My AC won't work because the compressor won't come on because, apparently, my compressor relay is not getting a TD (rpm) signal. I'm told this is sort of a safetly feature - the compressor relay compares the engine rpm to the compressor rpm and unless they are the same the compressor won't come on. So my compressor won't come on because it's not getting the engine rpm signal. I realize this is an obscure question but does anyone know where that engine rpm signal comes from, and what might be causing the failure. The wiring harness was recently replaced but I know the AC worked for a while after that.