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Author Topic:   E-TEC Rigging; Trim Gauge
jimh posted 10-25-2008 10:32 AM ET (US)   Profile for jimh   Send Email to jimh  
When a new 2008 E-TEC 250 H.O. was rigged on my boat, a standard adapter cable was used to mate the engine's wiring harness to the existing rigging on my boat. My boat rigging is a blend of c.1992 wiring harnesses and connectors, and more recent ones added when I installed a System Check upgrade kit. The standard adapter cable appeared to solve all the problems of interconnecting a 2008 motor with a boat rigged in 1992, and the 16-year gap in connector style, circuits, and components was bridged by the new cable.

Everything worked precisely as expected, until one gray afternoon I decided to turn on the gauge illumination circuit. To my great surprise, as soon as I pulled the knob on the gauge illumination circuit, the TRIM gauge reading jumped from its normal position to the full UP position. Not sure what was causing this malfunction, I temporarily solved the problem by immediately turning off the gauge illumination. The TRIM gauge reading returned to normal.

Investigating the cause of this malfunction some time later, I found the source of the problem. On my boat's rigging there is an isolated ground conductor used in the TRIM gauge circuit, but in the newer rigging this isolated ground conductor is omitted. At first one might expect that loss of the isolated ground on the TRIM gauge would prevent it from working at all, but it appears as if a sneak circuit which allowed the meter current to flow through the lamps of the gauge illumination circuit provided a path to ground for the small current needed to operate the meter. A few schematic diagrams will explain for the technically curious reader:

Schematic diagram of TRIM circuit
The original TRIM gauge circuit c.1992

In the original circuit the TRIM gauge was unique among the other gauges in that it used an isolated ground conductor. A BLACK with WHITE STRIPE lead carried the ground circuit of the gauge back to the engine where it was bonded to the common ground at the same point as the rheostat. This was probably done to prevent other ground currents from affecting the gauge reading.

When the IGNITION circuit is closed, current flows from the battery, through a series resistor, and then divides between the gauge's meter and the engine's rheostat. As the resistance of the rheostat changes, the flow of current though the gauge changes along with it, providing an indicator of the engine position. The circuit is configured so that maximum current flow through the meter corresponds with lowest position of the motor, and as the motor is raised, less current flows though the meter. The scale on the meter is arranged to follow this convention. The rest or no-current position on the meter is marked as the UP trim position, and the full-scale or maximum current position is marked as DOWN trim.

Schematic diagram of TRIM circuit
Gauge current flows via the isolated ground

The one flaw in this circuit is the illuminating bulb current will also flow on the common ground. It was noted that a small change in gauge indication would occur then the illuminating lamp circuit was switched on. This was due to a very slight change in the current flow created by the lamp current. However, this also created the sneak path which would be useful in the E-TEC rigging.

When the E-TEC rigging adapter was used, the BLACK with WHITE STRIPE conductor was not carried through the new harness. As a result the ground side of the TRIM gauge lost its return circuit. Strangely, however, the meter seemed to work normally. This was due to the fact that the current flow in the meter circuit is quite small, and the nature of incandescent lamps to have very low resistance when they are not at their normal operating temperature. These factors combined to allow the meter current to find a new path to ground. It flowed through the unlit lamp of the TRIM gauge, through the paralleled and unlit lamps of the other gauges, and then to ground via their more conventional shared ground circuit. Because the current flow was very small, a few milliamperes, the resistance of the cold bulb filaments stayed very low, and, even though they were in series, they did not influence the meter current much at all. This diagram shows the path:

Schematic diagram of TRIM circuit
Gauge current flow with broken circuit in isolated ground follows this sneak circuit.

Everything worked fine--and it did for many hours of use--until I happened to turn on the gauge illuminating circuit. The bulbs in the other gauges immediately warmed up and increased their resistance, but not the bulb in the TRIM gauge. Since it had no path to ground, it stayed cold and low-resistance. This had the effect of connecting the ground side of the meter back to the +12-Volt battery, as shown below:

Schematic diagram of TRIM circuit
Gauge current flow with broken circuit in isolated ground and lamp illumination turned on.

The TRIM meter indicator current flow actually reversed in this situation, as the normally negative lead of the meter is now closer to +12-volts than the positive. The indicator is driven against the rest stop and shows the highest motor position.

The solution to this problem is simple. Add a common ground to the TRIM gauge. Remove the BLACK with WHITE STRIPE isolated ground wire, and connect the G terminal of the gauge to a common ground.

seabob4 posted 10-25-2008 02:39 PM ET (US)     Profile for seabob4  Send Email to seabob4     
What I have found over the years, especially with trim and fuel gauges, is that when power is applied, and they "peg", there is a ground missing somewhere.
jimh posted 10-25-2008 04:56 PM ET (US)     Profile for jimh  Send Email to jimh     
A missing ground is precisely the problem I described above. As I explain above, the oddity is the gauge worked just fine until the gauge light circuit was energized,
seabob4 posted 10-25-2008 07:31 PM ET (US)     Profile for seabob4  Send Email to seabob4     
I think it is more interesting than that. You pointed out the "reversal" of current flow, hence 12V+ becomes 12V-, which, from a logic standpoint, doesn't make sense.

Several years ago I was doing systems check on a Stamas 37. Turned on the DC Main breaker, and I had dancing indicator lights on all my DC switches. What I learned was that DC will try, through whatever means possible, to find it's way back to the battery. That little "Whoa, what the hell's going on here?" moment reinforced the mantra of having a GOOD ground.

Thanks for posting that.

jimh posted 10-26-2008 09:25 AM ET (US)     Profile for jimh  Send Email to jimh     
Bob--Your comment got me thinking. The situation I describe above is not limited to this unusual case of the E-TEC wiring harness. It would occur in any situation where the TRIM gauge loses its ground connection but shares its illuminating circuit with other gauges. I imagine this sort of wiring is very common on boat dash boards. So this behavior could be seen in many other situations.

Yes, it is unusual that the current through the meter reverses when the illuminating circuit is energized. The dial pointer really pegs hard against the rest stop!

jimh posted 10-26-2008 09:33 AM ET (US)     Profile for jimh  Send Email to jimh     
If the motor is trimmed down in an operating position, the resistance in the rheostat will be low. When the illuminating current flows though the meter, it is only limited by the resistance of the bulb filament and the rheostat. This permits a lot of current to flow, much more than typically would be allowed in the circuit. The series resistor in the gauge circuit normally limits the current flow to whatever value the meter needs to read full-scale. The meter is now is series with a much higher current circuit than normal, and you can sense that by the way the dial pointer jumps hard against the pointer stop.

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