Without a scope you won't be able to find the problem, unless it's something trivial, like a broken Feedback-Loop.
What I would suggest to check, from personal experience with TL494-Based SMPS, would be these points:
- Transformer OK? - Shorted Windings due to overheat might be a possibility and cannot transfer power, leading the chip to run the MOSFETs constantly at 50% DutyCycle (btw. my FullBridge SMPS-Design blew the MOSFETs after the Diode-Bridge burned out due to overheating during a max current-test)
- Feedback-Loop OK? - No feedback-voltage = Constant 50% DutyCycle
- Chip OK? - Car Supplies are NASTY (Voltage Spikes are expected to be as high as 80V with A LOT of Power behind them, according to some Application-Notes I read!). If the Chip isn't protected, it might have burned out and the outputs have been shorted to VCC - Other Failure-Modes: Switching-Frequency too high, TurnOn-Overlap, TurnOff-Comparator damaged, internal Feedback-Amps damaged, etc.
If you have the patience to build an SMPS yourself, I'd be happy to post the updated schematic of my FullBridge SMPS, or even send you a few boards.
It requires only 4 MOSFETs, should be adaptable to a transformer with center-tap and includes all the suggestions T3sl4co1l made (I hope I did implement them correctly^^), as well as overheat protection and a resettable fuse.
Components can be found on Digikey, or Ebay and are less than 30€.