An accidental solution: When an issue resolves itself, but you don't know why - Transmission Digest

An accidental solution: When an issue resolves itself, but you don’t know why

Today I found myself working on a 2008 Chevy Silverado with a 4L60E transmission that wasn’t shifting correctly and whose speedometer was jumping around. After a short drive I was able to confirm the customer’s concern that the transmission had an erratic shift. I have seen faulty alternators that cause erratic speedometers, so I did a charging system analysis. The alternator was charging properly, and the AC ripple was normal.

As I drove the vehicle, I could see that the speedometer and the data from the scan tool did in fact match. I also noted that they both seemed to drop in and out at the same time, so my first thought was to look for a bad wire or sensor issue. (See Figure 1).

RR-Tech-September-FIG-1-600
Figure 1.

I was hoping to visually find a wire chaffed or touching a ground, but I had no such luck. After checking the wiring and sensor, the results showed no open circuits and no short to ground. The sensor’s resistance should be between 1,377 and 3,355Ω. The sensor checked at 2180Ω, so it was within range.

OK, not the results I was hoping for. At this point I was thinking that it could be in the PCM, so I put the scanner back on the vehicle and dove in a little deeper. Now that I had it connected to the scan tool, I was still having issues with the speedometer and the erratic shifting. I grabbed a screen shot of the Output Speed Sensor (OSS) with the vehicle stationary. There was activity showing on the sensor, albeit erratic activity. (See Figure 2).

RR-Tech-September-FIG-2
Figure 2.

I placed the truck on a lift with the tires just off the ground, running it in gear while monitoring the OSS signal. I was simultaneously performing a wiggle test at the PCM. There was no change moving wires or connectors at the PCM. It had been running about five minutes at this point. I stopped the wheels, shifted the transmission into park and noticed that I again had a signal while there were no tires spinning. The speedometer displayed 30 MPH in park. I unplugged the sensor, and it was still showing 30 MPH. It was beginning to look like I was dealing with the possibility of a faulty PCM.

In the process of removing the PCM, I placed a fender cover over the fender. Suddenly, there was no issue with speedometer, and now everything was working correctly. What was going on? All I did was put a fender cover on!

Read more stories in our R&R Tech series here.

I wiggled and pulled on the harness again and still was not able to duplicate the problem. I took all my equipment off and it was still working correctly. I took the vehicle for a long test drive and still nothing; the transmission was shifting correctly, and speedometer was working normally.

I took a break and about an hour later I was able to get back to this truck. After a long road test there were still no apparent issues. I thought I must have disturbed something and now wouldn’t be able to duplicate the issue further for testing. But hey, that doesn’t happen in the auto repair world very often now, does it?

I decided to clean up the wiring around the transmission and install some new wire loom covering, because as you probably know, they just crumble and fall apart on a lot of these older trucks. (See Figure 3).

RR-Tech-September-FIG-3
Figure 3.

After getting the loom on and the wires taped and secured at the transmission, I was moving the truck out of the bay. At that moment, it started acting up. Once again, the speedometer was reading 30 MPH. On the road test, the transmission had harsh engagements. I raised the hood for another look and to wiggle the PCM connector and wire harness. There was no change.
By now I was again thinking that the PCM is intermittently bad. As I was moving stuff around, my drop light fell on the engine wire harness and the problem went away. My focus was now directly on the area of the harness in question.

My light had fallen on the wire harness that runs along the valve cover, and this was the harness I needed to look at more closely. I wiggled the harness and this time the signal became erratic. I was looking for damage or a place that had rubbed through, and I found nothing. As I was moving the harness, I did notice that the spark plug wires were recently replaced and were wrapped around and laying on the wire harness. I rerouted the spark plug wires away from the wire harness and the issue went away. While I was there, I put new wire loom covering on the harness because the old loom was deteriorated. I wiggle tested the harness again and drove the vehicle a few more times. No more erratic speedometer and the transmission shifted smoothly.

