I have been troubleshooting a "no heat" call on a Trane YCD061 rooftop unit and it seems I'm at a point where I need further assistance. This unit is equipped with a UCP and CTI board, both of which were replaced this past summer for problems with cooling. Now, during a call for heat from the programmable space thermostat, the unit will come on for about 10 seconds and then cycle off. Other times, when I jump out LTB 14 and LTB 5 to bypass the thermostat, nothing happens, but I do notice the green light on the IGN board is not lit. When I check the voltage at the TH+ terminal on the IGN board, I get 12 volts AC, not 24. I also get 12 volts AC at the J1-22 pin on the UCP but get 24 volts at the J1-21 pin (either side of the K6 contacts). It seems as thought I'm getting a voltage drop in the UCP board at the K6 contacts. But, when I unplug the cable at the IGN board and check the voltage at PPF3-2, I get 24 volts. I've checked both TC01 and TC02 and they're ok. If I take 24 volts right from the step-down transformer and apply it to TH+, the heat goes through a typical start sequence and lights off ok. The system runs ok when in the test mode.
Very frustrating. Any help would be appreciated!