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In replacing the tanks on my 1997 F-350, I found there's some minor cases of the rear-tank wiring being different from the majority of the trucks out there. First of all, I want to thank Jamie at T-Rex auto parts (go find them on ebay). They were one of the most competitive spots I was able to find the new fuel senders, and Jamie is a wealth of knowledge. I found a solution for an issue on the plug for the rear tank being different; prior to talking with Jamie I was only able to (maybe) find one other mention of this on the internet, so wanted to get some info out there. This is a bit of a long post to provide some context (looks like a damn mom blog really) so:

TLDR: Ford is weird sometimes but the plugs are out there to fix it. Information at the bottom.

To start with, both of my tanks were reading "past full". Looking around I was able to find the wiring diagram and attempted to ohm out the sender. Seemed it was an open circuit which corresponded to the sensor thinking it was "very resistive" (aka high-level) so I set out looking for the right pickup. These apparently are becoming harder to come across, but I found T-Rex auto carries them and at a reasonable price. The descriptions for the senders that T-Rex has are detailed and they do a great job listing what the "correct" pickup and sender should be, based on the tank size and locations. It seems the variations Ford offered resulted in several options so its important to get the correct ones.

At this point, I measured and confirmed my front tank but was skeptical about the rear tank. I was able to find references to both a "19 gallon" as well as an "18.5 gallon" tank. After some more looking the consensus was for a steel tank, there was only one size offered and that was it. I later asked Jamie about this and they had (anecdotally) this to suggest on the nomenclature (This is secondary to the wiring but was interesting so I thought I'd share):

This is another great Fordism - the rear metal fuel tank had the same size/dimensions from 1980-1997. In 1985, Ford increased the sending unit hole size to accommodate an in-tank fuel pump for the gas engines, otherwise the fuel tank did not change. From 1980 until the mid-'90's, Ford referred to it as a 19 gal. tank but then for some reason in the mid-'90's they started referring to it as an 18.5 gal. tank even though the dimensions never changed.......fast forward to today and everyone gets confused/frustrated because most of us keep calling it the 19 gal. rear metal tank!
After replacing the side tank successfully and then transferring everything from the rear to the front (which deserves a whole 'nother post of its own from the series of events I've had with tis truck!) I got the rear tank dropped, new sending unit in a new tank, and then got everything wiggled up underneath. I went to plug in the sender and much to my dismay they were different connectors. Now i had tested the front before all of this but because the rear only had one size/setup I didn't give it another thought. Out it came and I sent an email to T-Rex asking for options and this was the insight they had on the matter:

....you are one of those lucky guys with the "small" plug that no one, including three different Ford guys I've spoken too, can tell us why they, in rare occurrences, used the small plug on the rear tank unit. We have sold somewhere in the ballpark of about 700 of these in the last 4+ years and I'd say we're around two dozen guys telling us about this small plug. I started to keep an excel sheet on the year, make and model to try and figure a pattern but there is none because I have a truck from all four years, the F-250 and F-350 as well as the Cab and Chassis versions. I'm starting to think it was someone bored at Ford just wanting to mess with people.... I know that's not the case and there is some reason for it, but it isn't an obvious reason.
The suggested solution was to cut/crimp the existing pigtail to the new sender which, would have worked but may have left me SOL in the future if it went out again. Hopefully that wouldn't happen for the lifetime of the truck but who knows. I set out to find some answers. Turns out, ford used these plugs in various applications (like many manufactures do). Looking around I've seen them referenced for EGR, fuel sender, cam position, o2, and a few others. I was able to find a nice resource looking for these, It's a pigtail list for (most) of the ford connectors. I've included the link below. Based on that I think the following part numbers are the correct compliments:

WPT-152 : Small plug, female
WPT-1008 : Small plug, male
WPT-171 : Large plug, male

Unfortunately, I've not been able to find the correct "WPT" number for the "large plug, female" which is what mates to the fuel sender unit. However, I was able to find a cross-reference based on it's use. The two below number are just a couple of the part numbers but will get the right direction:

S-677 - Standard motor products
88860522 - ACDelco

I was able to find a pigtail for the (S-677) part and then crimped it using a connector kit for WPT-1008 which mates to the factory harness. The particular s-677 pigtail i found was for a wb02 connection. As a result I had to re-pin the two smaller gauge wires. Otherwise it crimped on without an issue. All the above connectors were easily found having "pigtail" kits, so worst case the two pigtails could be crimped together for similar effect. My choice to use a connector kit for the WPT-1008 was simply to make a cleaner install.

Pigtail and connector list for Ford connectors:

When completing the pigtail I did notice that the wire colors deviate from most of the references i have been able to find. My sender wires on both the factory chassis harness and the reproduction sender were yellow and grey. The factory was a yellow/white and solid grey. Otherwise they were identical; for reference this is the tank and selector wiring (taken from another post here):
Rectangle Slope Font Parallel Pattern


Additionally this is the "small" plug discussed. It matches the plug used on the front pickup and sender:
Hand Finger Nail Wrist Body jewelry
Hand Finger Thumb Automotive tire Gas


Using the pigtail and connector kit, I made a small harness to adapt the two together. As mentioned, I had to re-pin the S-677 pigtail but that was easily done after removing the red plastic pin-lock and re-arranging the pins. The other two cavities were left empty for the time being, if you were fastidious about it these should be plugged to prevent water ingress.

Car alarm Finger Thumb Material property Nail
Automotive lighting Automotive tire Hood Motor vehicle Guitar accessory
Ac adapter Adapter Fluid Adapter Material property
 
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