In need of an Automotive Electrician

[Nickelcharlie]

in the Fort Worth area. I am electrical incompetent, there I said it. I am in need of someone that can look over my dual tanks and find out why the gauge is not moving on either tanks. Grounds are good and clean, new switching valve, new rubber hoses, new fuel pump, new sending unit in left tank, and rebuilt Carb by Mister Carburetor in Cedar Hills. If you need a rebuild on you carb and in the DFW area I highly recommend him. My truck runs real nice now, just need to know how far I can run! Thanks

[mongocanfly]

Do you have 12v to the gauge?...a fuel gauge us a fairly simple setup....power to the gauge...ground to the sending unit from the gauge....good ground from sending unit to frame.....the float arm acts like a reostat to provide different ohms to ground..
You could have a bad gauge....

[Bandit130]

Originally Posted by Nickelcharlie

in the Fort Worth area. I am electrical incompetent, there I said it. I am in need of someone that can look over my dual tanks and find out why the gauge is not moving on either tanks. Grounds are good and clean, new switching valve, new rubber hoses, new fuel pump, new sending unit in left tank, and rebuilt Carb by Mister Carburetor in Cedar Hills. If you need a rebuild on you carb and in the DFW area I highly recommend him. My truck runs real nice now, just need to know how far I can run! Thanks

I cannot offer up any help on the dual tank switching since I have only one tank. But, had my quadrajet rebuilt by Mr. Carburetor in July 2017 and have not had any carb issues since. Good rebuild job done. Good luck in getting your situation solved.

[Nickelcharlie]

Thanks Greg for the reply. I have the gauge out and will check for 12 volts at the gauge and work from there!

[Dead Parrot]

General rules for gas gauges:

Open circuit in wire to sender = 3'oclock on gauge(F)(this includes a bad ground from sender to frame). Short circuit in wire to sender = 9 o'clock(E).

Rapidly moving gauge: either a bad sender, intermittent connection, bad gauge.

A gauge that doesn't react at all: either fried gauge or bad connections in cluster.

There are many other ways for these to not work properly but this is a good starting list.

I didn't see a new switch in your list. The switch also changes which tank sender the gauge reads. At least on my 75 with the single wire 3 port tank valve.

[hatzie]

It's important to know what year truck you're talking about. Otherwise we're shooting in the dark...
The dash switch handles the gauge on the 73-80 solenoid valve systems.
The 81-91 motorized valve systems switch the gauge inside the valve.

Dual Tank Theory of Operation
Post #1 is the 73-80 systems.
Post #2 is the 81-91 systems.

[Nickelcharlie]

Sorry, it is an 86! I pulled the gas gauge and there are 3 connection points on the cluster! I put my black lead on the bottom connection/ground and the red lead on the left tangs and I got a reading of 11.86. When I touch the red lead to the right tangs, I do not get a reading. Am I supposed to be getting a reading from the right tangs? Thanks for all the help!

[hatzie]

The bottom clip is your gauge ground.
The LH clip is ignition power.
The RH clip is the resistance through the valve switch and sender rheostat to the frame ground. 0Ω=E 90Ω=F

You're obviously getting battery power to the gauge...

Check the sender...
Leave the ignition off and switch your meter to the 100Ω resistance scale. Touch the probes to the Ground and Sender clips. You should get a reading somewhere between 2Ω & 94Ω.
If you get no reading at all you have an open circuit so start chasing the wiring.
If you get 0Ω and the tank has more than a couple gallons you have a short or the sender is knackered.

[Nickelcharlie]

hatzie thanks, my meter only goes down to 200. When you say touch the the probes to the sender clips I am assuming that we are still talking about the 3 clips, right? When I touch the red probe to the left clip I get 2.78 and I get nothing on the right clip.

[hatzie]

200Ω range on a DMM is just fine for this circuit. The sender is nominally 90Ω max ohmage... meaning anywhere from 87-94Ω is in the workable full tank range. As long as you pick a range that's more than 95Ω but not wildly different you should be good to go. 1K is probably a bit much but 200Ω or even 500Ω is fine. It's more critical on analog meters like my Simpson 260 or Triplett 630 to pick the right scale than it is on digital meters.

You're reading ∞ Ω so you have an open sender circuit. It could be the switch in the valve or the wiring.

A common failure point is the frame to sender retaining ring ground wire. Usually the wire breaks off or the tang on the sender bung rusts off. You can test this fairly easily on 86 and prior non TBI vehicles... Clamp a jumper cable from the frame to the pinch weld on the tank.

