Case in point: One 1992 Chevy C10 (350 with throttle body injection) had been at several places for a low power concern, but nobody had been successful at straightening it out. As a last resort, the student brought it to the automotive department and we went to the trouble to check the fuel pressure, an exercise that is very difficult on a TBI system if you don't have the proper adapters. Well, we found 2 pounds of fuel pressure instead of the 12 to 14 psi that's supposed to be there, and the fix turned out to be pretty simple.
Another case: A man who had spent more than $700 on his van at several independent shops brought it to the Ford service department where I worked. I found a big wire harness behind the engine chafing on a lift bracket and fixed his problem with a tie strap and a roll of tape. He was stunned when the bill was only $20.
Troubleshooting is 90 percent of a successful repair in most cases, and it goes without saying that a surgical repair makes a lot more sense than replacing everything that might be causing the problem. But some problems can have strange and unusual causes.
Limping In
Jimmy, one of my department graduates, let me in on a transmission situation he encountered on a 2001 Power Stroke that was equipped with a 4R100. This pickup was flashing its overdrive light and going into transmission limp-in mode, and a local transmission shop had thrown some clutches and seals at it to no avail.When a Ford electronic transmission goes into limp-in mode, it raises the line pressure to protect the clutches from slipping, and it typically enters that mode when it believes slipping is happening or when it believes incorrect gear ratios are being obtained for one gear or another. Basically, the PCM compares engine speed, turbine shaft speeds and output shaft speeds as each gear is engaged, and it knows when things aren't as they should be. In addition to checking for slippage by comparing the speeds of the engine, turbine and output shafts, the PCM will enter limp-in mode if a speed sensor signal is determined to be untrustworthy, because in those situations the PCM doesn't know if slippage is happening or not, and it's better to be safe than sorry.
So what is the PCM looking for? Imagine yourself operating a vehicle you drive every day, and let's assume you're paying attention to the tach and the speedometer. Your transmission shifts the first time, then the second time, and when it shifts into third gear you pick up on the fact that the engine speed on the tachometer is far higher than it should be at that vehicle speed in that gear. You're doing the same thing the PCM does, only the PCM has a much tighter tolerance than you do.If a gear is slipping, the worst thing the PCM can do is leave the pressure low enough so that the clutches are only half engaged. Higher fluid pressure squeezes those fibers and steels a lot harder and makes them less likely to slip, so the PCM is programmed to reduce current to the EPC solenoid and pressure rises.
Slippage might happen as a result of a fluid pressure leak through a cracked or porous casting in the case or valve body, a stuck spool valve somewhere or a shift solenoid mechanical malfunction. Simple torque converter clutch slippage can cause limp-in mode activation on some Fords. Clearing the DTC that has been stored will return the pressure to normal until the code is stored again.
Very Simple Repair
Jimmy spoke with the transmission tech who rebuilt the unit at the other shop, and the guy said gently applying the throttle in gear on an incline would bring the problem on. He was right, but then, he had been trying for two weeks to figure out what the heck was going on, so he knew exactly what it took to make it happen.Jimmy duplicated the concern and retrieved a P0720 for a loss of the Output Shaft Speed Sensor (OSS) signal. A trip through the datastream window was quite revealing, but it was also somewhat confusing. The Vehicle Speed Sensor reading was 127 mph, and the Output Shaft Speed Sensor was reading 6,000 rpm with the vehicle sitting motionless in the service bay.
Out with the OSS sensor, and Jimmy inspected the tone wheel with a bore scope to find nothing wrong in there. The hurricane speeds coming over that Data Link had to be the result of electrical interference, he decided. Sure enough, disconnecting the generator took care of the problem!
But was it the generator?
No.
What Jimmy found was that three of the four battery terminals were loose, and when he cleaned and tightened those terminals, cleared the code and attempted to duplicate the concern, the flashing OD light and limp-in activation were gone.Spikes from an intermittently open charging circuit or bad ground can wreak havoc with today's electronics, to be sure! The problem was so simple it had been overlooked by the guy in the transmission shop. It was a nasty curve ball, but the transmission shop guy had made an easy job hard. In his defense, who'd have thought loose battery terminals would cause a transmission concern?
