I'm getting a variable voltage reading at my fuel pump relay. Truck will not start, while looking under the hood I could hear a tapping sound coming from the fuse/relay box on the drivers side (with key on). Found the noise coming from the fuel relay. Change out the relay with no success. Checked voltage where the relay plugs in, found 12 volts on one leg. But found voltage changing from 11.5 to 8.5 volts on two of the smaller pins. I figure this was causing the relay to make the tapping noise. I understand the voltage for the relay is first sent from the PCM. I just had the PCM checked out and its ok, Have had the ECM checked out and its OK, Just put new airdog fuel system on and it had been working fine. Any Ideas? Tried search but keep getting error

 

if you bypass the relay control, jumper the Airdog relay. do this for testing only. it leaves the pump running all the time. does the truck start.

if not then you have something else wrong.
if it runs check for bad grounds.

do you get the same voltage if you bump the starter? are you grid heaters on? if you disable the grid heaters what's the voltage then?

-dkenny