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[KWKloeber]

Randy

Good eye. Your observation/conclusion is correct (caveat:this is w/o being hands on there.)

I would verify the further by:

1) Verifying whether you have a good ground at the temp sender. Either with a meter or grounding the sender case to a good negative. But I think you probably grounded the S wire to a good known negative so that kinda eliminates that possible cause.
2) Before I tore into the harness I would run a long wire if you have something around (w/ alligator clips?) to the cockpit to bypass the S wire.  That will for sure nail down whether it’s the S wire being bad.

The gauge pinned high, if it’s the S wire, means it’s grounded (insulation off, pinched somewhere maybe) and ‘permanently’ telling the gauge that the sender has zero resistance (overheating.)

You could also check the resistance between the sender case and terminal. If it’s zero or very low (with a cool engine) then there’s something wrong w/ the sender. The troubleshooting guide lists some normal resistances at different temps. The relationship is not precise be aware, temp gauge can vary 15 deg or so, so resistance is just a guide.

[Rortega46]

I like the idea of running  a long wire directly from the sender to the gauge and bypassing everything else. I’ll give it a try in the next day or two and let you know what I find. Thanks for the suggestion.

[Ron Hill]

Randy : Make sure that you have a "rubber" protective boot over the connection on thermostat.  Any contact with the metallic insulation will screw up readings!!

A thought

[Rortega46]

Today I ran a wire with alligator clips on the ends from the sender (@ thermostat) to the S terminal on the temp gauge.  The gauge worked perfectly and did not show overheating or deviate with changes in engine rpm or extended runtime.  This confirms it is the yellow circuit/wiring from the sender to the gauge.  Now I need to find where the wire is cut, pinched, corroded, or otherwise bad connection.

The most likely problem seemed to be a bad looking ring connector at the sender.  After reconnecting the original wiring I changed the ring connector but the temp gauge problems continued. The most likely problem areas now seem to be end terminals, wire joints/connections, plug connectors, or tight turns.

So how should I go about finding the fault in the wiring?

Does anyone know wire routing once it goes down in front of the engine and behind the alternator?     

Where in the aft cabin does the harness run?

Is the a plug or other type wire connection between the engine and where it enters the SS pedestal tube?

Thanks for your ideas and suggestions.

Ron: There was and is no protective rubber boot at the sender.  What does this look like and where do I get one?

[Ron Hill]

Randy : You are probably better off just running a new wire all the way, rather than trying to spend time looking for a break!!

The RED rubber boots are a 90 degree cover that slides on the wire so it also covers the terminal connection.  look in the electrical section of a marine chancellery.

A few thoughts

[KWKloeber]

[edited]

Randy

The BC temp sender being on the butt end of the TStat cap might not be issue as far as shorting out but still, it will never hurt....   You want a Blue Sea # 4008 boot, WorstMarine, Defender, etc.
You did check it with the engine cover off [no insulation interfering]?

The Universal B engines sported a black RV plug on the harness (if you look at the cut sheet for the M25XPB on the WIKI you can see it.  I presume that CTY kept the switch [RV plug] in place.  Sorry, I can't link you to it cuz I haven't had access to the wiki for like 6 months.  See WIKI > MANUALS

The schematic for the BC engines is also on the Wiki WIKI  > MANUALS.

Again, if the gauge is pinned high then the S wire is permanently grounded somewhere, not a "bad connection" (i.e., high resistance or open circuit, which would pin the gauge low.)

The terminal on the high-temp switch is THE WORST for on a boat.  They make a switch that has a 1/4" quick disconnect terminal (which is only one step above -- I haven't found a screw terminal switch yet.)
 

[Rortega46]

Today I solved the riddle of the faulty circuit. The temp gauge is now functioning properly.  I planned to run a portion of a new wire from the sender to the temp gauge and started by opening up the harness in the aft berth, separating the yellow (temp gauge) wire from the others and then cutting it.  I used a jumper wire with alligator clips to determine the problem was from that point forward to the sender, and that everything from that point aft to the temp gauge was ok.  Before beginning the process of routing a new wire around and under the engine, I cut additional wire from the sender terminal and replaced the heat shrink ring connector again. Viola, that did the trick and the temp gauge is now behaving normally. Evidently the unprotected wire had been bent and compromised too many times so cutting it back further did the trick.  I’ve ordered a Blue Sea #4008 boot from WM and will install it for added protection when it arrives. 

Thanks Ken and Ron for your responses and assistance.

[KWKloeber]

 I cut additional wire from the sender terminal and replaced the heat shrink ring connector again. Viola, that did the trick and the temp gauge is now behaving normally. Evidently the unprotected wire had been bent and compromised too many times so cutting it back further did the trick. 

Randy

That's great that she's working now, BUT the cause (broken wire near the ring terminal) makes no sense.

Quote direct from the gauge troubleshooting:

3. If sender is shorted (0 ohms) gauge will read above 240°F.  (Your gauge symptom)
4. If sender has infinite resistance (Open) gauge will read below 120°F. ( Your sender wire condition.)
Oh well, it's a boat.

Comment -- your wire condition is why it's important to tie down any wire within 6" of a terminal (ABYC). 
Flex can destroy a perfectly good terminal connection.  I usually add a pc of heat shrink on the terminal/wire to act as a strain relief where there may be some flex.  Also, because the 14 ga (BLUE) terminals are not as hefty as the 10-12 ga (YELLOW) terminals, I use the yellow ones on engine connections (skin back the copper 2x long, fold over and crimp it doubled-up in the 10-12 gauge terminal.)
 

[Rortega46]

Ken, I think I understand what you’re saying in that I may have fixed the problem but it may recur because I didn’t get to the source of the problem.

I believe the problem is in the first 6-8” of the yellow sender wire since it is unprotected there and repositioning it after cutting an inch or so seemed to fix the problem.  Wherever was grounding the circuit was likely moved enough to eliminate the short.  For now I will put a boot around the connector and split loom on the exposed wire as far as I can. I will also keep a handy dandy laser thermometer on board, so if the temp gauge shows overheating, I can check engine temp at the engine block.

[KWKloeber]

Randy

“Sort of but not really” what I’m saying.

When you say unprotected, I presume you do NOT mean no insulation on the yellow wire.

The way that the gauge would pin high (if it’s a wire issue) would be where there is no insulation (worn, burnt, wire broken and sitting against a ground) and there is a dead short to a ground (ie, the same condition as when you grounded the S terminal on the back of the gauge.). What you’re describing is the polar opposite of what **should** have happened. My mind is blown about the cause/symptom/fix/result. It just does not compute (but after all it is a boat.)

Could the sender terminal have been grounded to the engine compartment insulation when you did the testing?

PS, what kind of seawater temps do you run (high end)?

[Rortega46]

Ken

When I say unprotected I now understand this to mean no insulation on the yellow wire (worn, burnt, wire broken and sitting against a ground). 

I can't explain why it's working now and don't think it's the new terminal connection.  I suspect it's somewhere along the first ~10" of the sender wire which was likely previously grounded but no longer is because it has been shortened and moved.  I can't prove it because the exact reason/spot of the grounding has not been isolated.

I don't think the sender terminal was grounded to the engine compartment insulation when I did the testing.  Seawater items lately have been ~ 74°.