The Malibu
Lloyd is a guy I've known for nearly 40 years, and he called about the 2002 Malibu. This car belongs to a woman with whom Lloyd works, and she had taken her car to several places trying to get it straightened out. But nobody had been able to do anything except spend her money, and she had gone through about $2,000. When I spoke with her on the phone, she told me if I wasn't able to figure her problem out she was going to trade it. But she really liked the car and really didn't need a car payment.Her story was that the Malibu had almost no power at all — cars traveling behind her on curvy country roads where it's hard to pass would tailgate her until an opportunity came to move into the passing lane and leave her behind, because she could only poke along at about 50 mph.
"The engine makes a lot of noise when I try to accelerate, but the car doesn't go very fast," she told me. It was a challenge, and I rose to it, not having any idea whether I could do her any good or not. Those "I've been everywhere" jobs are always interesting (whether we can figure them out or not), and just about everybody has some notches on his or her pocket screwdriver handle representing their victories over these problem children. Would I be able to add a notch to mine? We'd see.
First I drove the car with one of the students, and we both agreed it felt like a clogged catalyst. But the engine wasn't quite as noisy as it should have been for a plugged exhaust problem. Most of the students drove the car as well and said it felt like a transmission concern. So I took the time to explain how an underpowered engine, reduced road speed and a foot deep into the throttle will cause a transmission to shift late and hard.In the service bay we checked engine vacuum — it was about 19 inches and recovered well on the acceleration test (holding the throttle up to see if the vacuum stayed low – it didn't.) I have to admit that I was surprised.
Next we checked the fuel pressure, and it was a solid 55 pounds (52-59 is the spec).
With the scan tool there were no diagnostic trouble codes and nothing out of the ordinary on the data stream, not even in the fuel trim PIDs.On the lift, a temperature check of the pipe ahead of the catalyst showed 225 degrees Fahrenheit, and behind the cat we read 375 degrees. But I've seen those temperatures lie to me before, so we did a pressure check of the exhaust stream between the converter and the engine that revealed less than 2 psi of pressure, even with the engine at 2,000 rpm and holding. This wasn't a plugged catalyst. So what was it? I had no idea, and quite frankly, I could understand why several shops fought with it unsuccessfully.
Well, we were studying transmissions this semester, and this was one of those funky GM transaxles with no dipstick (seems like there were a lot of those popping up around the turn of the century), and I was determined to show my students the right way to check and fill the transaxle on one of these 4T40Es.
There is a ⅛ pipe plug on the passenger's end of the transaxle that looks like a line pressure test port, but it's actually the fluid level check point. You're supposed to remove the red filler cap on the upper side of the transmission case and then remove that ⅛ pipe plug near the bottom of the transaxle with the engine running and check with a pocket screwdriver to see if the transmission fluid comes to the base of that hole. If it runs out of the hole, let it run out until it stops running out and the fluid level is correct.
When we screwed the plug out, there were about 2 quarts of fluid that came out before the fluid level reached its optimum. That was a lot of fluid, and usually an overfull transmission or transaxle will push excess fluid out the vent, but this transaxle won't do that for some reason. This one sure didn't! Somebody had obviously added fluid without removing the plug, probably figuring too much was better than not enough.
It was only a hunch, but I took the car out for a test drive after the fluid dump and was pretty astounded to find that it ran like a new vehicle — the lack of power was gone.
Conclusion
I certainly wasn't any kind of diagnostic superman on this job. The simple fact is that if I hadn't been so eager to demonstrate that silly fluid level check, we might have tinkered with the engine controls and the fuel system like everybody else did.
As it turned out, the lady who owned the car paid our basic charge and drove away with a big smile on her face thinking I was some kind of hero. In a word, we took a hard job and made it easy, not because we were smart, but because we decided to check something very basic that everybody else had overlooked, and that made for a very satisfying day.
Richard McCuistian is an ASE-certified Master Auto Technician and was a professional mechanic for more than 25 years. Richard is now an auto mechanics instructor at LBW Community College/MacArthur Campus in Opp, Ala. E-mail Richard at [email protected]
