Speedometer, Transmission, and Cruise control problems

[ryno]

So I think these three problems are interrelated. I've read some posts about similar problems, but I haven't seen a clear solution yet so here's my problem.

The speedometer needle on my car bounces at around 45 mph. It seems to work fine up until that speed, but after that it just bounces in a 5-10 mph range and doesn't read correctly. (The tachometer, fuel gauge, and odometer work correctly).

Also my transmission occasionally doesn't shift into overdrive. The only way I can fix this is to shut the car off and start it back up. That usually makes the problem go away, but it obviously doesn't fix the problem.

My cruise control is the other glaring problem. The cruise control struggles to maintain a constant speed. Even on flat ground it constantly speeds up and slows down maintain the speed.

Any ideas what could be causing these problems? I have done a lot of research, but I still haven't been able to pinpoint the problem.

[D2Beetle]

I'm not familiar with the automatics, but I believe that in the past I have read something about a transmission control module.

[MichelleRoze]

I will agree with D2Beetle. I had an automatic, even though I never had that problem. Something is up with the tranny obviously. Cause that controls all three of your issues. I would check the control module. Maybe a sensor or wire is off? There is a bunch of stuff that the auto is connected to and it can cause a world of issues. Good luck. I hope you figure out what it is....

[ryno]

So I had the codes read at a local auto parts store yesterday and this is what it had:

P0722, P0715, and P2181.

I think the P2181 code is due to a recent problem with the fuse block on top of the battery melting, including the fan fuse. (waiting on the new part to ship)

However, wouldn't the other two codes lead you to believe a vehicle speed sensor is bad?

[gamefoo21]

Sadly, I'm at the point of having to test the TCM aka the Transmission Control Module aka J217.

I've replaced the VSS aka G68, I've tested G38 it reads the speed fine, but the Speedo still bounces, and as time goes on it's getting worse.

There are unfortunately only two tests available for the TCM a ground and a voltage test, if these fail you can fix the wire but if they pass those tests and the issue continues, th only option is the replacement of the TCM.

A side effect of the TCM packing it in for me, may also be the brutally long time it takes for VCDS to connect, and the random communication errors... "This module is using an unknown format for communication." error message. The connection time is around a minute where as all other modules take 10 to 15 seconds at the most.

Atleast the TCM isn't had to replace, it's just an expensive part.

[ryno]

Gamefoo21, you're having the exact same problems? I have yet to test the sensors to see if I can find anything, but it seems like the problem is extremely hard to diagnose to say the least. Sorry to hear you have to replace the TCM. If you do let me know how that goes.

[MichelleRoze]

Let me get my bentley and I'll see what those codes mean.....brb

P0722-Output Speed Sensor Circ No Signal

P0715-Input Turbine/Speed Sensor Circ. Malfunction

P2181- can't find that code. Sure it was 2181?

Nonetheless, looks like you might have a speed sensor issue.

[Smileybug]

I didn't see the first 2 codes in the Ross-Tech wiki at all. I think the Bentley shares the codes. Generic readers often will give the wrong codes on VWs. This reminds me...I need to get my Bentley back from Chris.

[ryno]

The P2181 code is for a cooling system malfunction I think. I'm sure it registered that code when the alternator wire to the fuse block on the battery heated up and melted everything. Since the fuse to the fan was also melted it took out the fan and the ac.

I am thinking that it's the G68 speed sensor, but I can't be sure. Anyone know how to test that sensor? It'd be nice to be confident that that's the problem before I buy it.

[gamefoo21]

Originally Posted by ryno

Gamefoo21, you're having the exact same problems? I have yet to test the sensors to see if I can find anything, but it seems like the problem is extremely hard to diagnose to say the least. Sorry to hear you have to replace the TCM. If you do let me know how that goes.

Well, the Bentley DVD shows the TCM mounted backwards, and it's a total PITA to get out of the car, the book makes it look easy, it's under the dash by the interior air filter.

I pulled the TCM out today, cleaned out it connector, sprayed some ignition wire dry/lube on the connector and clicked it back in, then I went to the otherside where the ECM is located, lots of leaves and dirt and general nasty dirtyness. I found the primary ground point for the ECM, right by it, it grounds in the dash, pulled the nut. Very corroded the ground wire connectors were, so I cleaned em up, put some copper anti-sieze on them, and put em back into use.

Sadly, it was a whole pile of work, took me about 4 hours to do, and it did piss all to sort the issue.

G68 aka the VSS does go bad on these cars, they are cheap and easy enough to replace(or so I say after having pulled mine out a dozen times), just make sure you pull the air box and main breather tube out of the engine bay, it'll make it much easier to reach down there.

[ryno]

I replaced the G68 speed sensor today. The wire was cut and the plug was slightly melted (I'm assuming it grounded out to the transmission at some point). I cleared the codes after I got it all back together, and it seems to have worked.

For the first 15 miles at least the transmission has shifted perfectly. The P0722 and P0715 codes are now gone.

Unfortunately the speedometer wasn't fixed. The needle still floats although now it's a 70 mph