Fuel gauge help please

First time poster on this site. I bought an 82 Seca 750 last week and although it's in very good condition, I've discovered an irritationg issue with the fuel guage. At rest with the key on, the guage slowly goes up to 3/4 and bounces on and off to full, which is where it should be with only 80 kms on a full tank. When on the road, it will occasionally go down to empty, which triggers the warning light, then it fluctuates between empty and 3/4, but of course, not consistantly. I thought it was a dirty ground so I've taken off the tank, and made sure the ground wire was well connected, as well as the connection about 2 feet down the line. My question is - assuming the ground wire is ok, will the sending unit either work - or not work. Seems the electrical componant is a simple movable resistance scale, and depending on how much gas is in the tank, it changes the resistance. I can't see it being sporatic. - Any other ideas?
thanks, Rick, in Duncan, British Columbia.

 

Have you ever SEEN one (the fuel sending unit that is?)

"Primitive" is a compliment. Your sending unit could very well be flaky.

From a 550 Seca (similar technology



There's a real basic rheostat inside the metal box; in this pic, the lines and arrows are in regard to the tiny signal lead wire and the fact that its insulation had crumbled away. What you can't see is a long rod with a float on it, very similar to a toilet float but smaller, attached to the other side of the metal box.

Need I go on?

 

Inside view


And yes the wiper having a poor connection in certain spots can cause erratic readings.

 

NO FAIR.

Yours is all clean and shiny. Mine's original.

 

Well, I shouldn't have, but I took the tank off again and double checked my ground wire, and even added a second ground 'just in case. Just letting it run for a few moments created the irratic readings, (so I went wire hunting again and found the power wire to the sensor. My multimeter goes kinda crazy close to the engine). The 'energy' wire looks like it's only putting out 6 volts ( hey - a great way to get power to the battery testing sensor instead of rewiring to a new resistor), so I thought - must be a bad power connection, so I ran a good hot wire (12 volts) to the sensor and fired up the curcuit. I immediately heard what sounded like a wire cooking INSIDE the almost full gas tank, so 3 miliseconds later I turned off the key. Figuring that I'd fixed it so that no one else could fix it and trying to figure out how I could blame my wife for it - led me to decide I needed a drink. 10 minutes later I hooked up the origional wiring, turned on the key - and the frigging thing started to work again, if only for a few minutes, I I guess 12 volts @30 amps didn't fry the system. So - any idea if my local yamaha dealer can order a new one? Worse case senario is I get an answer to my other question - how many kms to a tank and I'll just use the trip meter in the future. - thanks, Rick

 

NO your Yamaha dealer can't order a new one, sorry. If only it were that easy.

Your Yamaha dealer can't order a new whole lot of anything, except expensive consummables.

eBay will probably be your solution; or take it apart and see if you can fix it.

Check with chacal (XJ4Ever, logo/link in banner at top of this page) and see what he might have. This particular bit is "unobtainium." You might want to post in "Sell, Trade, Swap, Wanted."

Oh, and have you checked the rear brake for delamination? You can't go by feel, you have to look, and it can wreck you: http://xjbikes.com/Forums/viewtopic/t=15874.html

 

That 6 volts was the wire from the gauge. You have just ran full battery current through the sender.

New old stock senders appear on ebay on occasion.

To work out your range, fill up and ride a set distance and fill again. Record the Klms and liters. Work out how many Klms you get per liter and multiply by your tank capacity (17L ???)

 

Really, all things considered, a fuel gauge on a motorcyle, at least an '80s motorcycle, is only for entertainment.

I've ridden street bikes for well over 40 years; my 550 Secas are the first bikes I've ever had with fuel gauges. Pessimistic little SOBs at that, although one goes to red exactly when I have to go to reserve, every time, which is downright spooky. The other one is just a foolish cartoon.

Highly unnecessary. Cute, but only marginally useful on a good day.

Don't flush too much money after this one. Do your brakes instead. (Stainless lines, the old ones gotta go anyway.)

