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95 Buick LeSabre V6 3.8L Low Speed Shuddering

Old 06-15-2018, 03:04 PM
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New thought: The problem could be a bad vacuum modulator on the transaxle. It would be an inexpensive and easy part to replace to see if it fixes it and/or further the diagnosis and/or rule out the vacuum modulator. First thing I'd do is take the vacuum hose off of it and see if transmission fluid runs out of it. If it does then likely there'* your problem, or at least there'* something that needed fixing anyways.
In order to run the diagnostic tests from the transaxle service manual I needed to disconnect the vacuum line connecting the vacuum modulator and use a vacuum hand pump to apply 18 P.*.I. for the minimum pressure tests. For max pressure tests there should be zero vacuum on the vacuum modulator. Unfortunately the vacuum pump I had did not have P.*.I. readings and it used in. hg. instead. I determined that 18 P.*.I. is approximately equal to a little over 30 in. hg. and was only able to achieve about 25 in. hg. which is more like 12 P.*.I. With all that said, my test results in various gears matched very closely the lower ends of the numbers ranges for pressures. The only anomaly I noticed was the minimum range test for D1. It was staying at around 75 P.*.I. which is acceptable for other gears (P, R, N, D4, D3, D2) but D1 is unique in that it should be showing 158-170 P.*.I. at the minimum range (with 18 P.*.I. applied). I don't know if this anomaly is due to me only being able to apply 12 P.*.I. or an indication of an issue specific to D1. It kind of makes sense to me it might have an issue considering D1 is where I'm noticing most of the slipping issues mentioned earlier in this thread. The pressure test with the vacuum pressure supply line from the engine connected to the vacuum modulator showed an instant increase in P.*.I. pressure when opening throttle as well.

I look forward to any feedback/thoughts on this.
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Old 06-15-2018, 03:05 PM
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First thing I'd do is take the vacuum hose off of it and see if transmission fluid runs out of it
I forgot to mention that no transmission fluid ran out when I disconnected the vacuum hose.
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Old 06-17-2018, 11:37 AM
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Pressure tests all checked out. I thought minimum range pressure test for D1 was wrong but it may of been my helper selecting the wrong gear. Upon testing it a second time it checked out. Pressure is good, no fluid came out of the vacuum modulator. It would appear from my limited knowledge that the transmission has an internal issue unless someone can think of something else I should be testing before throwing in the towel.
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Old 06-19-2018, 07:03 PM
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You could try driving it with the vacuum hose temporarily disconnected from the modulator (and plugged) and see if there is still slip there. Don't do this for long though, the transaxle will think you are at full throttle all the time.
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