Need help wiring Generac GTS to Cummins c25 genset

[Pdexter1210]

Hello, I'm looking for someone who might be able to help point me in the right direction on hooking the subject transfer switch and genset together. I have run contacts from 178 and 183 in the GTS out to the genset and landed those on TB5 (Remote Start) and TB1 (Ground). Have tested the generator in manual, it starts and runs just fine, generates electricity as it is supposed to. When we disconnect utility service and hold down the Fast Test Mode switch, the generator starts, comes up to speed and the GTS transfers power to the load. This has the appearance of working as it should, however if we don't disconnect utility service ahead of time, the transfer switch seems to feed power through both the utility and the generator (as noted by the nearby a/c unit ramping up dramatically when we tried the transfer). The transfer switch mechanism is working, so I don't how to explain why it seemed like we were sending electricity from both sources, but it does. We have disconnected the utility service, started the generator manually and manually transferred the GTS and that works fine. I have manuals for both the GTS and the Genset, but haven't been able to diagnose my issue yet, so I thought I would check the forum. Also note, when we put the GTS in Normal Test Mode, nothing happens (although I'm not sure yet whether this is simply a timer that we have set too long, or if it is a further issue). Licensed electrician working on this, just not one that is familiar with this GTS.

Any thoughts on where we might have stumbled or ideas on what to look for would be appreciated. Thanks in advance.