Mercury 1974 850 Ignition Problem

[High Plains Fisherman]

Hi, this is my first post, but not my first visit to this great site. I've learned a lot by reading the various threads.

The problem I am having is with an old, beat up 1974 850 Mercury's ignition system. I am not getting any spark on #3 cylinder. I have a 5772a series switch box and pulled the wires from the box to a coil that was firing and hooked them to the suspect coil. It fired great. Then I took the disconnected coil and hooked it to the wires for #3 cylinder and got no fire. This tells me the coils are working properly. That leaves me to suspect that the switch box is bad. Is it common to lose just one cylinder on the switch box, or do I have some other problem that I need to check out? Any help with this problem would be greatly appreciated.

 

[ricksrster]

Re: Mercury 1974 850 Ignition Problem

Post your serial #. I believe you have a 1976 or a 1977 by your description. 1974 and 1975 had different type ignitions. If you have 4 coils go to this link:
http://www.homestead.com/dolphinmarineservice/Ign5.html

 

[High Plains Fisherman]

Re: Mercury 1974 850 Ignition Problem

The serial number on the engine is 3877779. I thought it was a '74 model, but there are many inconsistantcies with this motor, so I am not positive of what it is. I looked on some websites with schematics, and that's the box that is shown on them, and the box that is on it, but my Seloc manual says it should be a different setup.

 

[Chris1956]

Re: Mercury 1974 850 Ignition Problem

High, That motor has either a battery/distributor and one coil ignition, or the ADI ignition with four coils. You need to use the correct troubleshooting guide. The year is not that important for what you need to do. They components that can go bad are the coil, switchbox, trigger, stator and rectifier. The manual should help you isolate it.

 

[ricksrster]

Re: Mercury 1974 850 Ignition Problem

I have that 5772 Switch box on my 850. Here are the test numbers.
You will need a voltmeter with DVA to check voltages. You can use a regular OHM meter for resistances.
Stator : Blue to Blue/White = 5800-7000 Ohms. DVA = 180V+.
Red to Red/White = 135-165 Ohms. DVA = 20V+
You should have 180V+ from each of the coil posts on the switchbox to ground.
Trigger: Brown to White/Black and Purple to White = 800-1400 Ohms. DVA = 4V+.

 

[High Plains Fisherman]

Re: Mercury 1974 850 Ignition Problem

Thanks for the specs and diagram. I appreciate it very much. I do have a question about it though. In the upper left side of the diagram it shows that the pigtail on the coils should be grounded. Is this just when testing or all the time? Mine aren't as they are inside the rubber jackets that cover the bottom part of the coils.

 

[CharlieB]

Re: Mercury 1974 850 Ignition Problem

There should be a black wire on each coil connecting the primary side to ground.

A green/? wire also on the primary side.

The plug wire.

The DVA meter tests must be done to the trigger(s) to determine if it is the triger or the ignition module at fault. You've pretty much proved that the module is not firing the coil, no test to determine why, is the trigger failing to trigger the module, if the trigger output tests OK then you can eliminate the trigger, and then double check the box output again with the DVA meter once more before ordering a box.

Make NO assumptions, these parts get expensive fast.

 

[High Plains Fisherman]

Re: Mercury 1974 850 Ignition Problem

Thanks very much for the info. I am fairly ignorant about outboard CDI system, so I have a quick question. Does the trigger fire four distinct and seperate times (one fire for each coil), or does it fire once per revolution of the flywheel?

 

[Motor Boater Bill]

Re: Mercury 1974 850 Ignition Problem

The pigtails on the coils should ground to the metal mounting bracket that holds the coils. The wire should be sandwiched between the rubber jacket and the bracket.

 

[High Plains Fisherman]

Re: Mercury 1974 850 Ignition Problem

Thanks Bill, I was curious about that as none of mine are grounded that way. They are all inside the rubber jacket.

 

[Motor Boater Bill]

Re: Mercury 1974 850 Ignition Problem

Thanks Bill, I was curious about that as none of mine are grounded that way. They are all inside the rubber jacket.

Really? Are these the 339-5288T2 coils? If so, I think the pigtail is the only ground for the secondary winding. Someone please correct me if I'm mistaken on that. However, if I'm correct, these need a good, positive ground. I have those coils on my 1976 Merc 200 and the metal bracket they mount to was taped off when the motor was painted at the factory to leave a bare metal surface where the coils mount. In fact, if you look at the schematic posted above it clearly states "Lead on back side of coil must be grounded" (upper left corner of schematic). Try grounding those leads. The ones that are firing may just be arcing somehow.

 

[High Plains Fisherman]

Re: Mercury 1974 850 Ignition Problem

Thanks again for the response Bill. Yes, those are the same coils. I pulled the whole coil assembly off today, and looked at it closely. The whole mess was corroded and full of dirt, etc. Anyway, three of the coils had the pigtails outside of the jacket like they were supposed to be (it was hard to see them from all the crud), but the coil that isn't firing (even though the coil seems to be good) wasn't grounded by the pigtail. That particular coil had been removed and replaced by a previous owner I'm sure as the pigtail was taped to the back of the coil. I am soaking the housing in solvent and will get all the crud off tomorrow. I wonder if the ungrounded coil fried the portion of the switch box that it connects to. As stated before, hooking the wires to a different coil produced no results, but the suspect coil fired when hooked to a set of wires from a coil that was sparking. Again, I sure appreciate the response about the pigtails.

 

[Motor Boater Bill]

Re: Mercury 1974 850 Ignition Problem

I've heard that a bad coil (ungrounded in this case) can damage the CDI box--but I'd say clean up those contacts and try it! Keep us posted.