diff --git a/icu4c/source/common/unicode/platform.h.in b/icu4c/source/common/unicode/platform.h.in
index e1dcfdc148..fc6249a153 100644
--- a/icu4c/source/common/unicode/platform.h.in
+++ b/icu4c/source/common/unicode/platform.h.in
@@ -201,7 +201,7 @@ typedef unsigned int uint32_t;
 /* Character data types                                                      */
 /*===========================================================================*/
 
-#if defined(OS390) || defined(OS400)
+#if ((defined(OS390) && (!defined(__CHARSET_LIB) || !__CHARSET_LIB))) || defined(OS400)
 #   define U_CHARSET_FAMILY 1
 #endif
 
diff --git a/icu4c/source/config/mh-os390 b/icu4c/source/config/mh-os390
index c4a6913fe0..2927f6b1f0 100644
--- a/icu4c/source/config/mh-os390
+++ b/icu4c/source/config/mh-os390
@@ -103,8 +103,6 @@ IMPORT_LIB = $(basename $(notdir $(SO_TARGET)))$(IMPORT_LIB_EXT)
 MIDDLE_IMPORT_LIB = $(basename $(notdir $(MIDDLE_SO_TARGET)))$(IMPORT_LIB_EXT)
 FINAL_IMPORT_LIB = $(basename $(notdir $(FINAL_SO_TARGET)))$(IMPORT_LIB_EXT)
 
-## Shared object suffix (switch to dll for shared library build)
-SO = dll
 ## Non-shared intermediate object suffix
 STATIC_O = o
 
@@ -146,6 +144,17 @@ LIBICUIO=	$(top_builddir)/io/$(LIBICU)io$(ICULIBSUFFIX)$(SO_TARGET_VERSION).x
 LIBCTESTFW=	$(top_builddir)/tools/ctestfw/$(LIBICU)test$(ICULIBSUFFIX)$(SO_TARGET_VERSION).x
 LIBICUTOOLUTIL=	$(top_builddir)/tools/toolutil/$(LIBICU)tu$(ICULIBSUFFIX)$(SO_TARGET_VERSION).x
 
+ifneq (${ICUDATA_CHAR},e)
+## We're in ASCII mode.
+CFLAGS		+= -Wc,"ascii"
+CXXFLAGS	+= -Wc,"ascii"
+SO = so
+else
+## We're in EBCDIC mode.
+## Shared object suffix
+SO = dll
+endif
+
 ## Special 390 rules
 
 ## Build archive from object
diff --git a/icu4c/source/icudefs.mk.in b/icu4c/source/icudefs.mk.in
index 331c8363e3..9fea259731 100644
--- a/icu4c/source/icudefs.mk.in
+++ b/icu4c/source/icudefs.mk.in
@@ -44,7 +44,8 @@ ICUDATA_DIR = @pkgicudatadir@/$(PACKAGE)$(ICULIBSUFFIX)/$(VERSION)
 
 ICUDATA_BASENAME_VERSION = $(ICUPREFIX)dt@LIB_VERSION_MAJOR@
 ICUDATA_ENTRY_POINT = $(ICUDATA_BASENAME_VERSION)
-ICUDATA_PLATFORM_NAME = $(ICUDATA_BASENAME_VERSION)@ICUDATA_CHAR@
+ICUDATA_CHAR = @ICUDATA_CHAR@
+ICUDATA_PLATFORM_NAME = $(ICUDATA_BASENAME_VERSION)$(ICUDATA_CHAR)
 PKGDATA_LIBSTATICNAME = -L $(STATIC_PREFIX)$(ICUPREFIX)$(DATA_STUBNAME)$(ICULIBSUFFIX)
 ifeq ($(strip $(PKGDATA_MODE)),)
 PKGDATA_MODE=@DATA_PACKAGING_MODE@