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.MAN
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.