It seems that we see more and more transmission concerns that are not caused by the transmission itself. With so many sensitive electronics involved with the control and sensing aspects driving the transmission, it opens these systems up to outside interference that can adversely affect the function. In this case, a clue could have been that the speedometer was erratic only with the engine running. The transmission control system was picking up inductive voltage from the misrouted spark plug wires. As they say, hindsight is 20/20. Perseverance and a little luck got the issue resolved.

You May Also Like

10L80 and 10R80 pump gear differences

You may have seen an article in the August 2023 issue of Transmission Digest called “GM 10L80: A new kind of pump noise,” which goes over how the front cover housing in the 10L80 is fitted with a converter drive gear and idler gear. The idler gear drives the pump’s driven gear, and is press

You may have seen an article in the August 2023 issue of Transmission Digest called “GM 10L80: A new kind of pump noise,” which goes over how the front cover housing in the 10L80 is fitted with a converter drive gear and idler gear. The idler gear drives the pump’s driven gear, and is press fitted to this housing by way of a ball bearing (see Figure 1). It is this bearing that goes bad, making the aforementioned new kind of pump noise. 

Shift Pointers: What to do when the 62TE TRS tab breaks

How frustrating it is when on a hot summer day, as you go to open a nice cold can of your drink of choice, and the tab breaks off? You are outside, away from any tools to remedy the problem quickly. It now requires a MacGyver mentality looking around at the resources available to get

Going the extra mile: Proving your transmission repair suspicions

A 2003 Honda Pilot with a five-speed three-shaft transmission came into our shop with a customer concern that the vehicle had no power, and the “D” light was flashing. I first did a scan for codes to see what it came up with, and the scan tool returned four DTCs: P1298 (ELD voltage high), P0135 (H02S

RRfeature-1400
Diagnosing Ford 10R60, 10R80 and 10R140 series speed sensor issues

Ford 10-speed 10R series transmissions utilize four two-wire, Hall-effect sensors — TSS, ISSA2, ISSAB and OSS — for providing speed signals to PCM or TCM. They are supplied nine volts by a PCM or TCM and assist in the control of clutch apply/release timing that is used in determining shift quality, including TCC. Related Articles

Jatco JF613E transmission quick reference material

For those working on the Jatco JR613E transmission, a widespread transmission with plenty of applications, the following should be a helpful guide. Related Articles – Outgrowing the walls: The story of EVT Transmission Parts – Spotting different 68RFE designs through the years to avoid issues – Valve body and component suppliers: A comprehensive list Domestic

Other Posts

Shift Pointers: Failures caused by incorrect tire sizes

For years ATSG has produced a wide range of issues related to improper tire sizes on vehicles. Even under-inflated tires have been known to cause issues. Problems such as premature failure with an active 4WD transfer case will occur with incorrect tire sizes. Related Articles – Understanding lube flow control valves in Toyota/Lexus UA/UB80 transmissions

Shift-Pointers-Jan-Figure-1-1400
Understanding lube flow control valves in Toyota/Lexus UA/UB80 transmissions

The Toyota/Lexus UA80 and UB80 transmissions first came out in 2017 in Highlanders and Siennas. The UA80 is used in V6 applications, and the UB80 is paired with four-cylinder versions. They have been called Toyota New Global Architecture type transmissions, and alternately referred to as the “Direct Shift 8AT” eight-speed automatic transmission. This transmission was

Tasc-Tip-December-Figure-1---LFC-Valve-OE-Partial-Circuit-Diagram-1400
How reading through service bulletins can turn a technician into the customer’s hero

Over the last 28 years of being a technician, I have developed the habit of checking for and reading technical service bulletins at the forefront of the diagnostic process, especially when an unfamiliar vehicle exhibiting blatant or straightforward concerns comes into the shop. I have found many valuable nuggets of information while reading over these

Sometimes, you should sweat the small stuff

It’s a common phrase: There may have been a time when you worried about something, and someone who knew what you are going through said, “Hey, don’t sweat the small stuff.” Sometimes, this may be good advice. But other times, it may be wise to handle the small stuff before it becomes bigger “stuff.”  Related