I have been working on a Trane Heat Pump and the problem I'm having is the thermostat won't engage the compressor for heat, but it does on cooling mode, and in fan only mode.

I jumped the thermostat at RH and G and the unit kicked on. I have replaced the thermostat and still it does the same thing. I checked the small wire from the Heat Anticipator to see if it was broken, changed the Transformer, I still can't get anything. What could be wrong or is it something I am missing?

Sounds like you need to take a closer look at the wiring of the tstat,, does the tstat require a jumper from rh to rc,, go back to tstat instructions

I failed to see where you posted the make and model numbers of the tstat or the heatpump

Yes it does jump across RH and RC.

It is a Robertshaw Universal H/C model # DA490-400.

Something isn't wired right.

It was wired the same way as before. I already put a new thermostat in there 2 weeks ago and it was working fine, and a previous guy before me did the same thing. The lady said one night she heard a loud noise and then the heat went out. When I looked at it the small wire from the anticipator was broken, I fixed that put it back together, nothing. New thermostat was the next step, turned it on, a bright light from the thermostat came on, then nothing. I'm wondering if the whole thing needs to be rewired, or is there a wire touching somewhere?

I think I figured it out. Today when I was working on it I changed the Tstat again and it worked fine. I got a call back this afternoon and she said it went out again. Checked the heat side and the cool side, nothing. What I did find out which I believe is the problem is the Tstat was loose from the mounting. As soon as I jiggled it around, it started to come on then stop, which I am pretty much sure it needs to be remounted because the contacts are not set properly because of the Tstat being very loose. I will mount it again firmly and level it, then hopefully I shouldn't have any more problems.

Thanks for you guys input. I believe the problem will be solved now.