What gizmo/doohickey is needed to make a later model (90 ohm) fuel sending unit work with a factory 30 ohm gas gauge. By gizmo or doohickey I don't mean a newer gas gauge or aftermarket sending unit, I was thinking of some type of an inline resistor? My allowance is limited and I just hate begging my boss, oh I mean wife for another advance!:hm:

90 ohms is what GM has used scince I can remember. A 90 ohm gauge, or a different sending unit is the only fix I'm aware of.

I had heard one time that here was a resister or something you could buy to put in the wire that would chanbe the ohms to the proper reading. I did the same as you for a long time and it worked but it finally went haywire and I could never get the guage to work right after that. I really do believe the guage work for some time.

I just happened across this thread.

I believe you can use a 90 ohm sending unit with a 30 ohm sensing fuel gauge. For it to work properly, you would need to place a resistor of a value of 45 ohms in parallel with the fuel sending unit, i.e. place the 45-ohm resistor across the sensing and ground terminals of the fuel gauge. The 0 to 90 ohm range at the sending unit will now correspond to a 0 -30 ohm range on the fuel gauge.

make sure its is at least of 1/2W or greater...that is a low resitance, which can mean hi current.

Excellent! I will be performing this "mod" VERY soon, seeing as how the last time I took the ol' girl out I ran her out of gas - on the freeway no less!!!

Update:

I added the 45 ohm resistor to the back of my fuel gauge , hooked up the sending unit to the brown sending unit wire and............ my gauge still pegs at full! Time to trouble shoot - I checked the wiring from the sending unit to the fuse block and to the back of the gauge and found it in good shape ( used an ohm meter ). Then checked the other wire at the back of the fuel gauge, it is red and reads 12.8 volts with the key on, 0.0 volts with the key off. I also cheked the sending unit itself and it reads 31 ohms ( the tank is approx 1/3 full ) and the gauge itself reads now reads 24 ohms from terminal to terminal with the resistor in place. I need to figure out why my fuel gauge pegs with the brown wire hooked up to the sending unit or disconected, as always any and all help is appreciated.

Oh, all of the above has been done to my 1966 C10.

Thanks,
Pete

Maybe this will help. Scroll to the bottom and click on the fuel gauge button.

http://www.6066gmcguy.org/Electrical-02.htm

Update #2

First off, I want to say thank you to VetteVet for the link to 6066 GMC guy, lots of wiring info. I tested my gauge by ground the sending unit wire and it does in fact go to empty so I am confident that it is working. I then monitored the ohm resistance as I filled my tank on sunday ( used my multimeter from ground to the sending unit wire ) and the resistance went from 31 ohms to 104 ohms nice and smooth as the tank filled. Soooo, I now know that the sending unit is working as it should. I removed the 45 ohm resistor from the back of the fuel gauge ( only two posts on the back of the gauge, one for the sending unit wire and one for the keyed 12V power ) and checked the gauges resistance - its 47 ohms.

Hooked up the gauge without the 45 ohm resitor and now my gauge pegs at the empty mark! :whine: At this point I pulled out half of my hair, started speaking in toungues, made obscene gestures towards my truck and called it a day! Am I beating a dead horse trying to get my factory 30 ohm fuel gauge to work with this newer (1978) 90 ohm sending unit? Is there anything else I can try to get this pieces to work or do I need to consider other avenues?

As always, any and all help/info greatly appreciated.
Pete

If I'm following you correctly, you had mounted the 45 ohm resistor across the signal post and the 12V post of the fuel gauge. If so, you're not placing the resistor in parallel with the sending unit. You need to mount the resistor across the signal (sending) wire and the ground (not the 12V source). It should work in this configuration. Next time you're filling up and while monitoring the sending unit with the multimeter, place the resistor across the leads of the multimeter (basically between the sending unit wire and ground). It should read nomally between 0 and 30 ohms.

BTW, you were probably reading the shunt resistance of the fuel gauge when you observed 47 ohms.

SUCCESS!!!!!!! Boxrodz, you are the man! I wanted to post an excited and sincere thank you to all of those that have offered there input, ideas, resources and advice. I added a 47 ohm resistor (closest I could find to a 45 ohm resistor) to the fuel gage sending unit terminal, then grounded the other side of the resistor to the back of the gauge cluster by using one of the fuel gauge mounting screws. My truck fuel gauge now reads full with a full tank of gas!:metal:

Good to hear.
Back at you. :cool: