A diagnostic 'choose your own adventure'

April 1, 2014
  • 2003 VW Golf
  • 97,000 miles
  • 2.0L (AVH)
  • 01M (4 speed automatic)

Complaint: Won’t go into fourth gear and has a lack of power at highway speeds when in cruise control after driving for 15 to 20 minutes. The speedometer seems funny at times, too.

The used VW that was just bought and had a surge during cruise control.

As a child, I used to love to read the “Choose Your Own Adventure” books. For those unfamiliar with them, they consist of one story — typically of the sci-fi or fantasy genre — that offers multiple endings. As you read the story, it gives you the option to “…go to page 8 to stay on the path to the right.” Or “…to fire the rockets, go to page 14.” If you ended up at a bad end of the story, you could start over, or just go back to the last bad decision and try another option.

Diagnosing certain code or no code complaints doesn’t seem much different on modern cars. Whether you are a follower of flow charts or come up with your own plan of attack based on certain symptoms or data, we all are led down a path that may or may not yield the fix for a given complaint. Though we can take many paths to get to the end, in automotive repair there can only be one true ending, and that is getting the car fixed right to get it back to the customer.

Recently I had a chance at my own choose-your-own diagnostic adventure. Just like a well-read book has earmarks and signs of wear and tear, this car wore its history — with its wiring pulled out of wire looms and mismatched splice and dice wiring connectors, not to mention the list of used parts already swapped.

A Golf apart

This is a screen capture of a VW SSP mentioning G68 as being used for speedometer and cruise control function. This turned out to not be the case on this vehicle.

The vehicle in question is a 2003 VW Golf with only 97,000 miles on the odometer. The complaint is a big lack of power and unable to get fourth gear when hot. “Hot” in this case is after a 15- to 20-minute drive. The customer had just purchased this vehicle, and the used car lot he bought it from already has spent a lot of time on it, up to and including installing a used transmission. Unfortunately, the current used transmission is one with even higher mileage than the car (this is a point of contention as well with the used lot, but that is another story).

The car, however, is still doing the same thing that he originally brought it back for, still only after being driven for the 15 to 20 minutes and while in cruise control. The initial code scan showed no Diagnostic Trouble Codes (DTCs) in either the Engine Control Module (ECM) or Transmission Control Module (TCM).

After a first glance at the complaints, my mind was aflutter with “what could cause that?” type of questions. The first path I usually go down in a situation like this is probably the same one we all go down — or should — and that is the path of confirming the customer complaint.

The end of that trail yielded some interesting results. First, it changed my mind of the description of the complaint. 

The customer was right about a couple of things. You could set a stopwatch to the 20-minute mark and this thing would act up. The symptom, however, would be best described as a severe surge at cruising speed, though, and not a lack of power. The transmission would shift from 4-3-4 repetitively, with huge surges in engine power as well. This happened only while in cruise control, interestingly enough. The symptoms took me by such surprise that it wasn’t until I canceled the cruise control that I noticed the erratic speedometer. 

This screen shot is of scoping G68 while under a steady cruise at 70 mph. The speedometer was bouncing, but this signal was very even and consistent — not the source of our surge, it appears.

The speedometer continued to bounce, and varied anywhere from 5 to 15 mph at times. It was worse at highway speeds going from 70 to 60 mph, and at slower speeds like 45 mph would move only 3 to 5 mph. If you drove slower, the bounce in the speed was less noticeable, and if you engaged cruise control, the surge was also less.

Based on these facts, it was time to come up with a plan, or at least a path to go down. The scan tool confirmed the erratic vehicle speed readings, so I was thinking of heading to the transmission side of things next.

A winding road (or two)

This is the scope capture of G22 while driving at a steady 70 mph.  Note the not-very-even switching of the signals. Some are wider or shorter than the very next pulse next to it.

This 4-speed transmission known as the 01M has been around for a while. It has three speed sensors on it, and the terminology of them can be a bit confusing. The first in VW-speak is called G38, and in English called “Transmission Speed Sensor.” It also can go by a more job-appropriate description of Input Speed Sensor. I ruled out this as a possibility, because it does not have an impact on the vehicle speed reading.

The next sensor in line is G68, or the transmission vehicle speed sensor, more appropriately named the Output Speed Sensor. This is where the plot thickens — and I veer down a path that, not to spoil the story, went nowhere.

Here is the G68’s new harness connector that someone already tried. Note the wire color change.

VW service information mentions G68 as a main input for vehicle speed signal and cruise control operation. There were signs that someone else went down this road, as the wiring harness connector was spliced in with one from a different car, judging by the wire color changes that took place at the splice area.

At the time, this felt like the right path to go down. But while scoping the signal and driving the car and 20 minutes later exactly, it started to surge and act up again. The signal on G68 however was steady as could be — at least, steady enough that it couldn’t be the source of the speedometer fluctuations and hence the surge under cruise.

I was starting to doubt this path by now. Most G68 problems I’ve seen in the past throw DTCs pretty easily when there are electrical issues, and performance problems usually result in gear ratio codes. This hindsight and the current data caused me to backtrack. 

There is another speed sensor on the transmission called G22. This one, it turns out, is labeled the speedometer vehicle speed sensor. It still is called a Vehicle Speed Sensor (VSS) in the wiring diagrams just like G68, but G22 was not listed on the transmission wiring diagrams. G22 was found only in the instrument cluster wiring diagrams on ALLDATA.

Borescope of G22 reveals the split drive gear ring of the speed sensor.

Upon closer examination, there was evidence yet again that someone had been down this road. G22 had some wires pulled out of its wire loom as if someone did some testing in this area already. Testing at G22 started to feel like the right path after the next test drive. The time it took to get the complaint to act up was less now, but that most likely was due to the fact the car never cooled down completely during the rearrangement of the scope leads.

But during the test drive at the time of the surge in cruise — or out of cruise, for that matter — the signal on G22 was showing some erratic signals. I had expected a steady frequency while maintaining a steady speed, but was getting anything but that. At this point, since the transmission already was replaced, I was thinking that maybe some of the sensors were swapped from the original transmission and reused on this one. Fortunately, we had a good used G22 from various cores lying around the shop. But alas, the same problem persisted.

Closing in

This is the first of two images of the transmission after it was removed. The differential side cover was removed for reference of the speed sensor to drive gear relationship. Also, the crack can be seen with the sensor removed.

By now it felt like the whole mystery had been read cover to cover, just like it was before by someone else. The only other thing that could cause an erratic switching of the Hall Effect speed sensor G22 other than a sensor issue itself would be an issue with the drive gears of the sensor. A borescope was sent down the G22 hole into the differential section to check out the drive gear. As the tires were rotated, a big split in the G22 drive gear was discovered.

When the customer was informed that his transmission would either have to be removed to put a new drive gear on the differential or be replaced yet again, he was a bit surprised. The car lot was a bit disbelieving, too, unfortunately, which makes me wonder how they decided to swap the transmission in the first place.

In the end, the car lot wouldn’t or couldn’t help its customer out further, and the customer opted for yet another used transmission. This one had lower mileage, at least. Fortunately, he still had his original transmission that started this whole ordeal and was kind enough to drop it off so we could take the differential cover off of that one, as well as his used one that currently was in the car when it was removed for the “here-we-go-again” swap.

This is the second of two images of the transmission after it was removed. The differential side cover was removed for reference of the speed sensor to drive gear relationship. Also, the crack can be seen with the sensor removed.

The results weren’t surprising; they both had split speedometer drive gears on the differential that would even spin when putting a bit of pressure on them by hand. When the “new used” transmission showed up, the cover was pulled off that to inspect and make sure a three-peat of the story didn’t happen — and that was revealed was a nice tight, split-free drive gear.

Once the next transmission was installed, the problem was gone. The speedometer was nice and steady at all speeds, and cruise control worked flawlessly as well, because the PCM no longer was chasing an erratic speed reading. In the end, the car was fixed and a satisfactory conclusion to this story was reached. 

Of course, with hindsight being 20/20, it was obvious what could have been done to reach the same results sooner, but when faced with certain paths in the heat of the moment, sometimes we head down the wrong path(s). Sometimes when there is evidence someone else has walked the same trail, it’s easy to convince ourselves to do the same thing as well.

In the end, though, if the answer or the path you’re on isn’t right, go back and start again on a new path until the right answer is found. And even if the path you’re on was well traveled, you still just might reach a different conclusion then others who went the same way.

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