Fuel gauge always reads full

[gene8084]

I recently replaced the original gauges on my 1974 Formula 180, Mercruiser 888 I/O with Teleflex gauges. (low end price point ~$200).​

The old fuel gauge worked, although over 34 years it was hopelessly deteriorated by the sun. When I draw the tank down 8-10 gallons (30 gallon tank) it still reads full.​

I don't think it's wired wrong (Ignition, Send, Ground).​

• Any way to bench test the gauge or test the sending unit in place short of reinstalling the old gauge?​
• Are there capatibility issues between sending units and gauges?​

I previously pulled the sending unit to clean the tank. It was a nightmare to reinstall because the two threaded plates are not attached to the inside of the tank and simply drop in. Binding in place with temp wire ties to reinstall was a job for houdini.​

Note: I gave up on the Teleflex trim guage because I could not sync it with the old Merc Gen 1 sending unit.​

 

[Mark42]

Re: Fuel gauge always reads full

Might not be compatible with the old sending unit. Most fuel gauges work on a standard 240-33 ohm resistance sending unit. I'm sure that is what is compatible with the new Teleflex gauge. Check the back of the old gauge and see if it lists a ohm range, or maybe on the sending unit be be sure they match. Or test the sending unit with a multi meter.

 

[gene8084]

Re: Fuel gauge always reads full

Might not be compatible with the old sending unit. Most fuel gauges work on a standard 240-33 ohm resistance sending unit. I'm sure that is what is compatible with the new Teleflex gauge. Check the back of the old gauge and see if it lists a ohm range, or maybe on the sending unit be be sure they match. Or test the sending unit with a multi meter.

ok, I see. 240 Full to 33 empty / vise versa.

Thank you. I now know what I'm looking for.​

 

[tboltmike]

Re: Fuel gauge always reads full

The first trip out when I got my boat. Ran some 20 miles and the gauge needle never left FULL.

I thought this is really great. About 5 minutes later, dead in the water...out of gas.

Come to find out it had a switch on the dash for the fuel gauge. Flipped on and the needle nearly bent in half trying to get to "E".

Since wired to the ignition switch.

If found the hard way "F" can mean no juice to the gauge. and can mean no juice to the engine.

 

[Mark42]

Re: Fuel gauge always reads full

The first trip out when I got my boat. Ran some 20 miles and the gauge needle never left FULL.

I thought this is really great. About 5 minutes later, dead in the water...out of gas.

Come to find out it had a switch on the dash for the fuel gauge. Flipped on and the needle nearly bent in half trying to get to "E".

Since wired to the ignition switch.

If found the hard way "F" can mean no juice to the gauge. and can mean no juice to the engine.

Wow, a switch on the fuel gauge? The gauge hardly pulls any juice at all. I see no real point to switching it.

 

[gene8084]

Re: Fuel gauge always reads full

I tested the sending unit with my ditigal multimeter and it's reading 112 ohms (I've probably got between 1/2-2/3 of a tank. So, now I'm really puzzled. Seems like the needle on the gauge should have moved, but the gauge reads full.

Now I guess I can put the old gauge back in. If it works I can either pickup a new sending unit and test with the new gauge before installing or look for a way to test the new gauge without buying a new sending unit.

Any other ideas?​

 

[ziggy]

Re: Fuel gauge always reads full

here's my engineering idea... yer already started on it. you know ya got 112 ohms worth of fuel. assuming mark42 is right. which could be if ya got yer 1/2 to 2/3 tank full. anyways, let her run empty, checking the resistance as the level goes down. if 33 is empty. then fill her up and see what it is when it's full. if the reading continues to change as yer full or empty or in between, then the sender must be working. then i'd find out what the range is on the telefex gauge. if compatible. find out from teleflex how to test the gauge. proceed from there.
i'd take my resistance reading right at where it plugs into the gauge. if i had erratic readings. i'd go back to the sender itself. eliminating the wiring in between those to points....

best i can come up with..

i used to have a van that the gauge didn't work on. i pulled the sender wire out so i could easy check the resistance on it. that was what i used for a fuel gauge.. just kept my meter handy..

 

[gene8084]

Re: Fuel gauge always reads full

Good idea to check the reading at the gauge to see if the wiring is a problem.

Then, since I'm not that gutsy as you to run her down/empty, the thought of drifting the CT River gives me the willies, I will top her off and see if it read 33.

Mark42 was right. I found the paper work for the new gauges:​

1. Empty = 240​
2. 1/2 = 103​
3. Full = 33​

Certainly makes it hard to explain the behavior I'm seeing.

Could be the new Teleflex gauge. I did have to exchange the tach because it was off by 300 rpm at idle, but jeez that would be 2 or 3 bad out of 7 new gauges.​

 

[Mark42]

Re: Fuel gauge always reads full

About 4 years ago I put a set of Teleflex Sportsman gauges in my MFG Niagara. They perform very well, but the gas gauge could be more accurate.

