Index: firmware/App/Services/SystemCommMessages.c
===================================================================
diff -u -r9a9d04b84f4345fca87fb14d26f09d497b08aae8 -r6c60d0d0300828750c76f2e397d4e7ccaa84bbb2
--- firmware/App/Services/SystemCommMessages.c	(.../SystemCommMessages.c)	(revision 9a9d04b84f4345fca87fb14d26f09d497b08aae8)
+++ firmware/App/Services/SystemCommMessages.c	(.../SystemCommMessages.c)	(revision 6c60d0d0300828750c76f2e397d4e7ccaa84bbb2)
@@ -110,7 +110,7 @@
     data[ msgSize++ ] = MESSAGE_SYNC_BYTE;
 
     // set sequence # and ACK bit
-#ifdef ACK_IMPLEMENTED
+#ifndef ACK_NOT_IMPLEMENTED
     // thread protect next sequence # access & increment
     _disable_IRQ();
         msg.hdr.seqNo = nextSeqNo;
@@ -144,7 +144,7 @@
         data[ msgSize++ ] = 0;
     }
 
-#ifdef ACK_IMPLEMENTED
+#ifndef ACK_NOT_IMPLEMENTED
     // if ACK required, add to pending ACK list
     if ( TRUE == ackReq )
     {
@@ -161,7 +161,7 @@
     return result;
 }
 
-#ifdef ACK_IMPLEMENTED
+#ifndef ACK_NOT_IMPLEMENTED
 /*************************************************************************
  * @brief sendACKMsg
  * The sendACKMsg function constructs and queues for transmit an ACK message \n
@@ -180,11 +180,13 @@
     // create a message record
     blankMessage( &msg );
     // send ACK back with same seq. #, but w/o ACK bit
-    msg.hdr.seqNo = ( message->hdr.seqNo < 0 ? message->hdr.seqNo * -1 : message->hdr.seqNo );
+    msg.hdr.seqNo = message->hdr.seqNo * -1;
     // ACK messages always have this ID
     msg.hdr.msgID = MSG_ID_ACK;
+    // ACK messages always have no payload
+    msg.hdr.payloadLen = 0;
 
-    // serialize the message (w/ sync, CRC, and appropriate CAN padding)
+    // serialize and queue the message for transmit on broadcast channel
     result = serializeMessage( msg, COMM_BUFFER_OUT_CAN_HD_BROADCAST, ACK_NOT_REQUIRED );
 
     return result;