When i loaded Raspberry Pi 3 B+ with the SD Card from Raspberry Pi 3 B, it didn't boot later that it needs latest kernel and i have the old one. I could have installed rasjpbian stretch, but my app currently not ready for stretch so i have to use  jessy.

I finally update ran rpi-update wow, my Raspberry 3 B+ is up withe jessy, but later i was unhappy because i don't see wifi adapter, after a debug found dmsg yelling at me saying driver failed to load.


[    3.495761] brcmfmac: F1 signature read @0x18000000=0x15264345
[    3.504770] brcmfmac: brcmf_fw_map_chip_to_name: using brcm/brcmfmac43455-sdio.bin for chip 0x004345(17221) rev 0x000006
[    3.505168] usbcore: registered new interface driver brcmfmac
[    3.514427] brcmfmac mmc1:0001:1: Direct firmware load for brcm/brcmfmac43455-sdio.bin failed with error -2
[    5.574616] brcmfmac: brcmf_sdio_htclk: HT Avail timeout (1000000): clkctl 0x50

Existing driver is not working with B+, so i installed new driver which fixed it for me. Let show you.
First download the .deb from raspberry archives and install with dpkg. 

root@raspB+:~# wget https://archive.raspberrypi.org/debian/pool/main/f/firmware-nonfree/firmware-brcm80211_20161130-3+rpt3_all.deb
root@raspB+:~# dpkg -i firmware-brcm80211_20161130-3+rpt3_all.deb

Now reboot the Raspberry Pi to load the driver on next boot. After reboot you can check ifconfig/iwconfig to list the wlan interface. 

root@raspB+:~# ifconfig -a
eth0      Link encap:Ethernet  HWaddr 11:27:eb:1d:a6:22
          UP BROADCAST MULTICAST  MTU:1500  Metric:1
          RX packets:0 errors:0 dropped:0 overruns:0 frame:0
          TX packets:0 errors:0 dropped:0 overruns:0 carrier:0
          collisions:0 txqueuelen:1000 
          RX bytes:0 (0.0 B)  TX bytes:0 (0.0 B)

wlan0     Link encap:Ethernet  HWaddr 11:27:eb:1d:a6:22  
          UP BROADCAST MULTICAST  MTU:1500  Metric:1
          RX packets:0 errors:0 dropped:0 overruns:0 frame:0
          TX packets:0 errors:0 dropped:0 overruns:0 carrier:0
          collisions:0 txqueuelen:1000 
          RX bytes:0 (0.0 B)  TX bytes:0 (0.0 B)

Awesome you fixed it, let's also see how dmsg doing to confirm driver is loaded without any errors in case.

root@raspB+:~# dmesg |grep brcmac
[    4.417688] brcmfmac: F1 signature read @0x18000000=0x15264345
[    4.424231] brcmfmac: brcmf_fw_map_chip_to_name: using brcm/brcmfmac43455-sdio.bin for chip 0x004345(17221) rev 0x000006
[    4.424788] usbcore: registered new interface driver brcmfmac
[    4.763266] brcmfmac: brcmf_c_preinit_dcmds: Firmware version = wl0: Feb 27 2018 03:15:32 version 7.45.154 (r684107 CY) FWID 01-4fbe0b04
[    4.763840] brcmfmac: brcmf_c_preinit_dcmds: CLM version = API: 12.2 Data: 9.10.105 Compiler: 1.29.4 ClmImport: 1.36.3 Creation: 2018-03-09 18:56:28 
[  884.068594] brcmfmac: power management disabled

See driver loaded and registered...