 

Yeah, I know how (un)accurate they can be. I'll probably pull the plug on it and short out the wire so that the computer screen thinks it's full and doesn't keep flashing at me. My question in my other post about mileage is purely for a rough idea. I've always got the reserve switch, but if someone could tell me they get 200 kms on a tank that would be a starting point. My 'wing usually outdistances all my other bike riding buddies so I guess now I'll be stopping for gas when they do ( if not sooner)!

Thanks for the help guys ( and gals)

 

I get about 320k's from full to empty on mine, that includes a bit of turn it on here 'n there.

 

OK thanks for that. That will give me a starting point at what I can expect.

Anyone know how long Yamaha used that sort of fuel sender? We've got a few bone yards around here and if I can track down a rough idea what bikes used them, I might have better luck finding one.

 

Those sensors are getting hard to find. I looked all over for mine and ended up buying another tank that had a good one in it. I did a little experimenting with mine and had to bend the arm so it was more acurate.
Since I removed the old stock gauges (the Seca one with all the pre-check lights in it) I went with an automotive type gauge that worked at the same resistance as the stock gauge. It actually worked backwards so I had to change the E and F around so it works correctly now.

 

You do know you could have turned the windings over in the sender?

When I rewound my sender for the koso dash, I had to reverse it for the same reason.

 

Well, my '81 Seca 550 has a fuel gauge; and a quick peek through the (USA) parts fiche shows various bikes up through the early '90s with similarly-appearing sending units.

But there's the rub. Similarly-appearing.

We discovered a couple years back, when PainterD and a couple of other members, including myself, were fiddling around with fuel gauge sending units, that although similar in construction, the ELECTRICAL specs differ from bike to bike.

Some are high resistance full, some are high resistance low. There seem to be at least three different resistance ranges, too.

You're probably going to need to narrow your search to the same family/series of bikes.

 

Yes, it would've worked to turn the windings around in the sender, but just changing the letters around on the face of the gauge was a hell of alot easier.
I've found a gauge that works opposite than mine so now I will have one that works right out of the box.

 

I was at the local yamaha dealer today and they gave me the part number for the sending unit - and the price $96.00 - then told me it was 'unavaiable'. I took the bike out for a long run today and strangely, the fuel guage settled down after about 20 kms and didn't flucuate at all. I'm wondering if the flash of 12 volts and unlimited amps through the system shook any dirt off the sender, or if maybe it just had a flat spot in the reostat that is only effected at a certain gas level in the tank. In any event, both of my 'new bike' problems ( also the self cancelling turn sigs) have turned up to be non issues so I think I'll stop looking for parts to fix, take apart, break and fiddle with and just ride the damn thing. - Rick

 

I had the tank off to do the carbs anyway, so I removed the sending unit. It was actually very clean. My ohm meter showed that it was working, except for when the float was at the top of it's limit, then the meter was showing no connectivity. I loosened off the small screw which holds the assembly together and put a bit of pressure on the spring, tightened up the screw and rechecked the multimeter. - Good connection all the way though the float's range of motion. I can also feel just a bit of vibration as the arm moves over the windings.

 

Yes once they're shot, your going to have to get an act of congress to find one that's good. A rare part indeed and I can't seem to find any other one that would work with these bikes.

 

If you have the Skill and the Patience of a Saint, ... you can "Rehabilitate" a Fuel Tank Sending Unit.

The "Rheostat" is very basic.
A "Spoon-shaped" Contact which moves across a Simple "Grid" of Bare Wires which are progressively wound on a small Field Unit upon which the Spoon glides across.

Undo the Locking Tabs
Replace the Windings if the Spoon has worn through them.
DRESS the underside of the Contact Spoon to make it GLIDE without catching on a wire.
Test the Float
Make sure the mechanism pivots.
TEST for variable resistance using a Multi-Meter.

If you have a friend that ties his own Flies for fishing, ... have him help.