Mikentucky
Member
- Jul 31, 2020
- 61
- 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!
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!