<PAGE>
                       SECURITIES AND EXCHANGE COMMISSION
                             Washington, D.C. 20549

                                  SCHEDULE 13G
                             (Amendment No.    2  )


                    Under the Securities Exchange Act of 1934

                                  UNISYS CORP.
                       ----------------------------------
                                (Name of Issuer)

                                  COMMON STOCK
                       ----------------------------------
                         (Title or Class of Securities)

                                    909214108
                       ----------------------------------
                                 (CUSIP Number)

   Check the  following box if a fee is being paid with this statement / /.  (A
fee is not required only if the filing person:  (1) has a previous statement on
file reporting  beneficial  ownership of more than five percent of the class of
securities  described in Item 1;  and (2)  has  filed no  amendment  subsequent
thereto reporting  beneficial ownership of five percent or less of such class.)
(See Rule 13d-7.)


                        (Continued on following page(s))


                              Page 1 of  4  Pages



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CUSIP No. 909214108                   13G                 Page  2  of  4  Pages


- -------------------------------------------------------------------------------
 (1) Names of Reporting Persons
     S.S. or I.R.S. Identification No. of Above Person

     THE TCW GROUP, INC.
     (FORMERLY KNOWN AS TCW MANAGEMENT COMPANY)
     95-3703295
- -------------------------------------------------------------------------------
 (2) Check the Appropriate Box if a Member     (a)  / /
     of a Group*                               (b)  / /
- -------------------------------------------------------------------------------
 (3) SEC Use Only

- -------------------------------------------------------------------------------
 (4) Citizenship or Place of Organization
     NEVADA
- -------------------------------------------------------------------------------
Number of Shares              (5) Sole Voting
 Beneficially                       Power                       17,449,919
 Owned by                    --------------------------------------------------
 Each Reporting               (6) Shared Voting
 Person With                        Power                                0
                             --------------------------------------------------
                              (7) Sole Dispositive
                                    Power                       17,449,919
                             --------------------------------------------------
                              (8) Shared Dispositive
                                    Power                                0
- -------------------------------------------------------------------------------
 (9) Aggregate Amount Beneficially Owned by Each Reporting Person
                                                                17,449,919
- -------------------------------------------------------------------------------
(10) Check Box if the Aggregate Amount in Row (9) Excludes Certain Shares*

- -------------------------------------------------------------------------------
(11) Percent of Class Represented by Amount in Row (9)
                                                                     10.0%
- -------------------------------------------------------------------------------
(12) Type of Reporting Person*
                                                                        HC
- -------------------------------------------------------------------------------



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CUSIP No. 909214108                                       Page  3  of  4  Pages


ITEM 1(A)   NAME OF ISSUER:
UNISYS CORP.
- -------------------------------------------------------------------------------


ITEM 1(B)   ADDRESS OF ISSUER'S PRINCIPAL EXECUTIVE OFFICES:
P.O. BOX 500, BLUE BELL, PA  19424
- -------------------------------------------------------------------------------


ITEM 2(A)   NAME OF PERSON(S) FILING:
THE TCW GROUP, INC. (FORMERLY KNOWN AS TCW MANAGEMENT COMPANY)
- -------------------------------------------------------------------------------


ITEM 2(B)   ADDRESS OF PRINCIPAL BUSINESS OFFICE OR, IF NONE, RESIDENCE:
865 SOUTH FIGUEROA STREET, LOS ANGELES, CA  90017
- -------------------------------------------------------------------------------


ITEM 2(C)   CITIZENSHIP:
NEVADA
- -------------------------------------------------------------------------------


ITEM 2(D)   TITLE OR CLASS OF SECURITIES:
COMMON STOCK
- -------------------------------------------------------------------------------


ITEM 2(E)   CUSIP NUMBER:
909214108
- -------------------------------------------------------------------------------


ITEM 3.  IF THIS STATEMENT IS FILED PURSUANT TO RULES 13d-1(b), OR 13d-2(b),
         CHECK WHETHER THE PERSON FILING IS A:

    (a) / / Broker or Dealer registered under Section 15 of the Act

    (b) / / Bank as defined in section 3(a)(6) of the Act

    (c) / / Insurance Company as defined in section 3(a)(19) of the Act

    (d) / / Investment Company registered under section 8 of the Investment
            Company Act

    (e) / / Investment Adviser registered under section 203 of the Investment
            Advisers Act of 1940