It does seem odd that there would be multiple bad gauges in a set. But stranger things have happened.

Sure the gauge is connected right? Nothing shorted?

 

[gene8084]

Re: Fuel gauge always reads full

Update:​

1. I ran all the wires to make sure I didn't miss anything. Solid!​
2. I have the same ohm reading at the sender and at the gauge.​
3. Installed the old fuel gauge and reads right where you'd expect for the 112 ohm reading. It's gotta be the new gauge.​

Took my second new Teleflex gauge back to the shop for replacement under the warranty.​

1. Tach - off by 300 rpm at idle, replaced under warranty​
2. Trim - reinstalled the old gauge, tossed new on the bench in disgust​
3. Fuel - to be replaced under warranty​
4. Voltmeter - good​
5. Oil Pressure - good​
6. Speedometer - fair, not vary accurate at low speed​
7. Watertemp - good​

I'll let you know what happens with the replacement.​

3 of 7 = 42% defect rate... Must do random testing at Teleflex.​

On a positive note, you really learn how everything works when the root cause of your problem is the most unlikely scenario.​

 

[Silvertip]

Re: Fuel gauge always reads full

Simple Test: Short the "S" terminal on the gauge to the "G" (ground) terminal. That tells the gauge there is zero ohms resistance in the circuit. The gauge should pop over to "F" (full). If not -- the gauge is either not seeing +12 volts with the key in the RUN position, the gauge does not have a good ground or the gauge is simply dead. During this test the sender is essentially out of the picture.

 

[gene8084]

Re: Fuel gauge always reads full

I see.

My problem has been that the gauge jumps to full as soon as the key is turned to run and the needle does not drop as the fuel level does.​

 

[captharv]

Re: Fuel gauge always reads full

Guage tester.
Go to "the Shack" or whatever they call it, and buy 3 resistors: 33, 100, and 240 Ohms. Buy a 3 position rotary switch. Wire the resistors one to each switch position and the other ends together to a clip lead. The wiper of the switch to another clip lead. I made one 15 years ago and its aboard the boat.
Remove the "sender" wire on the guage. Put one clip lead here and the other on ground.
The guage should read:
240 Ohms 0 or left side index.
100 Ohms center scale
33 ohms right side index.
Since the fuel, temperature and oil pressure guages are the same except for the meter scale, this works for all these guages (trim senders are different).
left side index is: 1008, 0 oil pressure, empty gas.
right side index: 240-250 *, 80 PSI oil, full fuel/

Note: Teleflex and Faria guages use the same resistance senders and are interchangable.

Next, loose the switch in the gauge. Not necessary, and is a potential souce of a problem. Wire the 12V side to the ignition switch, the gound to a GOOD ground, and the sender wire goes directly to the sender. Use the KISS method. You guage problem acts like a pinched/shorted sender wire.

The trim guage specs very from manufacturer to manufacturer. You have to ask for the guage that fits X manufacturer Y model engine.

 

[Silvertip]

Re: Fuel gauge always reads full

Then disconnect the sender from the "S" terminal on the gauge. If the gauge still shows full, the gauge has a problem. With the sender removed that gauge is essentially seeing a very high resistance so it would normally show empty. If it doesn't, something is obviously wrong with the gauge.

 

[Mark42]

Re: Fuel gauge always reads full

Guage tester.
Go to "the Shack" or whatever they call it, and buy 3 resistors: 33, 100, and 240 Ohms. Buy a 3 position rotary switch. Wire the resistors one to each switch position and the other ends together to a clip lead. The wiper of the switch to another clip lead. I made one 15 years ago and its aboard the boat.
Remove the "sender" wire on the guage. Put one clip lead here and the other on ground.
The guage should read:
240 Ohms 0 or left side index.
100 Ohms center scale
33 ohms right side index.
Since the fuel, temperature and oil pressure guages are the same except for the meter scale, this works for all these guages (trim senders are different).
left side index is: 1008, 0 oil pressure, empty gas.
right side index: 240-250 *, 80 PSI oil, full fuel/

Note: Teleflex and Faria guages use the same resistance senders and are interchangable.....

Great idea, great info. Thanks for posting!

 

[DaNinja]

Re: Fuel gauge always reads full

Thanks for the info folks!
I got towed off the lake yesterday after the "MO Belle" suddendly died.
The mechanic at the marina was getting ready to embark on some labor intensive work when I suggested that, "just for the fun of it", let's see how much gas she'll take.

1) My fuel guage was grossly inacurate. It still showed full with an empty tank.

2) I knew the guage had a bad history and and wildly underestimated my fuel consumption.

So, I paid my "stupid tax" in the form of 50 gallons of premium marina fuel and now I'm "spark chasing" the wiring for the guage and the sending unit.

BTW...A public Thank-you!! to the kind gentleman on LOTO in the SeaRay Pachanga who went out of his way to turn around and offer assistance.