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  Voltage Drop When Engaging Tilt and Trim

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Author Topic:   Voltage Drop When Engaging Tilt and Trim
Newtauk1 posted 07-01-2010 09:48 PM ET (US)   Profile for Newtauk1  
I notice that my voltmeter drops to 12.0-volt from 13.8-volts when underway and pushing the Tilt-Trim switch. Is this normal for an 1998 Newport 2001 with Mercury 75-HP two-cycle and a 12-month-old battery?
where2 posted 07-01-2010 10:13 PM ET (US)     Profile for where2  Send Email to where2     
Sounds normal to me. I know my 1985 Johnson with power trim and tilt does the same thing to my analog dash mounted volt meter when I activate the trim switch while underway. In my case, my engine only has a 6A or smaller charging circuit output, while the trim motor may draw twice that much current when operated while the boat is underway.
David Pendleton posted 07-01-2010 10:59 PM ET (US)     Profile for David Pendleton  Send Email to David Pendleton     
Yes, it's normal.
Newtauk1 posted 07-02-2010 08:28 AM ET (US)     Profile for Newtauk1    
Thanks. still dealing with a pasky tachometer problem so I am paranoid about everything.
jimh posted 07-02-2010 10:02 AM ET (US)     Profile for jimh  Send Email to jimh     
Typically the tilt-trim device runs from an electric motor that creates the hydraulic pressure for the actuator. When you engage tilt-trim the electric motor draws current, probably a substantial load and around 20-amperes or more.

The voltmeter is probably connected in a branch of the power distribution which is feeding electrical power to the outboard motor. It is not monitoring the battery voltage directly, but rather the voltage at the outboard motor as supplied from the battery.

In normal operation the battery voltage is being pulled substantially higher than its actual terminal voltage by the charging current from the outboard motor. The typical battery terminal voltage while charging is in the range of 14 to 15-volts. When the trim motor operates, the charging current is much less than the load current, so the battery terminal voltage immediately drops to its actual terminal voltage without any charging current. As current is drawn from the battery the terminal voltage sags lower. The sag in terminal voltage under load depends on the size of the battery, its state of charge, and its internal resistance. A drop to 12.0-volts would be typical for a load such as the 20-amperes of a tilt motor.

Since the voltmeter is probably not connected directly to the battery terminals, its reading reflects any voltage drop in the power distribution wiring, too.

number9 posted 07-02-2010 08:51 PM ET (US)     Profile for number9  Send Email to number9     
quote:
electric motor that creates the hydraulic pressure

The motor drives a small hydraulic pump.
jimh posted 07-03-2010 10:25 AM ET (US)     Profile for jimh  Send Email to jimh     
Gosh--Do you think readers thought there was a magic motor that turned electricity directly into movement of hydraulic fluid?

I think you mean the electrical motor creates the mechanical energy which is converted into a rotary motion which couples to an input shaft of a pump which causes a flow of hydraulic fluid under pressure in the desired portion of the trim actuator system. If you want to get pedantic about how things works, I will be glad to play along. Nothing of that has much to do with the voltage drop.

dfmcintyre posted 07-03-2010 03:31 PM ET (US)     Profile for dfmcintyre  Send Email to dfmcintyre     
Jim -

You've got too much time on your hands. Very slowly, move away from the keyboard, shut down the link and get out on the water. It's a beautiful day here in Michigan.

What's my excuse? Just got back from the boat!

Don

jimh posted 07-04-2010 08:11 AM ET (US)     Profile for jimh  Send Email to jimh     
ASIDE to Don: The above was posted from my boat. A good Wi-Fi connection in northern Michigan made it possible.

Back to the voltage drop. If the voltmeter is wired into the engine harness--and most of them are--it also exaggerates the voltage drop. When the engine is supplying charging current, the voltmeter is wired to the source of the charging current, not the battery itself, and the voltmeter shows the voltage at the alternator, which will be higher than at the battery. When the trim motor is operated, the voltmeter is at the load, and now it shows a lower voltage than at the battery. As a result the actual voltage change at the battery itself (as would be measured if the voltmeter were connected right across its terminals) would be less than shown on the voltmeter wired into the engine harness.

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