Temp gauge stays cold, water temp light on dash?

Mikentucky

Member
Jul 31, 2020
61
Louisville, Kentucky
Boat Info
1994 400 Express Cruiser, "Lollygagging"
Engines
twin inboard Mercruiser 7.4L Bluewater, straight drives
OK, this one is stumping me. I recently bought a 1994 400 EC with 7.4 carbureted Bluewater inboards. The port engine temp gauge was inop--it sat at about 130 with key off, bottomed out to below 120 with key on/engine running and stayed there no matter what the engine temp might be. Before I bought the boat I did a thermal scan of the warmed up and running engine which showed 145 degrees at the thermostat housing, 175 at the exhaust manifold, and 102 at the elbow, so it looks like the engine is cooling ok.

Yesterday I went below to troubleshoot the inop gauge. First step was I started at the source and ohmed the temp sensor out--no variable signal coming out (open and showing 1 on the ohmmeter) so a defective sensor. I put a new one in thinking I had an easy fix (using no teflon, just a small dot of conductive Permatex copper gasket maker to seal). Ohmed it again, I was now getting about 700 ohms out of the sensor on the cold engine, so things were looking good. Started it up to congratulate myself on something going right LOL :) No luck--the gauge STILL bottoms out and stays below 120 degrees with the engine on/warmed up, but now the port "water temp" light on the Sea Ray systems monitor is on any time the engine switch is on, as if the port engine was overheating? I checked at the back of the gauge expecting to find either no signal or full continuity (high temp) meaning a faulty gauge and it's showing an open--1 on the ohm meter, which means low temp, not high. So somewhere the signal wire is going straight to ground, and then not making it to the gauge?. In addition...I then ran a jumper from my port gauge's signal lug to the starboard signal lug (starboard engine is key off and cold)....the port gauge started working, showing the correct port engine temp, and the starboard gauge still shows nothing since it's key is off and engine cold! I know they have different negatives, but how does hooking the port signal to the starboard gauge make the port gauge start working?

My thought is to totally swap the gauges rather than just running a jumper wire, to see if it's a faulty/grounding gauge. If that doesn't show anything, my thought was to run a test wire straight from the temp sensor to the gauge, theoretically making the gauge work (and possibly backfeeding backwards to the trigger for the high water temp light before it hits the break/short in the wire, making the monitor light also go out). My concern is that I'm not sure how the current flows or what connections are hidden between the temp sensor on the engine and the signal lug on the back of the gauge. I know some of the system monitor lights work by applying a triggered positive signal (bilges, shower sump, things that use a float switch to send a positive trigger to a pump and also to the corresponding system monitor light). So if I just run a test jumper (which will be negative) to the gauge, is the other end of that sending wire at the gauge hooked to a positive source somewhere that controls the monitor light, not directly hooked to the negative-trigger temp sensor on the motor? Or is the monitor light "always positive", and yes it's the temp sensor negative signal that completes the circuit when the ohms drop low enough?

Sheeesh!! Thoughts, cautions, suggestions? Thanks!
 
Not sure on your boat, but on mine, the gauge is a Variable resistive sender, the SeaRay Monitor is a temp switch.

Do you have the schematic and parts list for the boat? It will specifically tell you how it’s wired and you should be able to identify the right part. It might be available on searay.com, but a lot of that data went MIA lately.
 
Awesome and thanks for the tips guys--I don't have schematics handy today but I think the PO included original engine manuals so I'll see when I get home. I didn't think about the possibility of separate sending units, assuming both things operated off of one signal. But if that's correct, they may both have been bad. I do remember there being two different (and different looking) sensors threaded into the thermostat housing...but if so, why would installing a new sensor with correct ohm readings make the high temp light come on but the gauge still be inop? I obviously changed the one that affected the system monitor light, since that's the only thing that changed...unless it's maybe that the sensors are also redundant? The system monitor light isn't indicating high temperature, it indicates a temp differential between the two sending units? I'll do a little more troubleshooting ;)

When I went to the local marine parts store they pulled up my engine by specs but not serial number and it didn't show either of them--just showed plugs in the housing, so he gave me the generic listing for a water temperature sensor which matches one of my two. I'll ohm out the second sensor and see if it's faulty too, and also ohm out the wires between the sensors, temp gauge and monitor to see what goes where. Thanks again!
 
