, I have a 91 Buick Lesabre, 90,000 miles with a 440T4 (4L60) transmission. When driving from a cold start, the transmission will drop into neutral when coming to a stop. It may do this 1 or 2 times within the first 5 miles of driving. Once warmed up the transmission works fine. When it does drop into neutral, it will re-engage within 5-10 seconds. Manually shifting in the 1st will also re-engage the transmission.

I have replaced the vacuum modulator – no change. I have also removed/cleaned the pan, changed the filter and refilled to the proper fluid level – no change. The fluid is clean, not discolored and no burned odor.

Is there anything else I can do to service the transmission in the car or am I looking at a rebuild to correct the problem?

Thanks in advance!

[curtis73]

1-2 servo crack, cracked input drum, or worn clutches are my first guesses. Those are the more common 4T issues.

First thing to do is get a pressure gauge on it (or have someone do it) and see what the line pressure is.

[nova72]

I also noticed that the transmission only drops in neutral when in OverDrive. In Drive the transmission is OK.

[curtis73]

Again, I would want to see the pressures. When you say that last thing, it makes me suspect valve body. Only way to be sure is test pressures.

[nova72]

Hi, Pressure test results:

With vacuum modulator connected:
P: 70
R: 70
N: 70
D4: 65
D3: 65
D2: 65
D1:150

With vacuum modulator dis-connected:
P: 235
R: 230
N: 235
D4: 180
D3: 180
D2: 180
D1: 150

Hope this helps.

[curtis73]

That helps a lot. Did you happen to drive it with the vacuum disconnected? If so, did it solve the neutralization problem?

At any rate, the pressures look great... which means that it seems to be time for a rebuild

[nova72]

Hi, I did not drive the car with the modulator disconnected.