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Old 30-03-2012, 18:37   #1
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Can't Seem to Make Sense of Preheat Solenoid Resistor on Westerbeke 20B Two

BI have two 20B two engines in my boat, so at least I can reference the side that works. However, I cannot seem to get the preheat solenoid to close wired as per this diagram (page 35 in this pdf ). I know it's barely legible. As long as the button is pressed, the solenoid "clicks" very rapidly, never completely closing the work side. But the control side of the solenoid is trying to ground through a resistor. The lift pump is powered off this post, activation when the preheat is pressed, then powered when the oil pressure switch closes. Now, of course the solenoid works fine if just grounded. But when the oil pressure switch closes and applies power, it would be a short and hence the resistor I assume.

Not sure if this is a common way to do things, I am new to marine diesels, or if this is something..... special.
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Old 06-04-2012, 09:25   #2
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Re: Can't Seem to Make Sense of Preheat Solenoid Resistor on Westerbeke 20B Two

On many of this type of solenoid they have an "end of stroke" switch and the solenoid has two coils.

When you first apply power the switch is closed so BOTH coils are used in parallel to operate the solenoid.

Once the solenoid is closed it only takes a small amount of power to hold it on so to save overheating and using power, the end of stroke switch disconnects the "main" coil and just leaves a holding coil operating.

If your solenoid is cycling the holding coil may be shot so when the end of stroke switch opens there is nothing to hold it closed and it starts to release which re-engages the main operating coils and it oscillates back and forth.

Depending on how it is constructed you need to diagnose the circuit to the "holding" coil if it is accessible. If it is all internal you will need a new contactor.

Oops, tried to add more information, apology if this messed up. The Edit function was not allowing edit.

On your solenoid the end or stroke switch could be removing the short across a resistor so now the resistor is in series with the coil to reduce power but still have enough current to hold it in. If the resistor is bad, then when the switch opens the current would drop to zero and cause the cycling like described above.
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Old 08-04-2012, 19:51   #3
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Re: Can't Seem to Make Sense of Preheat Solenoid Resistor on Westerbeke 20B Two

That appears to be a common older Ford started rely. As best I understand the semantic you have to turn the ignition switch to on, push the preheat button and while holding the preheat button press the start button. It appears that if you are not depressing the preheat button you can not get the starter to engage. If this is not the way it works then the rest of this may be false.

You will need a voltmeter and a test lite. Using the picture on page 35, the posts will be posting one, next down small terminal will be number two, next small terminal number three and bottom large terminal number four.

At rest terminal 4 should have battery voltage, all others should show no voltage.
Ignition switch on and pre- heat button depressed, all terminal should have battery voltage. ( if you follow the diagram terminal 3, is applying power to the lift pump until the engine starts and the oil pressure switch closes. At this point the lift pump gets its power from the ignition switch. The $64 question is why is there a resistor, the only thing I can think of is that it bleeds off the voltage spike when the relay opens. )

I would start by doing these readings off of the good engine and comparing them to the problem child. You might start with resistance readings on the relay coil and resistor. ( Don’t forget to disconnect the wires while you do this.) It sounds like either in the starter solenoid has gone bad or you have low voltage ( corrosion). Either way somewhere along the line you will find the difference between a good engine and the problem child. I guess this is one of the other good points of having two engines. Good luck. Just my two cents, Mike.
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