If that ain't it then check the wiring in small sections...

I keep a handful of Molex GT, Metripak, Weatherpak, Deutsch etc Male and Female terminals in small ziploc plastic baggies in my canvas Meter probe bag along with an assortment of back probes and even some alligator clip test leads to avoid damaging connectors when I test them.

Get a Weatherpak Male terminal and insert it in the B terminal of the valve plug.
Run a test wire from terminal B to a clean spot on the frame.
Check the Ohmage between the Bottom and RH clips. If you're still reading ∞ Ω you've narrowed down where your open wire is. If they read 0Ω or maybe up to 4Ω then the wire from the valve B terminal to the gauge is intact.

You can test the sender wires in the same manner. A is the sender wire from one tank C is from the other. The GM sender rheostats are 0Ω to 90Ω. If you have more than 5 gallons in each tank they should read more than 10Ω... 90Ω is full.
Put your Male Weatherpak test terminal in valve plug tower A and check Ohmage to a clean spot on the frame. Do the same with valve plug tower C.

If the senders to Valve A & C and the Valve B terminal to gauge wire test OK then the switch in the valve is borked.

[Nickelcharlie]

hatzie, thank you very much for taking the time to post a very informative reply. I appreciate it and I know the next person that has this same problem will find this one and the other responses very useful.
My problem may be the circuit board on the back of the cluster. I ordered a new one from LLC, hopefully it will be here Friday!
After I took the gas gauge out and tested the connector tangs I thought I am this close I might as well take it all out and grease the speedometer cable as it was making some noise and replace some of the light bulbs that were not working. So when I put it all back together and started testing my left directional light would not work. When, I pulled the head lights on my right directional indicator light came on!
If my math is right this 86 truck is 32 years old and the plastic making up the circuit board for the cluster may now be brittle and is breaking down causing electrical problems that may be related to the gas gauge not working properly. I'll keep you informed.

[hatzie]

No problem. Glad I was able to help.

If you have corrosion on the bowed copper spring terminals on the cluster plug you can gently clean em up with a soft Pink Pearl drafting pencil eraser. The pink eraser is too aggressive for the thin copper plating on the plastic printed circuit contacts.

You can use the Pink Pearl drafting erasers on the spring clips and lamp socket fingers where they make contact with the plastic flexible printed circuit. It's less aggressive than other methods but still fairly effective.

I've used this cleaning technique to clean corrosion from printed circuit card edge fingers on some of my antique test equipment.

[Nickelcharlie]

I hate it when a thread ends and there is no posted finale results so I am reopening my thread on not getting fuel from my left tank .
I have learned a lot about electrical testing most of which has been after I attempted the repair. I am still not getting fuel from the left tank and my fuel gauge is not working! So here is what I have done:
New fuel pump, my old one did stop pumping! Replaced all rubber hoses, new left sending unit, new tank switching valve, and dash switch.
I have power at the back of the gauge. I have good clean grounds for the wires coming off the sending units.
I stripped a small section of the wire coming from the rt. tank sending unit, the gauge went to full side when I cut the wire in half. I then grounded the wire going into the harness, the gauge went to Empty. Gauge must be working. While the wire was bare I checked ohms on the wire coming out of the sending unit and it read 62ohms which I feel is right since I had been drawing gas from that tank.
Left tank I did the same, cut the wire got full and empty readings and 91.6 ohms when I tested the wire coming from the sending unit. I had just filled the left tank so I am thinking both sending units are working.
I think at one time I was drawing fuel from the left tank after making these changes but I am gettin on up in my golden years so I am not positive. Now I am thinking my new sending unit is stopped up. I can hear the tank switching valve moving when I depress the dash switch but I run out of fuel when I switch to the left tank.
On both tanks my gauge will not move past the 1/4 mark on the Empty side of the gauge. I read an article that states if the tank stays on 1/4 after passing the other 2 tests (ground and open tests) the gauge is bad. I ordered a 100ohm potentiometer to test if the gauge is not working, it should be in Friday. I will let you know the results if the gauge is good or faulty.
I am also going to pull the hoses off the tanks that feed into the tank valve and to the fuel pump and see if air will flow through the valve from the left tank. Then I may know if if the gauge is open along the left tank route.
If anyone makes it through this long read and has anymore suggestions or if I preformed a test procedure wrong I would like to hear from you, thanks.