Fuel Level Sender Getting 8.5 volts from Fuse Block?

by relic7680 Fri Apr 28, 2023 12:21 pm

'76 GMC Sprint 350/TH350

The first time I filled the tank to full since having the car down for several years, the gauge stayed on full even after 50+ miles of driving. Prior to this, partial fill ups were reading properly on the gauge. The sender itself is a Liland Global (that's what was available) and the tank is 100% clean inside.
Did troubleshooting and found the following:
- Sender ground to body above tank is good.
- Circuit integrity (single tan wire) from fuse block under dash to sender is good (passed continuity and ohms testing)
- Creating OPEN circuit to sender makes gauge read MAX OVERFULL and GROUNDING the tan wire makes gauge read EMPTY.
- Voltage available to tan wire at fuse block (harmonica connector) is only 8.5 VOLTS with ignition on and car not running. Connection looks free of corrosion. Voltage at battery is 12.5 volts.
- I have the GM service manuals and they don't show any separate resistor between the fuel level gauge and the sending unit in the tank. The troubleshooting flow charts do not discuss this issue.

Should I be seeing full battery voltage at the fuse block on this circuit? Or is there built in resistance in the gauge itself that is supposed to create a lower voltage here? I have a NOS GM sender on the way shortly, so I could just hook that up outside the car and see what happens....and I also have a nice used, low mile cluster with another gauge in it that I could take readings on. Anybody see this before? Thanks in advance.

by jaybee Fri Apr 28, 2023 11:24 pm

Hello

You could do a volt drop test on the circuit from the fuse block to the sending unit, this is the most accurate way to check an electrical circuit. It MUST be done on a live circuit - meaning powered up. Using a volt meter on the DC volt scale. Put one lead at the power source and the other lead at opposite end of the circuit being tested. The theory is that for each connection, in these cases 2 + 1 for the length of cable / wire. You would only want to see .3 tenths of a volt drop from your supply voltage. Anything more than that and you have excess resistance in the circuit. Remember to take into account any factory connections in-between fuse block and sender. This form of electrical testing can be done on any circuit in the car.

As far as supply voltage at the fuse block, I'm not aware of GM using any resistor from battery positive to the fuse block. Now if your car feeds the fuse block from the starter positive, you might want to check for a bad fusible link at the starter end or corrosion on starter terminal.

As always consult a factory wiring diagram if possible.

Hope this helps

by relic7680 Sat Apr 29, 2023 12:26 am

Already accomplished the voltage drop test; harness all the way up to the fuse block is fine. Fusible link and entire engine compartment harness have been gone through. This is the sole electrical issue on the car at present.

by g3chevy / Mr Pontiac Sat Apr 29, 2023 1:28 pm

Try pulling the fuel gauge out and test the power connection at the cluster. It should read battery voltage. On the back of the fuel gauge itself is a resistor block between 2 pins. This is probably dropping the voltage to the sending wire circuit to the 8.5 volts you mentioned. From your previous troubleshooting tests it sounds like the gauge itself working properly. My best bet is that the float in the tank is stuck. Try hitting the tank when you know its below its full mark and see if it drops.

by bracketchev1221 Sun Apr 30, 2023 9:00 am

I’m having fuel gauge issues in my 70 as well and at a loss.

by zucchi Mon May 01, 2023 9:27 am

You know what you have to do and, trust me when I say this, I feel your pain and share your dread…

You need to drop the tank and remove the sender assembly.

The most common cause of something like this is oxidation/corrosion. I've seen this happen on cars that don't get driven very much and are parked for long periods with a tank that isn't completely full. Moist humid air gets drawn into the tank and the moisture condenses onto the rheostat coils on the sending unit causing oxidation and, in bad cases, corrosion. Once you have the sending unit out (assuming it's not rotted), gently use 000 or 0000 (the finer the better) steel wool to clean the oxidation from the rheostat coil wires. On reassembly, thoroughly clean your ground connection.

