Electrical puzzle

[M99]

1994 Sea Ray 300DA

Here’s an electrical puzzle I have on my boat that someone might have seen before. The Port side Fuel gauge I haven’t been able to make work reliably? I have spent a lot more time screwing with it and I am pretty good with electrics. The previous owner also had problems with it because he had mechanics replace the sender for him, with the wrong sender. I changed that out a long time ago. The sender is sending the correct resistance when float is up or down. 34 Ohms when the tank is full and around 270 when empty, just like it should. Starboard sender and gauge work fine. After changing the Port sender out and checking the wiring to the back of the gauge, it worked intermittently. Usually read empty or ¼ full regardless of a full tank. The sender float is not hitting a baffle in the tank. I’ve checked that many times. Next I found the main 12 pin connector that had the wires to that gauge under the panel. Took that apart and cleaned it up, used Stabalant for the contacts and put it back together. Voila, it worked fine. Thought I had the problem fixed with a high resistance connection that I cleaned up. High resistance makes the gauge read low. I also had checked behind the gauges by switching the sender wires from one gauge to the other. Problem follows the sender wires, so the gauges are fine.



Now comes the real puzzle. For the last couple of months, it would seem to work fine 10 times in a row at the dock. Didn’t matter if the engines were running or not. It just worked. I thought it was fixed. But as soon as I headed put. It went right back to reading a constant ¼ tank, but very occasionally would work correctly even on the water for awhile, then go back to reading that ¼ tank. I finally found most of the correlation to that and this happens consistently. When the Shore Power cord is plugged in, even if the Shore Power Breaker on the Dock Box is OFF, the gauge reads correctly. Unplug the cord, it reads ¼ tank. Same thing if you leave it plugged in on the boat, but disconnect it at the Dock Box. Works when both ends are plugged in, and stops working as soon as either end is unplugged. I took the boat shore power connector out and looked around there for a bad or pinched wire. All looks perfect there. OK thinks I, the Dock Box breaker makes no difference on or off, and the breaker only switches + V on and off. So it has to have something to do with either the Earth Ground or Neutral which is always connected when the cable is attached at both ends. So yesterday I took an hour or so looking for grounding problems on the boat. Keep in mind that everything else, including the Starboard Fuel Gauge works fine regardless of the Shore Power cord connection. I tried grounding the two engines together just to see if that made a difference. I also tried connecting the Port Engine and even the Port Fuel sender ground, directly to one of the Green Earth Ground points in the bilge. Nothing made any difference at all.



I do have a Galvanic Isolator, but if that was bad and causing the problem, it would affect both sides.



Any ideas? I'm a licensed aircraft mechanic, but maybe a marine guru has an idea I am missing?

 

[Bt Doctur]

you need 2 things for any gauge to work properly, the feed wire and the ground wire. one or both are compromised

 

[Lazy Daze]

Boy, wiring gremlins can be a real pain in the butt. Try running a new sender wire directly to the gauge. For now, it doesn't have to be run neatly - just bring it up any way you can - doesn't matter if it looks messy. You might want to even leave the gauge (or the whole panel) sitting on the dash. You can do the same thing with ground, but for that you can get the feed from a bus bar behind the dash. Don't do these both at the same time - you want to continue to narrow things down. Before I did that, though, I would start by waiting till it's acting up and then play with the wires behind the dash -- to see if you can locate a broken wire (inside the sheath).

 

[M99]

That is my next step running a long jumper direct from the sender to the gauge. What I find so strange is that the problem follows the Shore Power Cord. Plug it in and all is well. Unplug it and it quits working. But I can find no connection in the wiring diagram that would create that type of situation. It must be a ground rather than Power problem, because with the shore power breaker off, there is no power it yet it works. But the shore power ground and neutral are always in circuit when its plugged in.

