If your WDT output is 5 Volts DC, your timer isn't timing at all. Sounds like your output line is tied to your 5 volt supply. If the timer was operating correctly the output voltage will be around 2.2 to 2.5 volts whether you measure it as DC or AC voltage on your multimeter. If you reversed the order of resistors R1 and R2 on the timer circuit, your output frequency will change to around 290Hz instead of 150Hz, but your output voltage should stay close to the same. It seems that your connections to the ECU are correct based on Jasper's picture, so I think something is wrong with your timer circuit.