Gauges dieing
 

Have had a couple of my Gaffrig (Gaffrig Precision Instruments, which ever "Gaffrig" that is, I can never keep them straight) redline gauges go beserk. The problem is they will itermittently go off scale on the high end and at the same time trigger the low limit red warning light.

The stbd fuel gauge has been doing this for some time, but has been too intermittent to actually test anything. I'd been assuming that it was the fuel sender, but never taken on the task of swapping it out.

Then the port oil pressure gauge started doing the same thing. I was sure it wasn't an actual oil pressure problem as my mechanical gauge on the engine was dead steady (and off scale high and low indicator at the same time is somewhat impossible). The oil gauge was a little more consistent when it whacked out, so I was able to wire it to the stbd gauge (both gauges on the same sender) and verify it was the gauge and not the sender. I replaced the oil gauge and yes, the problem has gone away.

Anyone else having problems with these gauges? I'm wondering if there could be anything in the wiring that is triggering them to fail or just bad luck with them.

Thanks - Greg

GregP
Had what sounds like the same problem last year. I cleaned all of the connections on the gauges (ground and positive) as well as all of the connections at the junction blocks. This fixed the problem, and infact it also activated some cockpit lights that hadn't worked in a long time. The funny thing was at a glance they hadn't looked that bad prior to cleaning!

Tom - I tried cleaning and even replacing the connectors on the fuel gauge when it first started acting up. On the oil gauge I had them on and off a bunch of times while trying to decide if it was a gauge or sender problem and only the port oil gauge ever acted up, and I didn't have a problem when I switched the port wires to the stbd gauge. Guess I could hit them with some contact cleaner as extra assurance though.

-Greg

Check the ground wire on your Starboard motor. Or substitute another ground wire between the battery and the engine. You might also have a bad crimp on the lugs on the wiring. Also make sure you are not using any pipe dope or sealers on your sending units, unless it is the conductive type. (Very rare unless you bought it specifically for an electronics installation.)
Good Luck

Remove the ground from the instrument in question and see the reaction you get.
Pegged out or dropped off then back-track that line of thinking.
Good idea on the engine ground! Most mechanics miss that

The Gaffrig's in my budys 97 Fountain do the same chit and sometimes when you bang on the dash they fix them self.

Most cases it is a bad ground or connection. Loosen them all and retighten.. if the problem changes you are close..

Good luck !!!!

Dick

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Sorry to take so long to reply, been out testing at work a lot lately.

I'm sure it isn't a ground or other wiring as the first thing I did was check all the leads with the DVM and also ran jumper lines from the ground and sender straight to the gauge as a test.

When I put in the new gauge, (or wired it to the one for the other motor), it all worked fine. I'm sure it's the gauges, just don't know why the early death (only 20-30 hours on them since the new dash install).

An intersesting side note: When I wired the new dash panels I originally tied the port and startboard grounds together. The motors are also grounded to each other. When the problems first started I removed the dash ground tie to see if I had a ground loop problem. It didn't effect things, but I did measure a 5 mv potential difference across the grounds at the dash, while the grounds at the batteries and engines show no cross-voltage. My local avionics guys are always warning about ground loops and saying everything should go back to ground at a single point ... guess they are correct.

-Greg

Be careful using a DVM. They will "LIE" to you.
I used a Fluke 77 for years and if the problem was involved we always went back to the "good old analog"
DVM's are prone to EMF and RF interfearance.
Also reading a DVM in ohms checking for continuity....if there is 1 strand that is touching it will show good. Where if using an analog it puts more voltage through the wire to get it's readings and will be more accurate with respect to integrity of the wire.
Prevents........ good readings in continuity but circuit unable to pass current.

no one eles has asked, are you running msd ing. ?
last year i ran into a problem with the msd wires . they were putting out a strong elec. field causing gauges to go bad.i rerouted the wires no more problems.

Yes, I have MSD boxes. I've never heard of the trigger lines being a problem. Anyway, the MSD wires run to the transom along the oil filters, well away from the bundle going to the harness plug with the gauge sender lines.

I will take another look just incase something got re-routed and I didn't notice a change.

-Greg

BOTH engines MUST be wired independently of each other for many reasons. You are creating what is called a Ground Loop by tieing anything electrical together between 2 motors, or terminating several grounds in different places. All grounds must terminate at the same spot on each motor, and is usually done with Bus bars (straps that hook several wires to the same source) or bolt. I have fixed hundreds of freaky electrical gremlins in boats by removing ground wires from 23 different locations, and terminating them all @ the same place (on each engine, port engine or electrical to port engine, and starboard to starboard with no splicing to save wire, or tieing any common power or grounds together port to starboard). I have a real good article on this subject, and if I can find it, I will post it, as it makes sense after reading and thinking about it. Again, SEPARATE any electrical between motors, including gages, charging systems, warning systems, battery cables, trim pumps, MSD etc. (this ground loop also causes gages, fuel senders & tachs to go into "booty lock"), or your premature failure. Now I gotta find that article... lol

YIKES... what a serch and rescue mission...lol
 

This is not written as being marine, or boat specific, but I promise you, if you follow these guidelines with your Offshore boat, you WILL solve many problems you refer to as "having a mind of its own". I have more on this, but it's time for beddy bye, so I will look later this weekend if you want, or need further conformation that what you have ain't gonna work......


