Help! Checking Voltage and Ground on a Ford PCM

Wouldn’t a bad checksum on the PCM point straight at the PCM itself? In the computer world (PCM is a computer) a bad checksum means there’s a fault in the firmware coding, i.e. the software running on the PCM is corrupted. If there was a power or ground issue at the PCM, seems like the vehicle probably wouldn’t run at all. ?

 

I guess an intermittent power or ground issue at the PCM could trigger a checksum error, but seems like there would be a serious drivability issue too. I defer to @Mike N though.

 

A checksum is a rudimentary but effective way for a computing device like a PCM to self-validate the integrity of its data or firmware programming. During manufacturing a known-good calculated checksum value is stored in the device's non-volatile memory, along with the rest of the programming data. At boot time (or whenever needed) the device re-computes a new checksum by scanning all its memory, and comparing the new computed checksum against the pre-stored checksum value. If they're not exactly the same, you have a checksum error. If even a single bit of data has changed, like due to a flakey memory device, the checksums will not be the same and you have a checksum error. In a car, a single bit of bad data out of probably hundreds of thousands of stored bits could be, well, not good.

Checksum Error

 

PCM to car’s master computer: “My mind is going. I can feel it.”

 

Every component under the hood sends a value to the computer. The computer knows what they should all add up to. If the total of the values doesn’t match the expected total, then the computer knows that one of the components is bad or going bad.

 

Yeah, I would think a bad checksum indicates an internal corrupted firmware or something like that.

Grounding issues can cause false readings on sensor voltages, which will tend to make the PCM do funny things. That's because the amount of current that flows among the chassis, negative bettery terminal, and the engine block, so even a couple tenths of an ohm of resistance can cause significant noise voltage in the ground reference of the sensors. I once was getting occasional random sensor error codes, and the engine was sometimes a little rough and hesitant. With the engine idling I measured a couple tenths of a volt--and fluctuating--between the chassis and the alternator body. I found the ground strap connection at the bellhousing had corroded (so much so that I had to cut the wire right at the terminal in order to unscrew the bolt because the terminal and bolt head were frozen together). After I replaced the ground strap and bolt the voltage drop was only a couple hundredths of a volt and the error codes never happened again.

 

If it's the whole car reporting a value into the Checksum it's pretty darn crude. It's normally a check for a data set transmitted correctly.Two computers can quickly check they 'got it' by comparing only the checksum rather than sending entire datasets back. Too bad if multiple errors combine to the correct checksum. I am sure there is raft of mathematics to analyse that.