Fuel gage or sending unit problem?

Bob K · May 30, 2006, 01:47:06 PM

My fuel gage recently stopped working - pegged on E. After some use, it occasionally popped up to the correct level, about 3/4 tank. My first thought was that I bumped the wire at that the sending unit when I replaced the exhaust hose earlier this spring. Today I measured the voltages at the gage while it was pegged on Empty, and I measured, 12.5V (battery voltage), 0V ground, and 10.5V. I assume this 10.5V is from the sending unit, so I am now thinking the gage itself is the problem. Tapping it didn't change anything. Has anybody ever measured the sending unit output voltage? How bout price and availability of a new gage?
Thanks,
Bob

Stu Jackson · May 31, 2006, 12:16:50 PM

There's another post on this topic within the past week.

Many of the issues have to do with, first, the ground, then the sender.

I found a completely disconnected ground wire that had failed at a butt connection on another C34. Simple fix. Check the ground wiring first.

I also, and this is a repeat, use engine hours to determine how much fuel I have left and how much I need to fill up with. We use 0.423 gallons per hour, this over almost eight years. I keep track on a spreadsheet. I refill every twenty hours with somewhat less than 10 gallons. Our fuel gage has been out of whack for all the time we've had the boat since a month after we bought her in 1998.

Please also be aware that the fuel gage is not linear like your car. The tank is narrower at the bottom, and since the gage is linear, the last quarter of your use is gonna go much faster.

Bob K · May 31, 2006, 05:51:50 PM

I checked ground at the gage, and it seemed OK. I was hoping to diagnose this without having to check things at the sender, but will probably need to dive in there anyway. Thx for the info.
Bob

Ron Hill · May 31, 2006, 06:52:09 PM

Bob : Fuel gages are like light bulbs. If the ground wire is OK you may need to replace the sending unit.
See my Mainsheet article and get a Tempo unit.
I use a simple .45 gal/hr fuel consumption, but unlike Stu never let it get below 3/4 tank.
If you ever flown 15 minutes on a 20 minute warning light over enemy territory, you don't feel easy when the gage moves too far off of FULL!!

Ken Heyman · June 01, 2006, 05:19:24 AM

Stu/Ron,

How would one's estimated fuel consumption change, if in similar wind/sea conditions, we steam at 1500 RPM for 1 hour vs. 2800 RPM for 1 hour? Coincidentally, I was just working on my "consumption" spreadsheet.

Ken

Bob K · June 01, 2006, 08:40:30 AM

Ron,
I usually cruise at 2000 RPM. I have only filled the tank 2x since owning the boat, but I think my consumption rate is quite a bit higher. M35 engine may be one reason. Will doublecheck. Do you remember which issue of Mainsheet your article was in? I still think I should be able to diagnose this problem without going further than the instrument panel. Knowing what the "correct" voltage or resistance range for the sender would help, but I will likely end up testing the meter alone by subbing a variable resistor in place of the sender.

Stu Jackson · June 01, 2006, 10:17:22 AM

FUEL CONSUMPTION

Ken

Probably not much different. The reason is that our 0.423 gallons per hour are based on our usage which consists of a very short ride out from our slip to the estuary, where we rev up to cruising speed at about 2000 to 2300 rpm. So our calculated average fuel use is based on using the boat at cruising speed almost all the time. Therefor, lower rpm should result in greater fuel conservation.

tstrand · June 02, 2006, 07:46:28 AM

I'm having the same problem right now, but I can't access Ron's Mainsheet Tech Notes article. Actually it shows up in the Tech Notes index in both November 2003 and November 2004. The '03 won't open and the '04 is a different article. Everything else opens fine.

Any ideas?

Tim

Stu Jackson · June 02, 2006, 08:17:20 AM

Tim

You are right Nov 2003 is broken Nov 2004 has the wrong index. We are working on a solution, plus a few other notes that are not quite correct. Won't be immediate, though. Anybody in urgent need, email me at mraquaq@aol.com

Ron Hill · June 02, 2006, 07:27:21 PM

Guys : I first should have said that my .45gal/hr fuel consumption was for an M25XP (23hp) engine. The M35A will consume more and the M35B will consume slightly more than the A. As I recall the M35A is about .55 gal/hr.

The fuel consumption is an average per hour based on some idle and mostly operating (80%) rpm at a high torque setting.
I believe that some of the Universal spec sheets have curves that define fuel consumption vrs. engine hp. Let me look when I get home and if I have them I'll post them on the web site.