 

[Lazy Daze]

That is my next step running a long jumper direct from the sender to the gauge. What I find so strange is that the problem follows the Shore Power Cord. Plug it in and all is well. Unplug it and it quits working. But I can find no connection in the wiring diagram that would create that type of situation. It must be a ground rather than Power problem, because with the shore power breaker off, there is no power it yet it works. But the shore power ground and neutral are always in circuit when its plugged in.

That's got me stumped, too. I agree - I don't understand the connection between the two systems, other than the ground. But the gauge still has the same path to ground, regardless of the shore power. For now, I'm thinking it's just a coincidence and maybe the slight moving around on the boat is doing it. From what I'm reading, the part where you cleaned the contacts and it started working holds the most merit.

 

[M99]

Right. I find no corollary either. I'm 100% certain that it is not just coincidence, because it is completely repeatable over different days even. The one point that I keep coming back to, is that the Shore Power cord ties Earth Ground and Neutral together when plugged in through the Shore connection. I keep wondering if it might have anything to do with the Galvanic Isolator since that also is supposed to tie those together. That's why I tried alternate grounding jumpers. There is no +V to the sender. Just resistance to Neutral (Ships Ground) being measured at the gauge. But if the Isolator was causing the problem, it should also affect the Starboard Gauge.

 

[Lazy Daze]

I hear 'ya. I just don't know. I'll keep thinking on this, but this one might be above my pay grade. I can see the point about the shore power cord and grounding... but then all of those gauges shouldn't work, unless the cord was plugged in since they usually share a common ground bus bar. Plus, if the isolator was the problem, wouldn't there then be many other things that are hincky? There's something going on here that I'm not wrapping my head around. Other than the cord thing, which is just strange, all signs point to a problem with either the pink wire or the ground wire from the gauge to where it splices in with other gauges/grounds.

 

[ttmott]

When on shore power the ground and neutral are separated and only joined at the power source which is the primary breaker box on land. When the boat's power transfer switches are set to generator then the grounds and neutrals are joined on the boat. Doubtful that the AC power neutrals or the galvanic isolators have anything to do with the issue. Regardless, there should be less than one ohm resistance between the battery negative terminal and any ground on the boat including the sender body on the fuel tanks. Remember, the fuel sender is simply a ground bias that the fuel gauge references. So, with all that said I would trace the grounding from the sender and fuel tank to the ground terminal strip and on to the batteries. Also, corrosion on the terminals will do weird things to the electrical system.

 

[M99]

Thanks guys. Really good information that gives me some new starting points. I did know about how the Earth Ground and Neutrals tie together at the Shore Power box and that is what got me thinking about some odd interconnection on the boat. The Switch on the Boat to Generator power made no difference. I am thinking a corrosion problem somewhere along that Pink / Black stripe wire, but I have not been able to find it. I may just run a new dedicated wire around that ground wire and leave it like that if that solves it. All other gauges function correctly, and as you pointed out, they share a common ground point. Boas seem to make even stranger problems then airplane do.

 

[Mike Blake]

Just my 2 cents here did you try swapping the gauge's? or the wires behind them? That would drive me crazy!!! Good luck.

 

[Bt Doctur]

the gauges share the common ground but the gas tank needs a good common ground too.

 

[ttmott]

the gauges share the common ground but the gas tank needs a good common ground too.

Yes however, the gauge ground is only for the lighting. The ground bias from the tank sender is what operates the gauge needle.

 

[Lazy Daze]

the gas tank needs a good common ground too.

You're right, and I don't think I explained that well above. Pink wire, gauge ground or tank ground. Check those wires (and grounding points) and/or jumper a new one (temporarily), one at a time, to narrow this down.

 

[M99]

Just my 2 cents here did you try swapping the gauge's? or the wires behind them? That would drive me crazy!!! Good luck.

Yes, that was one of the first things I tried. The problem followed the sender and associated wiring. Both gauges worked fine if connected to the Starboard sender and associated wiring.

 

[M99]

You're right, and I don't think I explained that well above. Pink wire, gauge ground or tank ground. Check those wires (and grounding points) and/or jumper a new one (temporarily), one at a time, to narrow this down.

