fuel sending unit/gauge questions....?

[alec22]

I've recently replaced all the wiring for the dual tanks in my truck. (the PO had cut it all out around the fire wall) I have two new sending units, and they both read only 1/4 tank when the float is lifted to a full tank. Both sending units are new and I get the same reading from both sides even when I switch the units around.

Here's what I've done so far...I've grounded out both sending unit wires and the gauge goes to empty. Disconnect it and it goes to full. From what I've read these are the correct indications.

Now, the sending units that I'm using are listed for an 80-84, and I have an 85. My truck originally had 3 outlet sending units but since all emissions, and the return fuel line, have been removed I decided to go with the two outlet units. The two outlet units only a feed line and a vent line.

Is there something different about the gauges in an 80-84 that wouldn't be the same with my 85? I can only think the these sending units are on a different OHM range then the orginal 3 outlet unit.

I've got a lot of money tied up in two new tanks, fuel tank brackets and straps, wiring, and a fuel selector valve and new switch. I'm stumped and fed-up too!

Please help me!

[boxrodz]

Alec22 -

The 80-84 sending units should have the same range as the 85's. To be sure, use a VOM (ohm meter) and measure the range as you swept the arm of the sending unit. You should observe readings from 0 to 90 ohms.

Grounding the sending unit signal wire should give you an 'Empty' reading as you mentioned. But, disconnecing the signal wire should give you a past 'Full' reading.

I posted a wiring diagram for you in your post (http://67-72chevytrucks.com/vboard/showthread.php?t=228714) last month. My guess is that it may be a wiring problem, if the sending units are indeed 90 ohm units. Can you elaborate more on how you wired the sending units to the fuel selector valve and then to the fuel gauge?

[alec22]

Thanks for the reply! Sorry it took me so long to get back to the post.

The sending units checked out like they should. Also, the gauge does go past full when grounded out.

As for the wiring, I got the wiring out of another truck. It included the plug, and the wiring all the way up. The only thing I can think of is that maybe the used selector valve maybe messing up the wiring. However, I hooked up each sending unit while by-passing the wiring to the valve. The main gauge wire that plugs into the wire going to the valve is where I hooked each sending unit once I realized there was a problem. After that though I continued to get the same fuel gauge reading (about 1/4 tank) no matter what side or where I hooked up the sending units.

I hope that makes sense!

After all of this I don't know what to do!

[boxrodz]

Originally Posted by alec22

Thanks for the reply! Sorry it took me so long to get back to the post.

The sending units checked out like they should. Also, the gauge does go past full when grounded out.

As for the wiring, I got the wiring out of another truck. It included the plug, and the wiring all the way up. The only thing I can think of is that maybe the used selector valve maybe messing up the wiring. However, I hooked up each sending unit while by-passing the wiring to the valve. The main gauge wire that plugs into the wire going to the valve is where I hooked each sending unit once I realized there was a problem. After that though I continued to get the same fuel gauge reading (about 1/4 tank) no matter what side or where I hooked up the sending units.

I hope that makes sense!

After all of this I don't know what to do!

No problem...

So, I assume you are running a used 3-port valve selector instead of a 6-port, since you eliminated the return line into the tanks. I'm not familar with the wiring for a 3-port valve, so I'm not certain whether the wiring diagram that I posted applies. Regardless, I would try again to directly wire only one of the fuel sending units by connecting signal wire of the sending unit to the fuel gauge lead while grounding the sending unit. Leave the other sending unit and fuel selector valve disconnected. Apply power, then sweep the arm of the sending unit and see what type of readings you are observing. BTW, do you have a VOM (meter)?

[robertish]

Hey,
just found your thread in a search for anything about sender units. I have to replace the units on both the right and left tank of my 1982 sierra 1/2 ton and the factory apparently discontinued the part. Im wondering if you got a custom one and if so where.
Thanks