Through the first movie, The Terminator (1984), there are several allusion to the fact the cyborg was rotting over time. I found this particular scene in the script (I omitted some passage and replaced them with [...] to shorten it):
INT. TERMINATOR'S HOTEL ROOM - DAY 194
MEDIUM ON TERMINATOR
sitting on the edge of the bed. His appearance isn't improving.
A patch of SCALP is blown away, revealing CHROME underneath. A flap of skin dangles from his cheek, which exposes some of the DRIVE CABLES which move the lips.
his eyes tracking rapidly. His skin is waxy, WHITE, BRUISED, GANGRENOUS in places. He ignores the FEW FLIES crawling on his face.
A MIDDLE-AGED MAN with a torn T-shirt covering his paunch knocks on the door. He is wheeling a trash cart.
Hey, buddy, you got a
dead cat in there or what?
Why was it decaying like this? The others terminators, more particularly the ones based on the same model from Terminator 2: Judgment Day (1991) and Terminator 3: Rise of the Machines (2003) didn't rot like that.