Can someone explain what the PROM is inside the ecm and its purpose.

 

The PROM is the intelligence for the computer. It can be programed for modifications done to your car. My 93 also has the Knock module on the MEMCAL carrier. If you into old computers think of it as the BIOS?
PROM= Programmable Read Only Memory.

 

Google search: PROM inside ECM turned up:

www.bsecorp.com/files/techtips/bdtecb103.pdf

WHAT IS A PROM?
A PROM is a removable memory chip located within an ECM. PROM stands for
Programmable Read Only Memory. This memory chip contains the vehicle
characteristic information, as well as memory tables for such functions as spark timing,
fuel control, emission control strategy, etc. The information contained within the PROM
memory chip can be thought of as an instruction manual written for the ECM. Without
the proper PROM chip, an ECM may not function as intended for a specific vehicle
application. Also, if certain driveability concerns are traced back to the PROM, a new
updated PROM may be released calling for the removal of the old PROM followed by
the installation of the updated version.
PROM TERMINOLOGY
It is important to understand the terminology used when describing PROM chips. Three
terms that often appear are “PROM”, “CALPAC”, and “MEMCAL”. A PROM, as
mentioned above, is a memory chip that contains vehicle characteristic information and
certain control strategy information. A CALPAC is a small resistor-pack chip that is
used for “back-up”, or “limp” modes. The CALPAC is used by the ECM when it detects
a sensor signal loss or sensor signal error. Under these conditions, the ECM uses the
fixed value created by the CALPAC internal resistor as a default value for the specific
sensor signal lost. This “substituted” signal allows the vehicle to operate in “back-up” or
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“limp” mode
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. The term MEMCAL is typically reserved for a memory chip assembly
consisting of a PROM and CALPAC chip(s), and possibly other memory chips or
devices. Sometimes, certain instructional literature or documents may use the terms
PROM and MEMCAL interchangeably. On vehicles equipped with a carburetor, the
ECM will typically contain only a PROM. Most vehicles equipped with electronic fuel
injection will use an ECM that contains both a PROM and a CALPAC, or a MEMCAL
assembly.
PROM IDENTIFICATION
There are two ways of identifying a PROM. One way is by using a scan-tool to “read” the
PROM scan-tool ID number. This is a four-digit number stored in the PROM, and can
only be retrieved through electronic communication with the ECM. Another method that
can be used to identify a PROM is to read the identification markings that are printed on
the PROM, or on a label that is affixed to the PROM. The printed identification
markings consist of two or three different label markings. The first label marking is
known as a BROAD CAST CODE part identifier. The Broad Cast Code identifier is an
alpha character code that may be 2 to 4 letters in length. The second label marking is
known as an EXTERNAL ID number. The External ID number is a 4-digit identifier
number used in conjunction with the Broad Cast Code to properly cross-reference the
PROM to a G.M. Service Number through PROM cross-reference tables. The third label
marking that may appear on the PROM is the full 8-digit service number. This marking
may be in place of the 4-digit External ID number. On the following page are two
examples of G.M.’s label marking formats, both the old style and the new.

1985 - 1990 ECM Trouble Codes
http://www.iroczone.com/specs/TroubleCodes.htm