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Hello all, first post on the forum - long maintainer of a 2001 Malibu LS 3.1 with 192,000 miles.

This problem has been a very tricky one to diagnose. As stated, when applying anything more than a very soft foot results in the speedometer bouncing erratically and the transmission of course not knowing what speed the car is going and thus having hard time knowing when to shift. Eventually it smooths out it under light acceleration, and behaves normally. It should be noted that shifting the transmission manually via the PRNDL the trans shifts fine with no slips.

I have replaced the output Vehicle Speed Sensor (VSS) that plugs in on the passenger rear side of the transmission case, with no difference in behavior. Wiring checked fine upward even though the loom has started to crumble from age. The old VSS didn't have a ton of metal stuck to it or anything which I am assuming is a good sign.

From my GM Tech 2 I have a PCM code U1000 and a code regarding the signal from the fuel pump sending unit. The U1000 persists, but is part of a TSB for 2003-2005 Malibu's as they were built without some oil monitoring hardware (awesome hah) and the fuel pump sending unit is a known issue from a cheap aftermarket replacement as it'll always peg Full or Empty. There are no codes anywhere else ECM, BCM, CMC (Chassis), and the behavior has persisted long enough to where I would imagine it would trip a code.

I watched data from all ABS wheel speed sensors, which were all accurate and in unison. Watching speed from the transmission/PCM shows the fluctuation shown in the speedometer when it acts up, so the transmission and/or its sensing hardware definitely the source. I am aware there is an internal vehicle speed sensor, but wouldn't bad wiring or a bad sensor throw a code? Could the reluctor wheel inside of the case have gone bad? Possibly even something funky with the PCM? I am having a hard time grasping that a problem like this wouldn't throw a code even if it weren't electrical, differences in speed sensors or something should throw a flag somewhere.

Any insight would be greatly appreciated. Thank you for your time.
 

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Remove the connectors from the Body control Module and reinstall them all the way [latched]. Do this a few times with the battery disconnected. Check the wiring again and again.
Possibly related to ignition switch recall?
Interesting read here.....
 
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