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Author Topic:   Tilt and trim
gvisko posted 10-03-2012 05:10 PM ET (US)   Profile for gvisko   Send Email to gvisko  
Hi--On a 1999 Mercury 115-HP when I turn on the battery [five minutes] later the motor trims down all by itself without me toching anything. [Where] should I start [the diagnostic procedure to find the cause of the problem]?
jimh posted 10-03-2012 08:46 PM ET (US)     Profile for jimh  Send Email to jimh     
Find or make a sketch of the entire electrical circuit that controls the TRIM pump. Scan the circuit diagram. Post it somewhere I can see it. Then I we can look at it together and see how to investigate the problem. Perhaps there is a diagram of the system in your service manual for the Mercury outboard engine.

Since this discussion is in SMALL BOAT ELETRICAL, I will confine my comments to the electrical portion of the system. For help in making basic diagnostic discoveries or in trouble-shooting basic electrical circuits, please follow the advice I give in

Basic Electrical Troubleshooting Procedures

L H G posted 10-04-2012 12:29 AM ET (US)     Profile for L H G    
That is most likely a defective trim switch on the engine, or possibly in the control handle.
There can be a crack in the "potting" of the switch, causing it to short out and activate the trim either up or down, especially if it has gotten wet.

Try disconnecting it in the engine, and see if the problem stops.

gvisko posted 10-04-2012 07:50 AM ET (US)     Profile for gvisko  Send Email to gvisko     
Thank you Jim and Larry I will check it all out and keep you posted


jimh posted 10-04-2012 12:13 PM ET (US)     Profile for jimh  Send Email to jimh     
I think L H G and I approach the diagnosis of a failure in an electrical circuit from different perspectives. The method I prefer is that which I recommended, that is, to first investigate the circuit and sketch out the circuit, including all the elements of the circuit. I like to see how the circuit works, how it is configured, and, then, give some though to what elements of the circuit could cause the circuit to malfunction in a way that fits the observed behavior. I would call this method the thoughtful analysis method.

There is an alternative approach to electrical diagnosis that I use sometimes and call the close visual inspection method. In this method the components of the circuit under diagnosis are observed very carefully and in some cases with the aid of a magnifying glass and added illumination. In this method I look for signs of malfunction such as discoloration, burn marks, scorch marks, signs of fluid intrusion, leaks, and so on. The close visual inspection can often lead to discovery of location of the component that has failed in an electrical circuit.

There is a third method, and I think this is the method L H G is describing, which I call the poke around method. In this method you use some sort of insulated non-conducting probe--perhaps even your own finger in a low-voltage circuit--and you poke at--that is you prod or push or move--various components in the circuit to see if some sort of unusual physical movement of the component brings on or elicits or causes the malfunction. This method is called the poking around method.

A fourth method depends on being aware of history and tendencies for certain components in certain circuits to chronically and repeatedly and continually fail. When a diagnostician has this knowledge, they can sometimes discover the cause of a present problem by their knowledge of historical problems with the same devices or circuit elements. This method is called the experiential method.

In actual practice, you can combine all four methods: circuit analysis, close visual inspection, poking around, and experience.

L H G posted 10-04-2012 08:16 PM ET (US)     Profile for L H G    
My suggestion is based on experience. When you own a collection of various Mercury outboards for as long as I have, you learn what little things can go wrong (as in trim switches manufactured in Mexico), and what things go right (as in Mercury's highly reliable Oil Injection system).

People with questions come here to save time by getting quick help from an experienced contributor.

Now, this Mercury engine is a 1000 miles from me, and all I have to go on is a brief description. So it's entirely possible my suggestion will not prove out.

jimh posted 10-04-2012 09:42 PM ET (US)     Profile for jimh  Send Email to jimh     
Sometimes an individual's history is clouded with prejudice. Selective memory allows some events to be forgotten, overlooked, or minimized. That is why I like fresh, objective analysis of the problem. In badly designed systems where the same component fails all the time, experience can be a great help in diagnosis.
jimh posted 10-05-2012 09:02 AM ET (US)     Profile for jimh  Send Email to jimh     
Don't sell the poke around method short. It is a good method, particularly on a device that operates in motion like a boat. But poking at a suspect part is enhanced if you can figure out which component is most likely be the cause of failure is more efficient if you can first identify the most likely suspects.
6992WHALER posted 10-05-2012 11:57 AM ET (US)     Profile for 6992WHALER  Send Email to 6992WHALER     
I have to go with Larry.
I have had two engine mounted trim control switches fail on my 1991 Yamahas. When the switches failed the trim motor just turned on when it wanted to. Mine always trimmed up.

Unplug the engine mounted switch. If the problem goes away get a new switch, if it persists then you know it is not that switch.

outragesteve posted 10-14-2012 01:46 AM ET (US)     Profile for outragesteve  Send Email to outragesteve     
I agree w/ Larry, with one addition: Could be faulty solenoid. I worked on a MerCrusier (same basic system) that had an intermittent trim solenoid: It would go up all by itself, while the boat was runnung! I also had friend w/ a 30' Scarab w/ twin Merc 200's and one engine would "try to start" just sitting on the trailer! Bad solenoid. Try to replace with factory/OEM parts, the aftermarket stuff can sometimes be suspect. Good luck!
jimh posted 10-14-2012 09:04 AM ET (US)     Profile for jimh  Send Email to jimh     
I agree with all of you with this exception: it could be any component in the circuit that is causing the problem.

If there is a known history of habitual, chronic, repeated, continual failure of one component in the circuit more than others, that is the component to check first. This is an outgrowth of the experiental method of diagnosis.

With this component identified, the poking around method should be applied to the component. If physical agitation of the component causes the onset of the problem, you can can further test by another method, the substitute and replace method.

In the substitute and replace method, you temporarily remove the suspect component from the circuit and substitute a replacement that is know to work properly. In the case of a switch that is a normally-open circuit you can substitute no circuit connection at all for that switch.

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