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Author Topic:   OMC/Evinrude Trim Gauge Wiring c.1992

jimh posted 06-15-2006 09:23 AM ET (US)
When re-wiring my c.1992 Evinrude TRIM gauge last weekend, I noticed that the WHITE/TAN conductor (SENDER) was much shorter than the BLACK/TAN conductor (SENDER GROUND) as the two wires came from the trim/tilt cable harness. Last night I was browsing in the service manual and I noticed a pictorial diagram that showed the wiring. The pictorial diagram showed what looked like an in-line fuse in the WHITE/TAN lead. There was no fuse in my harness, but the absence of a fuse would account for that lead being too short.
In an intact OEM wiring harness, is there an in-line fuse in the WHITE/TAN lead? Or, if not a fuse, what is that device shown in-line on the pictorial drawing?
Also, the same pictorial diagram shows that on the TRIM gauge terminal marked G (GROUND), only the BLACK/TAN (SENDER GROUND) lead is connected. In the original wiring on my instrument panel, the BLACK (GROUND) lead from the tachometer harness was connected to the G terminal on the TRIM gauge.
Should the G terminal on the TRIM gauge have only the BLACK/TAN lead connected? Or should it also have the the BLACK lead from the tachometer harness connected?
The trim gauge meter is illuminated, and the negative branch of the lamp circuit is connected to the G terminal. If the G terminal is only connected to the BLACK/TAN lead (which is how I have it wired at the moment), then the current for the gauge's lightbulb returns to the battery via the SENDER GROUND path. This causes the TRIM meter indication to change when the gauge illumination is turned on. The change in reading is slight and can be tolerated. Even more unusual, however, is what happens if the gauge lighting circuit is turned on while the IGNITION circuit is OFF. In this situation, the gauge indication is changed quite a bit. The meter jumps from its quiescent position to a new position as a result of the flow of the lamp current. This is a bit odd, although the situation in which this occurs in perhaps not the normal operating condition.
I was thinking of mitigating the influence of the lighting current on the TRIM gauge meter reading by two methods:
--modify the gauge itself so the lamp negative lead is isolated from the SENDER GROUND terminal;
--add a battery negative (BLACK) lead to the SENDER GROUND terminal.
With regard to that second option, I do notice that all of the battery negative leads (BLACK) associated with the instruments seem to be isolated and come from the tachometer harness BLACK lead. That is a very good practice as it keeps all the battery negative leads on the panel isolated to a connection back under the cowling of the motor itself. In this way, if there is a problem with the main (high-current) connection between the motor and the battery negative terminal, there will not be any other paths in the wiring harness back to the battery negative lead. If such an alternative path were introduced into the wiring, it would try to carry the heavy current that occurs during engine starting if the main battery negative connection failed. This would burn up any wires in the harness, which would exacerbate the problem very quickly into a real mess. So for that reason, I would not connect a battery negative lead into the instrument wiring that was not returned to battery negative via the engine main battery cable itself, that is, back at the engine.
I welcome advice from old hands at OMC instrumentation regarding the missing fuse situation and the influence of the lighting current on the gauge reading.
swist posted 06-15-2006 04:14 PM ET (US)
Hmm. It would be interesting to see just what the SENDER GROUND wire is connected to on the engine end of things. On most other manufacturer's engines, the trim (and other resistive sensing gauges) only have one wire coming from the engine for the sender. The other side of the sender is connected to engine ground which eventually gets back to instrument panel ground. The gauge is effectively an ohmmeter which is calibrated to translate ohms into whatever the sensor is measuring.
It sounds like they may have floated the sender off ground for some reason. I would measure the resistance between instrument ground and the SENDER GROUND wire to see what conclusions you can come to.
The sender resistance should be 1 ohm with the engine all the way up and 88 ohms all the way down. See if you can get those readings on an ohmmeter between the SENDER wire and ground. If so I would not bother with the SENDER GROUND wire and would hook the instrument ground (black) to an adjacent instrument ground. That way the gauge light will have nothing to do with the gauge function.
jimh posted 06-15-2006 08:50 PM ET (US)
Swist--the gauge works fine. I am just curious about the missing fuse and the normal installation wiring, as my original was at variance with that which is shown.
The SENDER GROUND lead is connected to the outboard motor ground at the motor. By connecting the instrument to a separate lead which runs from the instrument to this ground at the motor, the gauge is isolated from other currents which might flow on the battery negative return lead from the instrument cluster.
I don't know about other brands of outboard, but it is clear that OMC did a good job engineering the TRIM gauge wiring to make it as accurate as possible and to isolate it from other influences.

