What is this part and what does it do?

[Dennis4b]

Hi,<br /><br />This question concerns a 1988 Mercury Black Max V6, 150HP.<br /><br />The symptoms indicating a problem were, at first, a lack of power, taking a long time to plane (which it really shouldn't with this prop and HP) and not exceeding 4000 RPM, even with a smaller pitch prop (roughly same RPM is reached). The tachometer goes berserk above 2000 RPM, either dropping off to less than a thousand, or jumping wildly between 500-2500 RPM. Lastly, the engine doesn't sound right when this happens indicating something is wrong.<br /><br />Further testing at the dock shows that when in idle, and the engine is cold, for a brief moment it will idle above 2000 RPM and all seems well. However, as soon as the engine heats up a little bit the tacho stops working properly and the engine starts to sound rougher above that RPM. Note that a strobe does show spark on every cilinder (compression is also good), and as soon as the throttle is released just a bit to below 2000 RPM the tacho returns to normal and so does the engine.<br /><br />Tracing the tacho wire leads to a small black box, attached on top of the engine between the 2 top cylinders and the flywheel (near a water hose coming out of the block), attached with 2 bolts and having a red and a yellow wire, the metal body providing also a ground to the box. One of the wires goes to a connection point that includes the tacho wire (meaning the tacho wire going to the controls) and a connection leading to the stator, the other one goes to a connection point involving the switchboxes and also to the stator. <br /><br />When using a scope to look at the "input" wire, i.e. coming from stator, and before leading out of the part to the tacho, you can see a good pulsetrain, for this engine I believe it is 6 pulses per revolution. This pulse train stays perfect even above 2000 RPM when the engine starts to act up. If the tacho is connected to this point, rather than the "ouput" of this box, it works perfectly at every RPM. <br /><br />On the other wire though, the pulsetrain will break up above 2000 RPM (except not usually when engine is really cold, which doesn't last long), and the tacho will behave badly. The engine will also suddenly break up, sound changes. There is still spark on all cylinders, but I can't tell if for example it's only sparking every second revolution. If you take the scope probe and touch the other end, all is fine.<br /><br />The box itself measures as a direct connection between 2 wires (yellow and red). The scope can not identify it, there is no resistance either way. Still, it appears to do something because the input pulse train is badly butchered above 2000 RPM and the engine suffers suddenly (suddenly enough to suspect an ignition problem rather than a fuel problem - also because of the tach signal being butchered as well).<br /><br />My question is, what does this box do (filter?) and how to test if it is working correctly. Also if it is faulty can it cause my ignition problems? The engine always starts turn-key, probably because all is fine under 2000 RPM. I can just not imagine anything being designed to butcher a pulse train (cutting pulses) above a certain RPM.<br /><br />Sorry for the long post, but I thought I'd include as many details as possible. I wish I had brought a digital camera, a picture is worth a thousand words <br /><br />Best regards,<br />Dennis

 

[Yepblaze]

Re: What is this part and what does it do?

You will probably need a peak reading voltmeter but.<br /><br /> http://www.homestead.com/dolphinmarineservice/Tech.html

 

[Trent]

Re: What is this part and what does it do?

Does the part look like this???????

 

[Dennis4b]

Re: What is this part and what does it do?

Yepblaze, the friend I borrowed the scope from said it has that feature, picking up the peak voltage, however in the current state I haven't needed that yet, it's about the pulse train so far <br /><br />Trent, size is about right, I guess the models changed a bit over the years. It's not an exact match, but basically a 2 by 1,5 inch housing with metal grounded bottom and sides, attached via 2 bolts, and having a yellow and a red wire connected. I really need to take that picture!

 

[Dennis4b]

Re: What is this part and what does it do?

Hi again,<br /><br />I have taken some pictures of the engine to illustrate better what is going on. It is turning out to be very difficult to find a proper shop manual for this engine over here (Finland), maybe in 4 weeks, but that's what they said 5 weeks ago also :-(<br /><br />

<br /><br />Let's start with a side view. <br />The blue-greenish area at the bottom comprises the switchbox, with another one stacked underneath. Underneath the tape are 6 connections, 3 that receive the trigger pulses from the stator, and 3 that send the CDI pulse to the primary winding of the coils.<br />The purple area is the starter solenoid. <br />The red area is the tacho wire, leading to the remote control box and from there to the tacho gauge. It is connected in the yellow area to a device that shows up in a next picture.<br /><br />Question: Is my assumption that it receives and fires a seperate trigger and pulse respectively for every cylinder correct? It does not do any timing (which is done mechanically), and so it just fires a coil as soon as it receives a trigger pulse?<br /><br />Question: With a scope or DVA, what is the reading for a correctly functioning trigger?<br /><br />Question: With a scope or DVA, what is the reading for a correctly functioning CDI pulse to the coil?<br /><br />

