I like to use typical everyday shop problems and fixes as examples when presenting a seminar to get my point across. Along with a power point presentation that has some humorous references and visual affects to keep the techs attention on the presentation and not on me. So that their not just listening to or watching some guy reading out of a book. Shop war stories so to speak. I try to make them fun to listen to and when you're having fun learning we retain the things we learn better. I know I've seen that written down somewhere. No, it's not listed in my Trans Tech Index.
That's not the only reason I do it. We can all agree that transmission tech is boring and sitting in a room for 6 hours or more on a Saturday. Especially after partying half the night before and eating a nice big lunch doesn't help. When I can make the seminar attendees laugh I accomplish three things; they're enjoying the seminar and paying attention, retaining the knowledge presented to them and waking up the techs that fell asleep. See, you at least smiled on that one.
With that being said here is typical problem that may at first seem not too typical when presented. I received a call from a local transmission repair shop near by that I often go to get my hands dirty and keep my diagnostic skills sharp. Another shop further away had been working on a 2000 Lexus RX300 with a U140E series transaxle for more than 5 weeks. The customer is in a rental car paid for by the shop in question. Needless to say the shop is frantic at this point and sent the vehicle to this shop and asked me to look at it.
The vehicle had a Crank Sensor code and the transmission would bind and shuttle in and out of gear during the 1-2 and 2-3 shift. 4th gear shifted fine. It was at the Lexus dealer for a week and they basically gave up on it. The car went to another local transmission repair shop known for doing electrical diagnostics. After a day or two decided they didn't have time for it and sent it back to the original shop. Original shop now tried the shot gun method 2 Crank Sensors w/o checking the sensor signal. Then 3 valve bodies, although the PCM was commanded this shift pattern. Another complete transmission and you get the picture by now.
The next thing that was said was "Are you sure you want to get involve with this nightmare". At this point I thought "WOW" this should be interesting to say the least. What do I have to lose; I don't own the car besides that mine drives fine. The first thing noticed is the computer is removed from the retaining bracket behind the glove box and lying on the passenger floor for easy access. Cool, less work for me. It was still connected to the PCM but several wires had small pieces soldered into them from several other techs back probing and had to be repaired. It looked like a porcupine rolled around in bed with it. I'll admit I did have second thoughts at this point.
The way the transmission shift pattern is commanded by the PCM is this; both shift solenoid SL1 and SL2 are turned on for 1st gear. SL1 is turned off during the 1-2 shift and SL2 is turned off for the 2-3 shift. The S4 solenoid is turned off for 4th gear. The PWM SL1 and SL2 solenoid signal from the PCM is Pulse Width Modulated (square wave). The S4 solenoid is an on/off type. A Snap On Vantage was set it up like a breakout box to read two graphing screens to monitor both the SL1 and SL2 solenoid square wave form. Knowing how the system works it was time for the road test.
During the 1-2 shift the PWM SL1 solenoid turned off with one voltage spike (normal). Then it started pulsing on and off again and that's when the bind and shift shuttle is felt (not normal). Then the SL1 solenoid turned off completely only after shaking the dash violently for a couple of seconds. Then the vehicle drove smoothly until the 2-3 shift was commanded. The PWM SL2 solenoid turned off, then the SL2 and the SL1 both started pulsing erratically at the same time almost causing the tires to bark. This was extremely abnormal and after just one block of driving it was time to head back to the shop. No one would drive this vehicle too far in this condition.
Back at the shop the hood was released and one of the installers was asked to unbolt the alternator feed wire from the battery. On the next test drive at the same distance it had the same problem. Back to the shop the hood was re-opened just to have a look see. It was noticed that the installer did not unbolt the alternator feed from the battery he only unplugged the field connector. With the connector plugged back in and using a 10 mm wrench to unbolt the battery feed to the alternator and road tested again. On this road test the transmission shifted normally. The vehicle was stopped and the battery feed wire was bolted back onto the alternator and the symptoms reappeared. Great, we're almost there. Now one of the red leads on the Vantage was moved over to the ground circuit of the PCM, testing for a voltage drop.
Electrical theory for any ground circuit is this; there's always source voltage on the ground side of every load device, whether it's a solenoid, fan motor or a light bulb until it's grounded (even a computer). There is always residual voltage left on every ground circuit when grounded. Normal residual voltage on any ground circuit should be no more than .1 to .2 volts. Some factory manuals say as much as .3 volts is ok, but at that point I don't trust the ground circuit. If 0 volts is seen, the meter lead is not making good contact with the circuit being tested or the meter isn't grounded correctly. The multimeter should always be grounded at the battery even if you have to run a lead from the battery to the meter. When doing a voltage drop test on the ground side of any circuit it should read approximately .02 - .03 volts or more (meter dependent).
Sure enough there was .8 volts showing, way too high, A temporary ground was jumped from the battery to the PCM. On the next road test the vehicle performed perfectly. A redundant ground was run to the PCM and the vehicle was fixed. Total amount of time spent diagnosing the vehicle was about 25 minutes. To this day the original shop does not believe the vehicle was fixed in that short time period. Then again, that's why it was at this shop for diagnosis and not theirs.
Oh yeah, that Crank sensor code also went away. When this vehicle’s electrical system was researched the wire schematics verified how the ground circuits worked. It was found that the Crank Sensor and the transmission Input (ISS) & Output (OSS) speed sensor along with the vehicle computer, share the same shielded ground at the back of the Intake Plenum. This ground location is impossible to see even with a mirror.
Knowing how basic electronics work (remember I'm no rocket scientist) led me to a ground problem with the alternator disconnected. Electro Magnetic Interference sometimes referred to as A/C ripple is a big problem with speed sensors. We get that problem everyday on the ATRA technical hot line. There is always some A/C ripple coming off the alternator even when they are working fine. If the shielded grounds on any circuit have connection problems they become radio antennas and pick up that interference. That's why the crank sensor set the code, although the speed sensors didn't. That would have been too easy if they did. Now wasn't that more pleasant to learn about doing voltage drop test on grounds and A/C ripple than the old fashion way................boring Zzzz.
Final note: When removing and installing a transmission on this vehicle, the engine has to be tilted at an unusually sharp angle. Now you know, (the rest of the story) how the ground wire can become loose or broken during a transmission repair.