Hey guys...
My 75 W200 has a dead gas guage. I installed another guage cluster and the guage still won't move.
The truck has a cab mounted and frame mounted tank. I also can't get it to switch from the bed tank to the cab tank. It has a push/pull knob below the steering wheel on the lower dash frame. I used a test light and found that it gets power. The proper spades seem to light up as the switch is operated. The electric valve on the frame has power to it. Maybe it is siezed inside? I have little experience with the dual tanks and all the associated parts. I am curious if the dead guage problem is tied to the fuel valve?
Tomorrow I'll try swapping in a known good gauge cluster from a truck I recently parted out. If the gas guage still shows nothing, at least I can eliminate the cluster as a problem. The wiring looks good and pliable.

check and clean the ground straps on the pickup/sender

you can short the wire at sender to ground to see if the guage goes full tank.

I have often removed the sender and clean the rust off the swing arm and coil spring and get the to working again.

the pick up tube shound have a clip on ground strap grounding the pick up tube to frame if not add one. both tanks.

could be switch valve is stuck.

I took Scratchys advice and tried grounding the sender wire. It did not make the guage move. As usual, I am not so swift with electrical stuff. I can replace parts, but the diagnosis is still my weakness.
I found a few more parts in the attic and shed. I found a guage that worked! The bed tank is full to the cap and the guage shows it. The cab tank was down to about 1/8 of a tank and when switched, it shows that as well. It still won't switch the valve to the cab tank. Rock Auto has an AC Delco tank valve that looks nothing like the one in the truck. Theirs is priced at $60 plus shipping. The prior owner did replace the valve at one time but couldn't recall where he got it. Knowing his frugal nature, I'm sure it was an economically priced unit. It has no name on it and looks like a generic part. I'll check the local wrecking yards to see what a stock one should look like. If it is cheap enough used I may get it.
Having a working gas guage and ONE tank isn't bad. Thats how every other vehicle I have is right now. Having the extra tank is a luxury. I can wait on buying a $60 tank valve now that the guage actually works.

http://products.pollakaftermarket.com/it...en-valve/42-159 seen this? they have others as well. easily hooked up, once you decide what you need.

Hi Craig. Is you valve like one of these? I have the shop manual for my truck that shows the switch wiring diagram. If yours has the push pull switch it should be the same. You say your getting power to the switch then something's wrong with the switch.

Mine looks like the bottom one.
I get power to that threaded stud but it doesn't make a noise. I tapped it with a Ford tool and it still did nothing.

The top one looks OEM the bottom one does not. I have an OEM valve that works if you need it. Make sure the valve is getting a good ground. They do not click very loud and you can bench test it on a battery to verify it switches.

Here ya go Greg. I got mine here.
Remember, it's an easy test to see if they're working. Just ground the switch on your - battery post and use a hunk of wire to go to the + post and threaded part of the switch. You should hear an audible click when you touch the + post with the wire!
Oh, one other thing, OEM (used) is fine. But unless they're new in the box, I found out that after 30/40 years on a truck, the plastic starts to leak. That's why you see so many of them with epoxy all over them. I finally just went and got a new one at
http://www.oreillyauto.com/site/c/search/Fuel+Tank+Selector+Valve+-+Universal/N2000/C0025.oap