Put the old sender you took out back in , see if monitor quiets down. Then put the new sender where the other sender is that you found and see what happens?
 
Put the old sender you took out back in , see if monitor quiets down. Then put the new sender where the other sender is that you found and see what happens?
Go figure....my wife emptied the boat garbage today, AND the dumpster company emptied the marina dumpster LOL I went home and do have my engine manuals with wiring diagrams. So yep, I got a temp sender and put it in the temp sensor position. So tomorrow I'll go get a new temp sensor, put them both in the correct places, and everything should be fine. Thanks for all the tips, and I'll update tomorrow, but I'm pet sure this will correct both issues :)
 

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So update on the water temp gauge and idiot light: partially solved :)

I stopped by the local Sea Ray dealership today and bought both the temp sending unit for the gauge and the temp sensor for the audible alarm. Mercruiser parts; less than $100 for both. Installed both....the gauge SOMEWHAT worked, rose to just a hair above 120 degrees even though the infrared on the thermostat housing showed between 141 and 154 degrees as the thermostat opened and closed. I decided to check for a good starting negative by grounding the engine block to the base of the sending unit...BINGO! Gauge reads correctly. That means weak conductivity between the thermostat housing and the block, so I'm guessing the PO had the thermostat replaced and whoever did it used an automotive gasket, not the marine gasket with the little brass ferrules made into the screw holes for conductivity. Anyway, for now I'll use a grounding wire with a worm clamp on the temp sender base, and on winter layup I'll properly replace the gasket.

The idiot light...still stays on all the time now, even with the good grounding. What I can't wrap my mind around is that the idiot light was not on before I replaced the audible alarm sending unit, but I don't see how the audible alarm sender can affect the idiot light? The schematic in my last post looks like the idiot lights on the system monitor MUST run off of the same signal wires as the gauges; those wires have dedicated purpose (inset D). The wires from the second set of sensors (shown in inset C) join into a common wire, so they must only go to the audible alarm that starts beeping with either a with water temp, trans temp, or oil pressure fault--they can't be for a symptom-specific problem. So I would assume this means I still have a short to ground somewhere between the gauge and the water temp idiot light, but why did it only develop AFTER I replaced the audible alarm sensor? More ohming out wires and troubleshooting in my future...sheesh!
 
And the more I think about it....I probably didn't have any defective sending units at all, nothing more than just a weak ground from a previous incorrect thermostat replacement that was causing the inop gauge :rolleyes: First rule in electrical troubleshooting (especially on boats and their wet/corrosive environment)...check for a good positive and a good negative, don't jump right to a defective part LOL
 
Interesting timing on this thread! Over the winter I had my turbos replaced (Cat 3208TA's) and the starboard water temp sensor lights up at 2,400 RPM or higher. The gauges read normal and IR readings support the gauge readings. So I searched this forum looking for ideas on where to look to see if the sender is bad. So it sounds like both the gauge and the idiot light come from the same sensor on the thermostat housing?
 
I can tell you that I was wrong in my previous thread--the gauge and the system monitor light do feed from separate senders on the 7.4 gas motors. When I finally got my gauge working properly by grounding the thermostat housing, the water temp light still stayed on (but still no audible alarm, the third protection device). As we were heading out of the marina, gauge reading normal but monitor light glowing red as it had for the last two days of troubleshooting....the audible alarm decides to finally start going off! I opened the bilge and pulled the spade connector off of the second sending unit that I knew definitely didn't control the gauge...both the light went off and the alarm stopped. So (at least on the gas motors) there is one sending unit for the gauge, and a separate sending unit that signals both the system monitor light and the audible alarm.

Hope this helps for now! I haven't made troubleshooting the light and audible an issue since my temp gauge is working properly, I'll probably do that over the winter :)
 

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