That is where I'm headed on this now. I did already try grounding the Fuel Tank with a secondary ground wire to see if it was a bad ground to the tank. That had no effect and when testing the tank ground to Ships Ground, it checks with no resistance. I even tried jumpering the ground side wire from the sender directly to ships ground to bypass the actual tank. Same result. My best guess at this point is a high resistance somewhere in the Pink / Black stripe wire that the Gauge uses to get its ground through the sender, but I have not been able to physically follow that entire wire chase. However, I can't figure out why that would have any corollary to whether the Shore Power cord was connected or not. I have a lot of experience tracing and wringing out wiring circuits, but the shore power connection is the ringer that makes me think there is something else going on that I'm missing. Boat electrics are a little different than what I am used to, especially because of the mix of AC and DC, but they don't look that complicated.

 

[SeaAyeOwe]

I am not certain of the full DC circuit here and cannot comment on why the AC side fixing the problem - but if it is important that the Tank is grounded well, it is also important that the flange plate for the sender housing is grounded. This grounding will usually take place by making a circuit between the allen head screws that fasten the sender unit to the tank, at least on my unit that is the case. There is a gasket between the sender unit flange and the tank top - but normally the threads of the allen screws and the head of the screw would complete the circuit (unless fiber washers or some insulator has been used).
I might try grounding the actual sender unit housing temporarily also (a simple jumper wire from the stainless part of the sender housing to the tank would do it). This should take seconds and would at least eliminate one potential issue.

 

[ttmott]

It could be as simple as a sender that isn't working correctly; it happens even when new. Regardless, Should look like this with dedicated ground wire to the sender:

 

[peterkvs]

Put a jumper between the neutral and ground prongs on the bulkhead connector that your shore power connector plugs into. See if that makes it work. I bet you have a bad sensor ground and it is finding a sneak path through the battery charger, through the neutral side of the battery charger and to the ground system of the boat. Or something odd like that. Take the gauge out of the dash and connect it directly to the tank sensor and sensor cover as Tom shows.

Good luck,

Pete

 

[M99]

Yep, that's how it is set up although the tank grounding wires on my tanks are in a different location on the tanks. I have swapped out two different new senders, and also tested the senders outside of the tanks with a VOM several times. They track correctly. I have also run temporary test ground wires directly from the sender ground plate to both the Ships Neutral and the Earth Ground and the results are the same. It is a very strange deal. At first when I discovered the correlation between the Shore Power being plugged in or not, I thought that it might be a pinched wire behind the shore power plug on the boat so I took all that apart but found nothing out of sorts there. Then I also discovered that if I unplugged the shore power cord at the dock box, it also tracked the same effect. Therefor it was not something physical on the boat related to the shore power cord. It strikes me as something similar to having what is called a "floating neutral" in a residence electrical system, but I haven't been able to locate anything of that nature either, and I have been using jumpers to various grounds trying to isolate what is going on. Its supposed to warm up a bit this weekend, so I'll be doing some more troubleshooting then. I appreciate all the suggestions.

 

[M99]

Put a jumper between the neutral and ground prongs on the bulkhead connector that your shore power connector plugs into. See if that makes it work. I bet you have a bad sensor ground and it is finding a sneak path through the battery charger, through the neutral side of the battery charger and to the ground system of the boat. Or something odd like that. Take the gauge out of the dash and connect it directly to the tank sensor and sensor cover as Tom shows.

Good luck,

Pete

Now that is something I have not tried. I have tied the Neutral and Earth Ground together and it didn't help, but not at the Shore Power connector. I'll give that a try. As I mentioned, I did try a jumper wire directly from the ground lug of the sensor to Neutral and also to Earth Ground without a solution. At the panel, I switched the wires between Port / Starboard gauges and the problem tracked with the wires, not the gauges, so that eliminated the gauge as the problem. That thought of a sneak ground through the Battery Charger is something I had not thought of. I'll trace that down and see where it leads me.