Overcharged
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One of the niftiest things about opening the hood on a car that has been at several other shops is that in many (if not most)
cases, they've done all the hard work for you. Be that as it may, it's wise to gather the data that you know is pertinent
and come to your own conclusions. For example, not everybody checks fuel pressure, especially when it's a pain in the fanny.
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
Disconnecting the alternator on the Ford diesel killed the DTC and the flashing OD light.
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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.
These beefy battery terminals can look tight even when they're loose. With three out of four of the terminals dancing on their
respective posts, electrical spikes were the order of the day for the PCM, and that nonsense isn't received well by the controller's
sensitive electronic innards.
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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.