Unless the car is a true daily driver (all season commuter car, grocery getter, etc.), the easiest way to prevent this from happening again is to get into the habit of filling the tank all the way before parking the car in the garage.

by bracketchev1221 Tue May 02, 2023 9:44 am

Mine is all new tank parts that don't work. The car didn't' have a gas tank in it when I got it. It had a plastic fuel cell and the gas gauge was not connected. the tank that was taken out of the car was given to me with the sending unit installed. So i put a new tank and sending unit and tested the gas gauge from the back bumper as you're supposed to. I forget, you ground it and it goes to full whatever, but it did. Put 11 gallons of gas in it and it only goes to 1/4 tank. Assuming it is only reading half capacity. I never checked the sender out of the tank. I know my mistake. So I pulled the wire from the sender and bought a new sender and hooked it up. GAUGE READS THE SAME!!! I hooked up the sender from the old tank checking resistance and same thing. So basically I've narrowed it to between the tank and the dash. This car has had the gauges messed with and I have not had the dash out yet, but I know there is a resistor on the back of the gauge, and it may or may not be there. So I plan on replacing the gauge cluster at some point and we'll see where that goes.

by zucchi Tue May 02, 2023 10:48 am

bracketchev1221 wrote:Mine is all new tank parts that don't work…

Oops, I intended my post to be for relic7680.

I've learned that fuel gauges don't always work well with random senders. For instance, back in the 1990s, one guy who I helped out had a rotted sender in his 1965 Cadillac. Not much internet commerce back then and couldn't find the "correct" sender so we adapted a "generic" sender. That thing would read FULL until the actual level in the tank was down to ~4 gallons. PITA.

Have you considered putting an interim aftermarket fuel gauge that works with your new sender until you get the issues with your gauge cluster sorted out?

by 73ss Tue May 02, 2023 7:05 pm

I checked the schematics in my 73 service manual. The gauge itself gets the full 12 volts. It has 3 pins. Ground, +12v, & the sending unit connection which is the tan wire you are referring too. 8.5 Volts on the tan wire sounds about right since the fuel gauge acts like a resistor. The tan wire itself makes its way through a series of connections, (dash harness, rear lamp harness, gauge printed circuit,etc) and ends up at the back of the fuel gauge. It's circuit # 30 in the 73 manual.

I've had fuel gauge issues in the past and it has always been a sending unit ground issue. Make sure you have a good solid body ground.

by relic7680 Wed May 03, 2023 12:36 am

[quote="73ss"]I checked the schematics in my 73 service manual. The gauge itself gets the full 12 volts. It has 3 pins. Ground, +12v, & the sending unit connection which is the tan wire you are referring too. 8.5 Volts on the tan wire sounds about right since the fuel gauge acts like a resistor. The tan wire itself makes its way through a series of connections, (dash harness, rear lamp harness, gauge printed circuit,etc) and ends up at the back of the fuel gauge. It's circuit # 30 in the 73 manual.

I have the '73 GMC Sprint manual, as well as the '76 supplement. This is how I checked everything against what I was seeing on the car. Did all the troubleshooting short of removing the gauge (don't want to take the cluster out again) and removing the sender (don't want to drop the tank again just yet). I have an NOS GM sender on the way, so I'm going to hook it into my harness and see what the gauge does. I also have a nice low mile cluster in storage here that I can test the gauge on for reference. Thanks for your reply....I wonder if anybody here could take a voltage reading at their tan wire to see if they have 8.5 as well?

by bracketchev1221 Wed May 03, 2023 11:56 am

zucchi wrote:
bracketchev1221 wrote:Mine is all new tank parts that don't work…

Oops, I intended my post to be for relic7680.

I've learned that fuel gauges don't always work well with random senders. For instance, back in the 1990s, one guy who I helped out had a rotted sender in his 1965 Cadillac. Not much internet commerce back then and couldn't find the "correct" sender so we adapted a "generic" sender. That thing would read FULL until the actual level in the tank was down to ~4 gallons. PITA.

Have you considered putting an interim aftermarket fuel gauge that works with your new sender until you get the issues with your gauge cluster sorted out?

\

No, the car is in paint right now, so not really using it much to worry about it. I plan on an entire cluster change when I get to that point.

by relic7680 Tue May 23, 2023 10:49 am

Update on this....
The fuel gauge started reading lower as I was driving back from the exhaust shop....went from full to a bit over 3/4. I got my NOS sender in, and found that it read somewhat inaccurately on the way down from full to 3/4 (tested outside car). I also bench tested a low mile fuel gauge and found that it sent 8.8 volts to the "tan wire". This was taken directly off battery voltage with only test leads in between.
So anyway, I'm going to put this to bed for now.

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