Temp gauge mystery

[tgr23]

So this is a new boat to us, 1987 Cat 30 (universal M25XP) this year.  I did some upgrades this winter before we splashed her and although everything is working great the temp gauge is not.  Here's where I'm at.  The temp readings were seriously erratic before I started "fixing" it.  I won't go into all the details but the gauge finally had a good laugh and the needle just dropped off. So that explained a lot.  Sigh.  So at this point I have a new sender in (with a grounding strap), new thermostat (checked that it was stamped for 160), and new gauge.  Now when I've got the engine warmed up the gauges reads about 182ish.  I also picked up a laser infared gun from HF to check from the outside.  It reads 154, pretty much everywhere, tstat cap, top of engine where the coolant is filled.  The gauge has been tested and responds full on when grounded. Sender is the right ohm range for the gauge (Seastar Solutions Eclipse).  The only thing I've not really tested is the ohms at the sender.  Does anyone have the readings I should look for?  And does anyone have any other ideas what could be going on?

Could it be that the IR laser is not accurate and the temp really is in the low 180's, but that leads me back to the tstat is stamped for 160.  Tstat had a green strip on one of the frames that points up, is that supposed to have a specific orientation? Boat lives in salt water in NY.

Thanks

[KWKloeber]

T-

The ("Stewart-Warner") standard that teleflex gauges use
"approx"

75F ~ 600-800 ohms
212F ~ 55 ohms
240F ~ 33 ohms
At 180F, gauge can be off +/- 15 F.

Forgot to ask ya,what # is stamped on the sender?
004, 61, 15000, 007, etc?

-k

[tgr23]

Actually not sure what # is stamped on the sender.  I'll check that this weekend.

[Ron Hill]

Tigr : Do you have a protective boot on the temp gage connection??  It might be that the Al foil engine insolation is touching that connection - when you put the C34 steps back in place!!!!
 
Wrote an article in the Mainsheet tech notes a few years back on the need for protective boots on the C34 engine!!   Great Info in those old tech notes!! 

A few thoughts

[Ken Juul]

I don't know if applies to the C30, but if you have an an original electrical harness like the C34 came with it might be the wires.  Read the articles about harness upgrades  in the 101 section near the top of the forum.

[KWKloeber]

Ken

Just to avoid any misunderstanding with that issue, it *can't* be the harness or gummy bear. It was removed @ engine end but regardless, if there was excessive resistance thru the plug, that would cause a low, rather than high gauge reading. (Open circuit it pins low, grounded circuit she pins high.)

T - ping me with the gauge number and sender order info (remember what happened with the first one!)

[tgr23]

Pinged you.

Here also:
Sender: Sierra International #TS26611
Gauge:  SeaStar Solutions Eclipse Series Water Temperature Gauge. #68401P

[Bobg]

This probably isn't the fix for you, but, my temp gauge was reading erratic ,  I have a 160 Tstat, but the temp gauge was reading between 175 and 190, erratically, my ignition switch went bad and after I replaced it,  the temp gauge settled in at 160 steady, been that way for 10 years.

[KWKloeber]

They both check out.  I guess the next step is room temp and boil some water, and your multimeter. Very strange indeed, but will end up some explanation like the sender you ordered isn't the one you received.