Been having this unique problem with my 2002 B4C lately. During mild accelerations, the engine/tranny seems to 'surge' some and the speedo swings plus/minus 5-10 mph (not tach, just speedo). At high speed cruise (80+ on interstate), I also notice a slight speedo swing plus/minus @ 3 mph. Its kind of annoying! No codes are thrown, looked for and did not find any vacuum leaks (used carb cleaner after all warmed up, no bumps in rpm at all). Fluids are all right-on, fuel clean (premium 93 octane) and stock tune (had a hypertech tune but reverted back to see if that was causing the problem). Fuel filter was changed last year only @ 5K miles on this fuel filter (would fuel filter even cause that speedo to swing like that?). Plugs and wires were replaced a few months back as I had thought that was causing this weird behavior, and they were due anyway. I have looked over the posts here and closest thing I could find is maybe a ground or some connection problems? I wanted to remove the two harnesses at the PCM, but not sure how to safely do this. Do I remove battery neg first, will I have to reset anything after removal/inspection of those two harnesses, just want to track this down because its getting worse over the months to the point where its time to ask this vast knowledge base here in LS1 tech.

Would appreciate any hints, on what I have done, what I can do, and if this is a common problem for these cars (have 97+ K on the car now).

 

Check for codes even if the SES isn't on, some trans codes (if yours is an A4) might not illuminate the MIL every cycle it sees a fault. If you want to check the PCM pins make sure the ignition is off, remove neg first then pos. I'd check for codes first because disconnecting the battery will clear any stored...and the readiness tests will reset and remember idle will be grumpy until the PCM relarns it. IIRC disconnecting the PCM with the ignition on might require a crank relearn procedure.

 

2001NBM, Everytime I check this after a ride, I check for codes. I have an innova 3110, with no DTC's present (0/0 codes, and green) and all drive cycles have run (no blinking icons on screen, that indicated all drive cycles ran from last time I disconnected the battery). I'll go ahead and disconnect the battery, and remove the PCM harnesses (two) and check all those pins. If anyone is in the Sumter SC area that has a more in-depth OBDII or GM tech tool that may read hidden DTC's, I would love a contact with you! I hope its a simple corroded terminal(s) at those two PCM harnesses, its very annoying and frustrating. Yes, it is an A4, sure is screwy!

 

Well, I disconnected the battery and removed the PCM (no corroded terminals at all very clean). Reinstalled PCM and surprise, it did not lose settings! Before I removed the connectors, I turned on the headlights for @ 10 minutes (drain any electricity) so there could not have been any energy in the caps on the PCM. When I hooked it back up, no relearning, nor were there any drive cycles needed (all green on the innova 3110 with no flashing sensor groups needing to run). Only thing I had to do was unlock the radio theftlock! That was surprising, that it didn't reset the internal memory. Anyway, while I was there under the hood, I cleaned/checked the 4 grounds (one each fender well top, and one each either side of radiator). Nothing out of the ordinary! I will be driving the car today, if I do not have to take the captured wild cat (that had been pawprinting my black beauty and my red P/U truck) to the ASPCA for sleepyby. Any other grounds that might be causing those speedo fluctuations?

 

OK, after doing a little more research, I keyed onto the VSS at the rear of the tranny. The tranny is a special build heavy duty upgraded 750HP one that I had a local speed shop install in Nov 07 because the one (in my old cop car) was acting up a little. Anyway, after searching the forums about that VSS, and seeing its intergral to a few other systems (ABS, TPS, and such) I decided to check that sensor out. I drove the right side of the car up on home made ramp to tilt the fluid away from the sensor location (and had ready a catch pan for anything possibly coming out) and preceeded on jacking the car up at the rear passenger side to access the VSS sensor (REMEMBER TO BLOCK THE DRIVERS SIDE FRONT/REAR SO IT DOESN'T ROLL OFF THE JACK/RAMP). Grabbing my small set of metric sockets/driver, I slid under the passenger seat area. I eyeballed the socket I needed (uh-huh, first one I grabbed worked), and set out to removing the sensor to see what (if anything) I could see wrong with it. Well, I immediately found a problem! The bolt was so loose the sensor wiggles freely. I used my frail old fingers, and that bolt came right off no resistance! I wiggled the sensor out of its hole and steadied myself for fluid leaking, but non appeared. (I hate making a mess on my concrete!). OK, sensor in hand, I cleaned it off with a clean rag and found out its magnetic. Found that out when I was relubing the "O" ring with tranny dipstick fluid (it kept snapping to the magnet inside the sensor). OK, all cleaned up, lubed, and back to putting it back in. Slid that sensor into the housing, screwed the bolt in with my fingers, and used the socket to tighten it up. Sensor doesn't move any at all now (no wiggle or anything). Cleaned tools, lowered it off jack, took wheel blocks off drivers side front and rear wheels, and backed off the one ramp. Assembled the right rear (put jack away) and took it for a test drive. Not sure, it was hot but not driven more than 5 miles, but didn't see the swinging/surging speedo. Tomorrow, will do a much longer trip and will report my findings.

I am cautiously optomistic that VSS was wiggling and causing the speedo to jump. If that loose sensor was the cause, it would explain the erratic speedo, shifting, and throttle hiccups I have been having for quite a while. That speed shop may not have mounted that sensor because it was in the tail that he transferred from my old tranny. It may have been loose on the old tranny as well. All he would have done was taken the tail off my old one, put it on the new before installing it, and plug the wire in (you had to be looking for that wiggle, like I was). Right now, that sensor is rock solid firm and not moving any.

OK, has anyone got an idea what kind of problems that could pop up if the VSS is loose in the tail of the transmission (automatics)? Please pop in and lets banter this about some. I am asking the experts to chime in, if the problem goes away over the next few days of driving the car, then its great I found that sensor before purchasing a whole slew of things. Remember, there were no DTC's, no warning lights (except for a quirky ABS light a few weeks ago) and it was very annoying and problematic! So, whats the result of a loose VSS sensor? Any system/symptom that this could affect? Thanks guys (and gals)!

 

Went on a 30 mile trek, lots of slowing to a stop, various shifting, all the way to 65mph. No more surging/shifting/speedo swings anymore. Who would have thought that one sensor being loose would cause the speedo to swing up and down, and especially cause the transmission to shift up/down ad-nausium. I'm more than cautiousely optimistic this problem is solved!

 

Got that buggy problem solved, reinstalled my hypertech tune and its good-to-go again. Kind of surprised no tech's chimed in, would like to have known the technical side of this and how these systems intertwined/worked together.