1972 K20 in cab tank fuel sending unit question

[Threerun]

Hey all.

My truck is a 1972 K20 Cheyenne LB, 350/TH350. My fuel sending unit has taken the long nap on me. Full tank of fuel- gas gauge reads past 3 O'clock. Never moves. Disconnect the brown sending unit wire, run a jumper from the brown lead to chassis ground- straight to empty...

Every on-line parts place shows a '72 automatic trans sending unit with two ports, and manual transmissions with one port. However the unit on mine (auto tranny) has only one port. My tank is not the ECC model..

I've done a search on here to just how a two port would be plumbed via a diagram, but I've come up blank.

Any ideas? Should I just get the sending unit for a manual tranny and be done with it?

[Threerun]

Oh and the truck has a Holley carb on it. The quadrajet was replaced by the previous owner about 4 years ago.

[HO455]

Have you checked the sender ground? Here is a thread from the Master to help.

http://67-72chevytrucks.com/vboard/s...d.php?t=640615

As far as replacement I would go with the single port. That would make things easier if it comes to replacement. I recommend finding the problem before buying any parts. For the most part senders don't wear out, but fail from rust or corrosion. For most other issues they are relatively easy to repair for not much money.
I may be wrong but I think that all 72's had at least 2 ports. They had non vented caps and charcoal canisters on all 72's. There are a bunch of threads on them. That yours is a single port may indicate the emissions were deleted at some point in the past. It was a common thing to do back in the day.

[Threerun]

I’m pretty sure it’s toast. When i connect the brown sending ground to a good chassis ground via jumper, the gauge goes below empty.

I have a full tank of fuel in the rig.

You maybe right on the charcoal canister. I need to find an emissions diagram to verify. I know lots of folks that yanked them out. Funny thing it simply adsorbed fuel vapor. Not really parasitic at all.

[Jcfcamaro]

Pretty sure the brown wire is not ground. The tank/sender should ground its self to the cab/body. The brown wire goes to the gage.

[Threerun]

yeah misspoke. Brown sending unit wire.

[HO455]

When I said check the sender ground I meant leave the brown hooked up to the sender, then run a separate wire from the body of the sender to a known good chassis ground. This will allow the guage to work if the sender has become isolated from the chassis ground due to dirt and corrosion between the sender and the tank, and between the tank and the cab.

[notsolo]

Possibly the 2 port version was a vent line.my 71 had a in cab vent up in the side behind the door.

[Ol Blue K20]

3/4 ton was a single port and used a vented cap.

[Threerun]

Ah! Thank you!

[cljimmy72]

Not meaning to bring up a dead horse (older thread) but i have the same issue in a 1972 GMC K2500. Single tube coming off the sending unit. Sure had me confused looking up replacements with the automatic transmission showing a vented system! Thank you Ol Blue K20 for clearing that up. My sending unit left town due to rust and corrosion! Now my question is with all the Chinese garbage flooding the market what have you guys found to be a decent replacement. This is in an all-original show truck that sees very little road time. Thanks!

[cljimmy72]

No one here has replaced their sending unit?

[Sheepdip]

Originally Posted by cljimmy72

No one here has replaced their sending unit?

I just did mine and got it from LMC. Nice unit, no red flags on the construction, the only minor issue, which I personally don't consider it an issue, was the angle of the dangle of the tube coming out of the tank was off just a smidgen but was very easily tweaked to fit for a perfect alignment to the existing 3/8 fuel line that goes out thru the floor to feed the fuel pump.

[cljimmy72]

Sheepdip: Was yours a single feed 3/8's line and not the vented one with two tubes?

[Sheepdip]

Originally Posted by cljimmy72

Sheepdip: Was yours a single feed 3/8's line and not the vented one with two tubes?

Yes I purchased the single feed 3/8" line as I have no evap lines and I also run a vented in/out cap.

[cljimmy72]

Thanks: Same as mine. I'll give it a go.

[FirstOwner69]

Whatever you buy read this regarding replacing the brass float with the Jeep plastic float mentioned before installing the new sending unit. My new one with a brass float leaked within 2 months.
https://67-72chevytrucks.com/vboard/...ght=jeep+float

[cljimmy72]

FO69: Thanks for bringing this to my attention. I dislike aftermarket products, and this is one reason why! That also is why I was asking who had good results with what brand!