Temp gauge broken on 1996 7.4 Merc? What did I miss?

[MightyEsquilax]

Alright, I've got another topic I just can't seem to crack on my own. I have a 1996 7.4 Bravo 3. The old temperature sender was junk and wouldn't ohm out so I replaced it. While I was replacing the sender I also tested the function of the temp gauge by grounding the S pole to the G pole and as expected the gauge went full hot. I installed the new sender but now my temp gauge is reading way hotter than I think it normally should. It will creep from below 100 all the way to somewhere in the 190-200 range ALL THE TIME regardless of plane or idle speed. First, I know the boat isn't overheating. I used an IR therm and I have manifold temp of about 130-140 when she's all warmed up. I have good water flow through the system and no leaks. The elbow temperatures are about 105 degrees and 110 degrees (one is always supposed to be a little warmer than the other).

I took ohms readings at the sender and when the engine was all warmed up I got 121 ohms which according to the Mercruiser manual is supposed to be the sender reading at ~ 140 degrees...that tells me my sender is good and the sender is telling me about the same temp as my IR therm.

I admit the next step should probably be to buy some resistors or a pot and test the gauge at the resistance described in the merc manual. I had some 220 ohm resistors laying around so I hacked together a small test bench and I got readings on the gauge I thought were weird. At 220 ohms the gauge read about 130 degrees. At 430 ohms the gauge read just a "smidge" above 100 degrees. Anyone know what the gauge ohms readings really should be? Obviously the gauge needs to match the sender installed and I replaced my merc sender with a Sierra 18-5897 which is *supposed* to be a direct replacement for the merc 97257A.

I'd hate to spend $50+ on a new gauge (if I can find it) and have that not be the problem. Anyone know a convenient way to check the gauge or does everyone think my gauge is toast and I should just spend the $50? Did I miss something else obvious?? Thanks for any help!

 

[thumpar]

The only other thing I can think of for testing would be to run the gauge right off the sender to see if there is a gremlin in the wiring.

 

[NHGuy]

Is your running voltage correct? How's the ground at the gauge and the sender? How's the continuity of the wire?

 

[MightyEsquilax]

Thanks for the replies. All good ideas. Ground is probably ok because all my other gauges are working but will verify. Never thought of the obvious to check the wire from the sender to the gauge. Stand by for more.

 

[MightyEsquilax]

Ground continuity is good, sender wiring rings out good also. I'm leaning on it being a problem internal to the gauge. I could take the sender out and boil the water etc... But if I've got the Merc manual Ohms readings at the sender AND the IR temp gauge telling me about the same temp it has to be something faulty with the gauge. Thanks for the replies I'm open to any last minute thoughts before I buy the new gauge.

Thanks!