Answer from your duplicate posting at QRZ.COMhttp://forums.qrz.com/showthread.php?400144-Motorola-Microphone-wired-as-Yaesu-MH-27A8J-for-FT-2400-7400-amp-2500
You will not detect the PTT line going to ground, as it's not a simple direct short when you depress the PTT button. It grounds through a transistor circuit, using voltage coming in off the "Mic Hi" lead. Leave it to Motorola to design something complicated when something simple would do the trick!
Microphone uses Elecret element, so the PTT and mic circuit permits switching of DC voltage to elecret element only during PTT. Schematic at bottom of this posting.
For Product Questions / Support of Motorola Solutions radio equipment, search BATBOARD / BATLABS.http://batboard.batlabs.com/viewtopic.php?f=1&t=2970
Motorola Mobility (mobile phone buiness) is now part of Google.
Repeater Builders is also a good Motorola documentation / support web site to answer queries.http://www.repeater-builder.com/rbtip/mojoindex.html
HMN1035C Palm-size microphone (see the HMN3596A microphone below).
HMN3596A Palm-size microphone (full documentation, extracted from the GM300 manual).
Motorola GM-300 Product Pagehttp://www.repeater-builder.com/motorola/maxtrac/maxtrac-index.html
HMN-3596A Schematic diagramhttp://www.repeater-builder.com/motorola/maxtrac/pix/hmn3596a.jpg
HMN-3596 Full documentationhttp://www.repeater-builder.com/motorola/maxtrac/pix/hmn3596-microphone.pdf