Temperature gauge problem

[fozzy]

On my secondhand runabout I have a 90hp 2 stroke merc and have a speedo, tacho, hourmeter and temperature gauge hooked up to it.

The temp gauge has never worked from when i bought the boat, but I wasn't worried as the overtemp alarm self-test sounds when you fire it up, so I figured I was covered anyway.

Last trip out, I noticed that the gauge actually reads backwards. The at-cold needle position is a hair over the "cold" mark and the needle actually dips anti-clockwise as the engine is put thorugh it's paces as I find out, i.e. at flat chat the needle is trying to drive itself off the lefthand side of the clock and at idle it returns to just above the cold mark.

Now there's only one cable coming from the temp sender unit and disappearing into the wiring harness. At the instrument end, I have pos & neg terminals that seem to feed the lamp, as well as a terminal marked "s" which has the same coloured wire as leaves the enginebay attached.

I haven't hooked up a multimeter, but I assume that there would be a decreasing resistance or something on this cable which drives the needle on the gauge as the temperature alters.

How do I check the polarity or whatever of this sender? If I swap the pos & neg cables around on the gauge (these are part of a daisy chain feeding all the instruments), would that have a damaging effect on the gauge or what?

Any clues people??

Dave

[FNQCairns]

Dave I am not in any way an electronics wizz, hope someone can help more.

A simple test for the sender is to earth the wire that leads to it and if the guage reacts with a fill swing and will not when connected to the sender the sender is at fault or faulty or poor sender earth - remove sender and refit it ensuring it earths to material surrounding it.

In regard to your last paragraph, once you have exhausted all other posibilities then taking the chance on reversing the wires at the rear is a no brainer IMO.

I fitted a temp guage on the weekend to my yam but it had 2 wires leading from the sender, it also had (in addition to where the 2 sender wires terminate) an earth at the guage and a power at the guage as well as both +- for the light.
Interestingly when I was rigging this engine I was forced to reverse the colour coded wires on both the start in gear lockout switch at binnacle and the engine fitted trim tilt switch to make them work as designed, all of the above was brand new parts so designed for later model engines.

Could your guage follow the same pattern??

cheers fnq