Can't enter Apple Hardware Test Mode on my macbook pro I have an Early 2011 MacBook Pro.  When I hold down D immediately after restart, it boots to the normal login screen.
When I hold down ALT+D (with a working wifi connection before the reboot) the same thing happens.

I'm running macOS 10.13.5 

I don't have an original Applications Install disc 2 as mentioned here.

Any ideas why I can't enter AHT mode? I'm trying to troubleshoot an intermittent fault where the mac won't boot, gives me 3 beeps on a black screen (suspect faulty RAM, but want to run AHT first to check).

Recovery mode works correctly, but doesn't give the option to enter AHT mode.
You can use the following terminal commands to determine the model and board ID of your computer:

sysctl hw.model | awk '{ print $2 }'
ioreg -l | awk -F\" '/board-id/ { print $4 }'


Then download the proper AHT from this link 

https://github.com/upekkha/AppleHardwareTest