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What number computer, ecu, ecm, pcm what ever they call it came on the 99.5 psd 4x4 6 speed.
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It could be any one of several. Click here.
Note that Ford (and your title) lumps all early '99s and '99.5 models into one model year called '99.
It's called a PCM code. Ford's main engine control computer is called a Powertrain Control Module = GM and most others' electronic control unit/module.
Even the code on the PCM might not be current if a dealer's tech has ever "flashed" your PCM with an updated program. The same way your older PC with a Windows 98 sticker on the front has beed updated to Windows XP. The only way to know for sure is to use a scanner that will read the PCM part number, then use a cross-reference table such as the one at click here
to determine the PCM code applicable to that part number.