Apparently the guy I bought my 1986 6.9L from decided to bypass the water/fuel separator filter, most likely to stop air from leaking into the lines. From what I've read, that is a very common issue with those filters. I know the sensor works and that the truck has one because, now that I've reconnected the electrical plug, the "water in fuel" light briefly comes on when I start the truck, despite nothing being connected to the filter. It seems stupid to keep the filter bypassed and potentially keep water in my fuel. Thus my plan is to eliminate the old filter and replace it with a newer style filter. I'm going to use a Wix 24770 Remote Mounting Base with the drain-able fuel filters that fit a 7.3L. But I need help picking the correct filter to use. I've selected a few options of drain-able filters with the same gasket dimensions. However, there is an option for a sensor port with a 1/2-20 thread or a 7/16-20 threat. I need to know what the thread of the original 6.9L's sensor is. NOT the thread on a 7.3L. That way I can install a filter that is compatible with the sensor that's already on the truck. Does anyone know what size it is?

 

It's like a self check to make sure it's working. Perfectly normal for it to come on for just a second.

What I did on my 85 was remove the top fitting with the pull pin and retap for a 3/8npt plug.

Fitting out of the bottom just pulls out, retap for 1/8" npt and a ball valve.

Problem solved.

 

IF you are talking about the sensor that screws into the top of the separator that was mounted on the firewall .....Then it will not work with the filters that use the screw in probe.

EDIT:
IF you use the one that screws into the bottom of the filter F4TZ-9S281-B then you will also have to add the Solid State Switch for it to function...I don't recall the Thread size but I think it was 1/2 X 20 UNF don't quote me on that I would have to search it to be sure but since you have the sensor then why don't you just measure it?

Here is the PN for the Solid State F4TZ-9S281-A