    (f) / / Employee Benefit Plan, Pension Fund which is subject to the
            provisions of the Employee Retirement Income Security Act of 1974
            or Endowment Fund; see Section 240.13d-1(b)(1)(ii)(F)

    (g) /X/ Parent Holding Company, in accordance with Section 240.13d-1(b)
            (ii)(G)
            (Note: See Item 7)

    (h) / / Group, in accordance with Section 240.13d-1(b)(1)(ii)(H)

ITEM 4.  OWNERSHIP *

    (a) Amount Beneficially Owned:
     17,449,919
    ---------------------------------------------------------------------------

    (b) Percent of Class:
     10.0%
    ---------------------------------------------------------------------------

    (c) Number of shares as to which such person has:

          (i) sole power to vote or to direct the vote
               17,449,919
              -----------------------------------------------------------------
         (ii) shared power to vote or to direct the vote
               0
              -----------------------------------------------------------------
        (iii) sole power to dispose or to direct the disposition of
               17,449,919
              -----------------------------------------------------------------
         (iv) shared power to dispose or to direct the disposition of
               0
              -----------------------------------------------------------------

*    THE FILING OF THIS SCHEDULE 13G SHALL NOT BE CONSTRUED AS AN ADMISSION THAT
     THE REPORTING PERSON OR ANY OF ITS AFFILIATES IS, FOR THE PURPOSES OF
     SECTION 13(d) OR 13(g) OF THE SECURITIES EXCHANGE ACT OF 1934, THE
     BENEFICIAL OWNER OF ANY SECURITIES COVERED BY THIS SCHEDULE 13G.  IN
     ADDITION, THE FILING OF THIS SCHEDULE 13G SHALL NOT BE CONSTRUED AS AN
     ADMISSION THAT THE REPORTING PERSON OR ANY OF ITS AFFILIATES IS THE
     BENEFICIAL OWNER OF ANY SECURITIES COVERED BY THIS SCHEDULE 13G FOR ANY
     OTHER PURPOSES THAN SECTION 13(d) OF THE SECURITIES EXCHANGE ACT OF 1934.


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CUSIP No. 909214108                                       Page  4  of  4  Pages


ITEM 5.  OWNERSHIP OF FIVE PERCENT OR LESS OF A CLASS.

          Not Applicable
- -------------------------------------------------------------------------------
ITEM 6.  OWNERSHIP OF MORE THAN FIVE PERCENT ON BEHALF OF ANOTHER PERSON.

          Not Applicable
- -------------------------------------------------------------------------------


ITEM 7.  IDENTIFICATION AND CLASSIFICATION OF THE SUBSIDIARY WHICH ACQUIRED
         THE SECURITY BEING REPORTED ON BY THE PARENT HOLDING COMPANY.

         The reporting person is the parent company of (i) Trust Company of the
         West, a California corporation and a bank as defined in Section 3(a)(6)
         of the Securities Exchange Act of 1934 ("TCW"), (ii) TCW Asset
         Management Company, a California corporation and an Investment Adviser
         registered under Section 203 of the Investment Advisers Act of 1940,
         and (iii) TCW Funds Management, Inc., a California corporation and an
         Investment Adviser registered under Section 203 of the Investment
         Advisers Act of 1940.

- -------------------------------------------------------------------------------


ITEM 8.  IDENTIFICATION AND CLASSIFICATION OF MEMBERS OF THE GROUP
          Not Applicable
- -------------------------------------------------------------------------------


ITEM 9.  NOTICE OF DISSOLUTION OF GROUP
          Not Applicable
- -------------------------------------------------------------------------------


ITEM 10. CERTIFICATION

    By signing below I certify  that,  to the best of my  knowledge and belief,
the  securities  referred to above  were  acquired in the  ordinary  course  of
business and were not acquired for the purpose of and do not have the effect of
changing or  influencing the  control of the issuer of such securities and were
not acquired in  connection with or as a participation in any  transaction
having such purposes or effect.

                                  SIGNATURE:

    After  reasonable  inquiry and to the  best of my  knowledge and  belief, I
certify that the  information set forth in this statement is true, complete and
correct.
                                       January 21, 1995
                                       ----------------------------------------
                                       (Date)
                                       /s/ Mohan V. Phansalkar
                                       ----------------------------------------
                                       (Signature)
                                       MOHAN V. PHANSALKAR,
                                       ASSISTANT VICE PRESIDENT - LEGAL
                                       ----------------------------------------
                                       (Name/Title)