Mercruiser 350 magnum gauge issues

[swcurtis]

I just bought a 1988 Larson Senza V220 with a Mercruiser 350 Magnum (carburated) Engine, Alpha One outdrive and Thunderbolt Ignition System about a month ago.

I am having some issues with the gauges. So far I know that the Temperature gauge is reading low, the tachometer doesn't go past 3,000 RPM (even at full throttle) the Trim/tilt gauge doesn't move, and the fuel gauge stays slightly under 1/2 tank with the ignition on (even after putting nearly 20 gallons of gas into it.) So far I have grounded the temp sending unit which pegged the temp gauge, (telling me the gauge and wiring work) and replaced the sender with a brand new quicksilver part. I also noticed that the tach was set to 6 cyl so I switched it to 8 but it still didn't get above 3,000 RPM the next time out.

I am a little curious what the next step is. The previous owner claimed to have taken it to a shop to have all of the wiring redone, which looks to be true, at least from all the gauges and switches behind the dashboard back to the plug on the top starboard side of the engine. Is it possible that something was messed up with the ground in the harness that is making the gauges act strangely? I believe I traced the ground back to the engine and to the Alternator but I'm not positive if that's the correct one. Is it worth completely replacing the entire engine wiring harness assembly? I hate to spend $300 if that's not the issue. Is it likely that there is still a problem in each of the individual gauges' sending units?

Any expert advice on the topic would be greatly appreciated.

Thanks.

Btw, I've been told that I bough a salvaged boat but I'm not ready to accept that it's a bad boat.

 

[HalfFish5087]

swcurtis,

First welcome to iboats.

If I were you I would start tackling this by selecting one instrument perhaps the temp gauge and isolate it by unplugging all the existing wiring. Then run all new wires straight from the battery and instrument, through a fuse on the positive side, and see what you get.

 

[glenn property of pam]

I wouldn't jump to the conclusion that the wiring was messed up as the fuel gauge is not pegged. There's no real surprise that on a 1988 boat with original gauges and sensors that some are not reading correctly and many owners learn to ignore one or two bad readings so buy the time you buy a boat it has messed up gauges.
I would turn the screw on the tack backwards and forwards a few times to clean the contacts inside the tach before setting it to 8 as a small amount of corrosion can mess with the settings. If you can get to the fuel sensor then testing is the best idea as the sender may be bad.

 

[swcurtis]

Thanks for the replies, I have almost all of the problems solved, or know the solution so far. When I pulled off the dash again I noticed when I thought I had rotated the dial on the back of the tach to 180 degrees from where it should be, turned it back and rotated it around a couple times for good measure to clean it up and it worked great. For the temp sender i pulled it out and tested it in a can of heated water with a food thermometer in it.showed to be pretty accurate with what I thought it should be. After taking it out again It ran cold so I think the thermostat is the next step to replace. I filled up the gas tank and the file guage still never left halfway, so I pulled the sender and hooked up an ohm meter. The reading were pretty erratic and I also noticed the float rotation point had too much resistance so I've ordered a new one. It should work because when I put the old one back in for the time being I made sure it was at the top of the tank and it read full. Still working on the trim/tilt but I'm fairly certain a new sender and calibration will solve the truck there. Thanks again for all of your help, I'm really glad I don't have to mess with any wiring too much.