<br /><br />In the light-blue-greenish area is once again the switchbox. The green area shows the box, of which I don't know what it is supposed to do, but that it does do something, and that that is a measurable place where the ignition pulses start to act up, above 2000 RPM and when the engine is warm.<br />The blue box next to it would be, rectifier/regulator? I don't know.<br />Yellow area are the plugs, purple area the coils. Every coil is connected to a switchbox.<br /><br />Question: what is the box in the blue area? How can it be tested to be working correctly?<br /><br />

<br /><br />Here is the box on top of the engine, green area in the previous picture. It only has 2 wires, the red and yellow, and through its metal housing a ground. In short, a signal travels from the stator through this box, then to a connection point involving the tacho sender wire. Below 2000 RPM, both sides of this box show a pulse train, tacho works properly and I guess the engine does too. Above 2000 RPM, the input side is still fine, and if the tacho is connected there instead, it also works fine. However the output side is badly misshaped, missing pulses, etc, causing the tacho to act up (drop off or jump around), and the engine starts to run rough as if an ignition problem (too sudden change either way to be fuel). Peak voltage on either side is +- 14 volts, minimum voltage is about 7-8 on the output side, and 0 on the input side. All cylinders are still sparking when this happens, but I can't tell if it misses beats or misfires.<br /><br />Question: what does this box do? How can it be tested to be working correctly?<br /><br />

<br /><br />Yellow area is the tacho wire, purple are the 2 wires connected to the box in the previous picture, one red and one yellow wire. The blue area are the top connections on the switchboxes. The leftmost of these connects both boxes together, and also has a connection to the stator. The other 3 go to the stator, both boxes independantly, i.e. not cross-connected.<br /><br />Question: What are the top connections (4) on the switchbox exactly? How to check (scope or DVA) for correct operation?<br /><br />

<br /><br />On the other side of the engine there is a box shown as a brownish area with quite a few wires connected to it. Since it is connected to the oil tank floater, pump, and pulse from the switchbox I guess it is the oil pump controller? <br /><br />Other notes: the engine charges the battery, no overvoltage, consistent through the break up that occurs above 2000 RPM. Cylinders never lose spark altogether, and the result of the ignition breakup above 2000 RPM do not show with a strobe (since I can't see if it misses or misfires). Symptoms while driving are lack of power and inability to climb above 4000 RPM, even when the prop pitch is dropped (in fact 21" and 19" get to the same RPM).

 

[clanton]

Re: What is this part and what does it do?

The brown box is warning module. It gets signal from oil tank, overheat, and oil pump sensor, also has one wire to power pack(Ithink). The little box with the 2 yellow wires is rectifier, and is open, should open one way closed the other.<br /><br />Timing sensor 4 volts<br />pack output to coil 180 volts<br />charge coil output low side 20 volts<br /> high side 180 volts.<br />There may be 2 other modules a timing, and a idle module.<br />You need an OEM service manual. There is one poster on www.screamandfly.com that posts some pages from oem manual.

 

[Dennis4b]

Re: What is this part and what does it do?

Clanton, thanks for the voltage info!<br /><br />However the box I am holding is not the rectifier, that one is located next to the starter solenoid roughly above the switchboxes. The box I am holding, which is installed on top of the engine (looks like an afterthought design) is wired in parallel with the yellow stator AC wires going to the rectifier. I guess it is some kind of filter.<br /><br />At above 2000 RPM one of the yellow stator wires (AC power) starts to break up, giving bad signal, don't know yet if it is the rectifier, but since it is the one providing the tach pulses I guess I can switch the tach wire to the other stator wire and it should work fine. I am curious though wether the rectifier output from one good and one bad AC stator wire can have influence on the ignition; I assume it will also cut away some of the battery charging power.<br /><br />Then I pointed the timing strobe at the flywheel to look at the timing marks.<br /><br />3 cylinders (in different banks though) fire perfectly, the marks are steady within 1 degree at low RPM, and totally steady at high RPM. Never blink, move, or hiccup.<br /><br />1 cylinder does the same for the high RPM, but i lose the "lock" every now and then at low RPM, or the marks show up in a completely different place, meaning it misfires and probably also too often or too little.<br /><br />2 cylinders fire at completely random times, the firing rate does increase with RPM, but it is never at the same moment of the engine revolution. I can never get a lock on the marks, it just seems to do what it pleases <br /><br />I guess all this can explain smoke, oil in water (Bad bad bad!!!), roughness, lack of power, but still a turn-key start every single time!<br /><br />Any ideas now?