TOPIC: Gas gauge problem.. How to trouble shoot?

Hey everyone...
Spent the weekend at the lake with my dad and played around with the new pontoon boat. The gas gauge doesn't work. (see photo below... lol) Anyway, It shows 3/4 tank, then it will drop to nothing for a while. Just like the key is off. It will down below E just lile it has no power then jump to 3/4 tank. Even if you don't have but a fourth of a tank. Then quicky go back to below E. Seems like toward the end it never did anything at all.

My question is how should I go about troubleshooting this thing to see if it's the gauge, sending unit, ground, etc? I've never had to deal with something like this so any tips or help will be greatly appreciated!

Thanks!
Doug

Is the top of the tank accessible? If so pull out the sender...hook a ohmmeter to the wires and slide it through its range of motion. The resistance reading should go down as the float travels higher. If it has a maker on the sending unit, you might be able to look up specific reading ranges online somewhere. You could also just fill it full of gas while checking the resistance if you dont want to pull it out but will make it difficult to test more than once unless you feel like draining the tank, filling the tank, draining the tank...

If your seeing a smooth and steady decrease\increase in resistance as you go through the range of motion, its probably the gauge...or a bad connection.

My money is on a bad sender unless theres a shotty wiring job or the gauge looks a bit on the weathered\old side of things.

Doug,

What are you doing pushing on that rope?

As for the fuel gauge, they all use a standard resistance. A range of 33 to 240 ohms comes to mind. Regardless, short the wires together at the tank & you sould have FULL. Disconnect them & you should have EMPTY.

Because you have empty or 3/4, I vote for the sender being bad. If it is the capacitance type (electonic with no float arm), you can't really test them.

Senders all use a standard size hole & screw pattern. Any replacement will work so long as the tank depth is correct.

Jim

Intermittent/ bad ground?

Thanks guys.. After testing it looks like it's going to be a bad sending unit. This is a pic of the sender I have. It has no float on an arm, but instead a float the is inside the tube and there is a circuit board with resistors all the way up the board. No, I know I can replace this with a standard float/arm sending unit, but I am worried about how accurate it will be.

The tank I have is a triangular one that fits inside the triangular motor mount on the rear of the pontoon boat. My main concern is the bottom of this mount is tapered upwards as it goes forward, and the gas tank does too best I can tell. Also the tank narrows as it goes forward. I've attached a pic showing the motor mount/tank setup I have. Sorry it's the best one I have right now. Anyway, how accurate do you think a standard arm/float type sending unit will be? I would rather spend $30 instead of $80, but if I need the tube type to be accurate I will go that route. Ideas? Thanks!
Doug

My defective sending unit:


The universal arm on a float type I am considering using:

Because your tank is irregular in shape on a horizontal plane, as you burn fuel down to 1/2 its depth you will have most likely burned more than 1/2 its volume. That is going to cause the quantity gauge to read irregular (or more correctly) non lineally regardless of capacitance type like yours or float type. The main problem with a float type will be if the arm will have free travel or will it contact the inner surface. All these gauges are just a rough guess at best.

There is no way you will get a truely accurate reading on an irregular shaped tank. Just for comparison, the aircraft I used to work on had multiple capacitance type probes (4 or 5 per tank) that all worked together to get the fuel quantity to read accurately due to the complex shape of the tanks.

Jim

Two questions Doug:

1 - Do you have, or can you get me the approximate depth dimensions on the tank? Also how much room is the swing arm going to have. (We carry several gas gauge senders where I work, I'd be glad to check on getting one at wholesale $ for you. We may even have the O.E. type sender, I'll check on this later today when I get to work.)

2 - What year model Force did you say that was? I've got a Clymer "official shop manual" I don't need any more that goes to '99 150HP you can have if you need it.

Very nice pontoon, looks like it'll make you a good family boat once you get the few little bugs sorted out.

Hey Mark,
I put a call in to a Crest pontoon dealer to see if they can match this one up and give me a price on a direct replacement, but I imagine it's going to be sky high coming from the dealer. The tank is about 10" deep I believe because the sending unit is 9 1/2" deep. I'll get down and check it today and get you some better measurements. There is a ton of rom for a float arm, probably 18" inches, so clearance shouldn't be a problem.

I do have a new universal sending unit here in the shop for my roadster pickup gas tank which is 10" deep so I may just stick it in and see how it goes. Especially since the roadster won't be done for a couple of years. lol.

The motor is a '98 Force 120. I would love to get my hands on a manual. Check your email! Thanks,
Doug

Ok, just measured the tank and it is exactly 10" deep at the mounting point for the sending unit and 12" deep at the rear of the tank. If I can get a sender that reads empty at about 9 1/2" deep that will leave a tad of gas in the rear for backup. The original sender was 9 1/2".

I tried the sender I had on hand for my hot rod, but it was ordered to work with '47 Caddy gauges and never did read right on the pontoon's gauge. I guess the ohms are different.

Doug, the manual is yours, I'll check my personal email when I get home this evening for your home address to send it to. I'm on my lunch break now, there are several different ohm ratings for the gas tank senders. We carry several that are adjustable depth, shouldn't be a problem there. Can you find out which is right ohm range for your application and send the info to my work email?

MArk,
How do I go about finding out the ohm rating? Will it be on the gauge?

Just so y'all don't think I left classicfins hangin', we were unable to get him one out of our inventory. He did, however find one elsewhere and has it ordered.

Thanks for the great effort Mark!

No problem buddy, glad to do whatever I can to help a fellow "Glassaholic". I'll try to get the manual out to you today.

Update: Manual shipped