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Author	Topic: OMC/Evinrude Trim Gauge Wiring c.1992
jimh	posted 06-15-2006 09:23 AM ET (US) When re-wiring my c.1992 Evinrude TRIM gauge last weekend, I noticed that the WHITE/TAN conductor (SENDER) was much shorter than the BLACK/TAN conductor (SENDER GROUND) as the two wires came from the trim/tilt cable harness. Last night I was browsing in the service manual and I noticed a pictorial diagram that showed the wiring. The pictorial diagram showed what looked like an in-line fuse in the WHITE/TAN lead. There was no fuse in my harness, but the absence of a fuse would account for that lead being too short. In an intact OEM wiring harness, is there an in-line fuse in the WHITE/TAN lead? Or, if not a fuse, what is that device shown in-line on the pictorial drawing? Also, the same pictorial diagram shows that on the TRIM gauge terminal marked G (GROUND), only the BLACK/TAN (SENDER GROUND) lead is connected. In the original wiring on my instrument panel, the BLACK (GROUND) lead from the tachometer harness was connected to the G terminal on the TRIM gauge. Should the G terminal on the TRIM gauge have only the BLACK/TAN lead connected? Or should it also have the the BLACK lead from the tachometer harness connected? The trim gauge meter is illuminated, and the negative branch of the lamp circuit is connected to the G terminal. If the G terminal is only connected to the BLACK/TAN lead (which is how I have it wired at the moment), then the current for the gauge's lightbulb returns to the battery via the SENDER GROUND path. This causes the TRIM meter indication to change when the gauge illumination is turned on. The change in reading is slight and can be tolerated. Even more unusual, however, is what happens if the gauge lighting circuit is turned on while the IGNITION circuit is OFF. In this situation, the gauge indication is changed quite a bit. The meter jumps from its quiescent position to a new position as a result of the flow of the lamp current. This is a bit odd, although the situation in which this occurs in perhaps not the normal operating condition. I was thinking of mitigating the influence of the lighting current on the TRIM gauge meter reading by two methods: --modify the gauge itself so the lamp negative lead is isolated from the SENDER GROUND terminal; --add a battery negative (BLACK) lead to the SENDER GROUND terminal. With regard to that second option, I do notice that all of the battery negative leads (BLACK) associated with the instruments seem to be isolated and come from the tachometer harness BLACK lead. That is a very good practice as it keeps all the battery negative leads on the panel isolated to a connection back under the cowling of the motor itself. In this way, if there is a problem with the main (high-current) connection between the motor and the battery negative terminal, there will not be any other paths in the wiring harness back to the battery negative lead. If such an alternative path were introduced into the wiring, it would try to carry the heavy current that occurs during engine starting if the main battery negative connection failed. This would burn up any wires in the harness, which would exacerbate the problem very quickly into a real mess. So for that reason, I would not connect a battery negative lead into the instrument wiring that was not returned to battery negative via the engine main battery cable itself, that is, back at the engine. I welcome advice from old hands at OMC instrumentation regarding the missing fuse situation and the influence of the lighting current on the gauge reading.
swist	posted 06-15-2006 04:14 PM ET (US) Hmm. It would be interesting to see just what the SENDER GROUND wire is connected to on the engine end of things. On most other manufacturer's engines, the trim (and other resistive sensing gauges) only have one wire coming from the engine for the sender. The other side of the sender is connected to engine ground which eventually gets back to instrument panel ground. The gauge is effectively an ohmmeter which is calibrated to translate ohms into whatever the sensor is measuring. It sounds like they may have floated the sender off ground for some reason. I would measure the resistance between instrument ground and the SENDER GROUND wire to see what conclusions you can come to. The sender resistance should be 1 ohm with the engine all the way up and 88 ohms all the way down. See if you can get those readings on an ohmmeter between the SENDER wire and ground. If so I would not bother with the SENDER GROUND wire and would hook the instrument ground (black) to an adjacent instrument ground. That way the gauge light will have nothing to do with the gauge function.
jimh	posted 06-15-2006 08:50 PM ET (US) Swist--the gauge works fine. I am just curious about the missing fuse and the normal installation wiring, as my original was at variance with that which is shown. The SENDER GROUND lead is connected to the outboard motor ground at the motor. By connecting the instrument to a separate lead which runs from the instrument to this ground at the motor, the gauge is isolated from other currents which might flow on the battery negative return lead from the instrument cluster. I don't know about other brands of outboard, but it is clear that OMC did a good job engineering the TRIM gauge wiring to make it as accurate as possible and to isolate it from other influences.