Search results

It should be pretty simple. Output of the TDM is 2 wires, one should be ground and the other should toggle between 0 and 24v to control the cadillac valve. If you interupt the latter wire it should make the fan turn on. Or inject 24v to make it turn off.

That would work, but would still be susceptible to a TDM failure.

Switches, sensors, valves, etc, can all fail in either the open or closed position. There is no guarantee that a failure will result in the fan being on. The system was designed such that the most LIKELY failure results in the fan being on. Edit: think of it like air brakes on a semi truck. If...

If power is applied to the cadillac valve, the fan will be disabled.

I would interupt the positive, not the negative. Otherwise it might be able to "find" a ground somewhere. The TDM in theory could fail either open or closed, meaning the fan could be stuck on or stuck off. I'm not sure which is more likely. The cadillac valve could also fail open or closed...