Solving Instrumentation Ground Loop Problems

Unwanted ground loops can cause inaccurate sensor readings by negatively affecting instrumentation signals.

Have you ever had problems with process controls and electrical instrumentation? The source may be ground loops. What's this? According to the ANSI/ IEEE Standard Dictionary of Electrical and Electronics Terms, it's a "potentially detrimental loop formed when two or more points in an electrical system normally at ground potential are connected by a conducting path such that either or both points are not at the same ground potential."

In layman's terms, a ground loop develops because each ground is tied to a different earth potential. This condition allows current to flow between the grounds by way of the process loop.

Basically, ground loops cause problems by adding or subtracting current or voltage from the process signal. As a result, the receiving device can't
differentiate between the wanted and the unwanted signals. Therefore, it can't accurately reflect process conditions. The probability of establishing multiple grounds and ground loops is especially high when you install new programmable logic controllers (PLCs) or distributed control systems. With so many connections referenced to ground within a facility, the likelihood of establishing more than one point is great.

You may have to ground at more than one point. For some instruments (such as thermocouples and some analyzers), it may not be possible to eliminate ground loops. That's because these instruments require a ground to make accurate rate measurements. Also, all analog control loops are
grounded at one or more points. And as we've seen, multiple grounds can result in a ground loop that can upset the proper functioning of instruments. You also may have to ground instruments to ensure personnel safety.

So if you can't eliminate the conditions for ground loops, what's your next step? You can use signal isolators. These devices break the galvanic path (DC continuity) between all grounds while allowing the analog signal to continue throughout the loop. An isolator can also eliminate the electrical noise of AC continuity (common-mode voltage).

Signal isolators use one of two techniques to do the job. One is analog signal isolation, which uses an isolation transformer to chop, isolate, and reconstruct the signal. The other is discrete signal generation, which chops, transmits optically, and reconstructs the signal. Signal isolators using the latter technique are referred to as "opto-isolators." The choice between the two depends on your circuitry requirements.

Regardless of the isolation method you choose, an isolator must provide input, output, and power isolation. If you don't have this three-way
isolation, then an additional ground loop can develop between the isolator's power supply and the process input and/or output signal.

Isolators, like most other transmitters, come in 2- and 4-wire versions. The 4-wire type requires a separate power source and is partially suited for back-of-panel mounting. You can power the 2-wire type from either the input or output loops.

The input loop type makes it possible to isolate a process signal when line power or output loop power isn't available. The output loop type solves the problem of interfacing non-isolated field signals with systems such as a computer, PLC, or distributed control system, which provide loop-power to their output devices.

You can find a signal isolator to suit almost any application. Here are just a few.

• Resistance input isolators for use as RTD, slidewire, strain, and potentiometer transmitters.

• Millivolt isolators for use as thermocouple and millivolt transmitters.

• Current/voltage isolators for use as alarm tripping, deviation alarm notification, and other special application transmitters.

So if an instrumentation system starts acting strangely or erratically, make sure you eliminate all unintended ground connections.


Instrumentation grounding

Almost all equipment used in a control instrumentation strategy makes use of a common signal ground as a reference for its analog signals. Introducing any additional grounds into a control circuit will almost certainly cause ground loops to occur.

To minimize the danger of introducing these loops into a complicated network, you should use a dedicated instrumentation system ground bus and connect grounds from the signal common, cabinet ground, and instrumentation AC power ground to it. The bus is tied to earth via the building ground and plant ground grid.

The cabinet ground is a safety ground that protects equipment and personnel from accidental shock hazards. It also provides a direct drain
line for any static charges or electromagnetic interference (EMI) that may affect the cabinets. This cabinet ground remains separate from the
DC signal ground until it terminates at the master ground bus.

The AC service ground is a single-point ground termination of the system AC power. This ground connects to the neutral-to-ground bond at the main
AC power isolation transformer. It also terminates at a single point on the plant ground grid (usually the grounding electrode).

How an instrumentation signal loop works

Suppose you have an instrumentation loop, as shown in the figure (in the original article). As you can see, it's basically a DC system that operates at a specific voltage (24V in our example) to a master ground reference called a signal ground. The instrumentation signals vary within a range of
4mA to 20mA, depending on the value of the variable (temperature, pressure, etc.) seen by the sensor.

Let's say a precisely calibrated circuit takes this mA signal and converts it into a 1V-to-5V signal for a chart recorder. At 4mA, the voltage measured by the recorder is 1V (250 ohms2.004A). At 20mA, the measured voltage is 5V. Normally, the recorder scale is calibrated so the voltage reads
directly in DegrF, psi, etc.