Ok.. I know I dont have the experience you do with transmissions but I do understand the basic principals of a torque converter but I also know it has nothing to do with the gear ratios inside and from my experience with driving and working on vehicles I think it feels like 2nd maybe 3rd but if it was trying to take off in OD (4th) I dont think it would get down the road as well as it is. It just feels down on power, not like a geo metro motor trying to pull a heavy truck. Just saying it feels like a second gear take off so if were sure it would be in 4th with the wrong computer then that may narrow it down.

 

"Limp Mode" or "Fail Code" Condition
"Fail Code" conditions or "Limp Mode" happens when the vehicle computer recognizes a problem in it's logic. When an expected signal value from a sensor is sent to the computer and is not within the computer's programmed specifications, "secondary" programs are activated by the computer to strive to protect the transmission from damage the improper sensor signal might cause to occur.
In other words, the computer is always expecting certain signal values from certain sensors i.e. the temperature sensor, the speed sensor, the throttle position sensor, etc. As long as these signals are what it would normally expect for the conditions and is normal based on all the other signals it is receiving from other sensors, it acts normally and accordingly.
If the computer, all of a sudden, receives some crazy signal from one of the sensors that is out of the normal range expected from this sensor, it will go to "emergency" or "secondary" measures.
These emergency measures vary depending on the severity of the defective signal. All this is preprogrammed into the computer's logic by the manufacturer. The manufacturer has decided that as long as a certain parameter of a particular signal is sent from a sensor to the computer, all is well. The manufacturer decided that if this signal is higher than their highest parameter or lower than their lowest parameter, something is wrong with that sensor and the computer should make someone aware of the situation and take action to try to "save" the vehicle systems or powertrain.
Perhaps the computer will simply cause the "check engine" light to come on. The signal variation wasn't severe or critical to cause any mechanical failures but the vehicle's operator is made aware that he or she should have the vehicle checked out electronically to see if a minor sensor has broken down or is starting to send the odd irratic signal. This type of condition is commonly referred to as a "soft code". Normal functions are not affected but if the repair is not made, performance or fuel efficiencies might suffer. Perhaps the sensor only malfunctioned one time and all other times was fine. This might be an early warning of a sensor that is beginning to fail or has a loose connector or connection.
Other times the signal needed to perform operations normally is so far out of specification that the computer has no choice but to go into survival mode. With transmissions, the computer will cause the internal tranny fluid line pressure to default to high to protect clutches and bands. The transmission also turns off the shift solenoids to cause the unit to default to a single gear, usually second or third. All normal instructions to control line pressure are overridden so a hazardous "slipping condition" cannot occur easily. This theoretically is so that the vehicle's driver can get the damaged vehicle to the next town for repairs. This condition is commonly called "Limp Mode" for this reason. You limp to the next town in second or third gear only, at full line pressure so the tranny guts won't slip on your trip in.
By the way, interestingly and just as a side note, if the cable harness going to your transmission was ever to become detached, severed or damaged, your transmission would also go to "limp mode".
The vehicle's computer would immediately sense that it has lost contact with the transmission and would set the codes and send "limp mode" signals to the tranny. But because the harness is severed between the computer and the transmission, no computer signals will reach the transmission. These sent signals, however, would have had the identical affect on the transmission as what taking away supplied power to the shift and line pressure solenoids has as in the case of a transmission harness being detached or cut. Due to the engineered voltage strategies of the solenoids, the transmission simply defaults to a single gear and line pressure defaults to high, all in order to "limp" you home.
A Throttle Position Sensor that improperly sends a reading that it is wide open when in fact it is physically closed would be detected by the computer when it compared this reading with the vehicle speed sensor that perhaps is showing very slow vehicle speed. The signal, by itself can't be considered wrong but when put against all the other sensor signals of the system might cause a computer concern. The computer, at this point, unable to "trust" the collection of signals because together they are not making sense in it's logic, will simply go to limp mode in the transmission to protect it and make the operator aware that something is wrong with one of the sensors and a mechanic's attention is needed to correct the situation.
This Fail Code Condition will show up as a "code" reading on a mechanic's scanner. This code will be cross referenced to a table from the manufacturer and represent a problem with a particular sensor or a group of sensors or system. It gives the mechanic a better idea of where the problem has showed up and which systems or sensors are involved in the malfunction. The table of codes and what each one means, is commonly programmed right into the scanner tool that the mechanic uses for easy reference.
i.e. the scanner tool might tell the mechanic that the computer has thrown a code "35" which is the "transmission fluid temperature sensor" and might give the mechanic the recommended values this sensor should provide and what it in fact provided...
In your electronic transmission, many important functions are controlled by the computer. Shift timing, sequence, feel, line pressure are controlled. The information from the vehicle speed sensor affects fuel injection, fuel mixture, ABS, transmission operation, etc. Load information of your engine is commonly taken primarily from the TPS (throttle position sensor) or the MAP sensor (manifold absolute pressure). This controls transmission shifting and downshifting when stepping on the gas or climbing hills. A regular scanning of the computer for any set "hard" or "soft" codes is something routinely done by most good tuneup shops these days.

 

Perfect, That could be the case. Wouldn't limp mode kick of a check engine light though? I have no CEL at all and all gauges read normal. I am going to pull codes on it tomorrow so hopefully will know for sure what is going on and I have the 5.0L computer I can plug back in if for some reason I have the wrong computer in there.

I am still confused as to why I cant just run the 5.0L computer and had to get one from a 5.8L. I know its speed density but its running the same size injectors, throttle body, ignition. The only actual difference is the slight bump in compression but having a O/2 sensor I would think if it were slightly lean there would be enough adjustment in the fuel maps to compensate. Unless there O/2 is just for informational purposes and there is actually no adjustment at all that can be done by the computer. Who know but I am thinking the computer swap is half my problem. I probably should have just went to mass air at the same time.

 

Speed-density relies on the computer being pre-programmed with the engine's known volumetric efficiency at a given RPM and manifold vacuum. The 5.0 and 5.8 have different volumetric efficiencies.

 

The 1994 and newer 5.0L computer is for an SFI (many more wires) and is also as pointed out above an MASS system.
The 1995 5.8L is MFI and SD unless of course it has California emissions.
An older 5.0L may work but not one from a 1994 or newer 5.0L.

 

That is not correct. Only California 94 and 95 5.0 trucks were mass air. The majority were still speed density.

 

My 1995 5.0L F150 truck is not a California truck and it is MASS air and SFI.
Also all the Ford diagrams for the 1994 -1996 show only the 5.0L to be Mass air and SFI except for the non E4POD and 4R70W transmission were SD.

Note if they did not have a E4OD or 4R70W transmission then 1994 & 1995 5.0L was not MASS, they had the MAP sensor.

 

Could have sworn I've seen 5.0 SD trucks of those years, but maybe not.

 

But the E4OD and 4R100 default to fourth gear, not second or third.

 

The non E4OD and 4R70W were SD in 1994 & 1995.
Sorry about that but the 1994 and 1995 5.0L were still all SFI and not MFI.