Dual tanks. Gauge only reads passenger side.

[Bronze Knight]

Hey everyone. sorry to post another dual tank thread but I did a search and found nothing on this particular issue.

So this is all a bit wonky but I think the answer is that I need a new fuel tank selector valve.

So here is what I've done, when I got the truck both tanks were leaking, rust holes in both. I dropped both tanks and put one new one in with a new sending unit, after all that I had no gas gauge I set the issue aside to work on other things because the truck was running and that was the important thing at the time, that was about last August.

Well I've recently been trying to figure out why I don't have a gas gauge, here is what I've done so far.

• I pulled my instrument cluster and pulled the gas gauge out, put some test leads on a 12v battery, the gauge moved from one side to the other depending on where the + and - was applied and it stayed where ever it was when power was removed.
• Put in a new tank selector switch I got the non-momentary version from an 87-91 truck it does not say "Depress Fully". I read (over on the other forum) that it was ok to do this but I did have to modify the switch a little bit to get it to fit into the 1985 connector.
• I dropped the drivers side tank (only one on it right now) today and removed the sender. With the sender plugged in and with a good clean ground to the frame I let it dangle from the truck and looked at the gauge, it still read way over full. I moved it back and forth holding it at the middle for a minuet for the gauge to catch up but it did not move.
• I repeated that experiment on the Passenger side moving the sender over and suddenly the gauge started reading empty (again the sender was just hanging off the truck so that was correct. I then moved the float by hand and could watch through the window as the gauge moved when I moved the float.
• I thought that was a bit weird so I moved the sender back to the drivers side and then unplugged the 6 pin connector from the fuel tank selector valve I got a multi-meter set it to 20 ohms and put the + on pin A and the - to the frame. It was reading 90 ohms at a full tank (float at the to of its travel) and close to 0 ohms at empty.
• I did this again wit the sender on the passenger side, same results.
• Suspecting the fuel tank selector valve was bad I got a 12v power source and put + on pin D and - on pin E i could hear a wirrrrrr *CLUNK* of the valve moving I then swapped, put - on D and + on E, again I could hear wirrrr *CLUNK*
• Then I reconnected the 6pin connector and got my lovely assistant to flip the tank selector switch while I sat under the truck to see if I could hear it when the switch was flipped, but I heard nothing.
• Pulled the 6 pin connector off again and set it to drivers side tank put the multi-meter back on set to 20 DCV with + on D and - on E and it was reading 3.4v set it to the passenger tank that changed to -3.4v

So it seems to me that I a combination of issues still. It could be that the tank selector switch is bad which is why it is not changing which gas tanks information is being set to the gas gauge. OR I have some other wiring issue preventing me from getting 12v at the 6 pin connector but I don't know if its supposed to have 12v at that connector or not.

At this point I'm thinking I'll get both a new fuel tank selector valve and the momentary 81-86 tank selector switch put them both on and then see what it does then.

 

[Bronze Knight]

Did some more testing with the new parts today.

It looks like I had three issues,

• the switching valve was bad, the tank switch part was kind of working, but only sometimes.
• I may have had a bad/intermittent/partial ground from the switch in the cab to the cab ground bus bar. If so this would be why I had only 3 volts at the 6 pin connector. after taking off the crimp connector and soldering it properly I had 12/11.3v at the connector.
• the 87-91 non momentary switch (BWD part# S9020) did not work with my new valve (BWD part number# FSV3) with that switch connected the new valve got hot to the touch and stopped reporting the gas level to the gauge after being plugged in after about 8 min. I unplugged it let it cool down swapped the switch out for the 81-87 momentary switch (BWD part# S743) and let it sit for about 10 min switching it every now and then and it was still working and felt cool to the touch.

So I now have a working gas gauge that swaps with the selected tank. Or at least I did when I had everything rigged up with alligator clips and whatnot. Finished the day with taking the old valve off the truck. Putting new hoses on and then I'll have to get the one tank I have put back on, that'll be fun as its 1/2 full. Still saving money to get the other tank, just can't swing it this month.