mercury 850 wiring diagram

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kenlynn

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I bought a 850 mercury s/n3741831 {don't know the year either } with distributer . It will not fire . the brown wire on the ignition side of brain box runs back to key switch but is not hook to nothing. The white wire same side is hooked to red, nothing were the white wire is supposed to be. Does someone happen to have a wiring digram for one and could someone tell me what year it is
 

emckelvy

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Re: mercury 850 wiring diagram

Looks like your motor is a 1973. The brown wire is the tach wire and for use with old Merc tachs the wire was connected to the switchbox. For newer tachs which use a signal from the alternator, the wire would be connected to one of the two yellow stator wires hooked to the rectifier.

The white wire on the same side of the switchbox as the red wire should be connected to the white terminal on the switchbox. The white wire provides switched +12V to the switchbox and may be the reason you have no spark. Otherwise I bet the trigger assy is shot.

Here's a link to wiring diag for your motor:

http://www.maxrules.com/oldmercs/Wiring/1966ona/30.gif

And here's some info on how to troubleshoot the Merc CDI distributor-type system:

MERCURY BATTERY-POWERED CDI TEST FOR DISTRIBUTOR MODELS

This test is for the 332-2986 switchbox used from 1967-1978 on all the inlines.

This test assumes your coil is good (problems with CDI coils are rare).

DISCONNECT BATTERY

1. Turn off ignition;

2. Disconnect all 3 distributor wires on the Port side of the switchbox (and the ?mercury switch? if present);

3. Remove the HV lead from the ign coil to the center of the dist. cap (remember it unplugs from the coil and unscrews from the cap);

4. Reconnect the HV lead to the COIL only;

5. Position the free end of the HV lead approx. 3/8" from ground (block, shrouds etc), and find a way to hold it there;

6. Jumper the brown and white terminals on the dist. side of the switchbox to each other.

RECONNECT BATTERY

7. Check that you have +12V at the red terminal (even with the ign off);

8. Turn on ignition and verify +12V at the white terminal (same side as the red terminal);

9. Ground the black terminal on the distributor side of the switchbox - this should cause a spark each time you touch ground.

If you get spark with the distributor bypassed, and it won't fire with the distributor connected, the trigger is bad and the entire distributor housing assy must be replaced.

If you get no spark using the test, the switchbox is probably bad. In that case, be sure to check for correct power on the switchbox, check all connections, and check the coil's resistance to make sure it's OK.

HTH............ed
 

kenlynn

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Re: mercury 850 wiring diagram

They had the white wire back feeding key switch. Moved white back to igition box did not have no voltage on white wire. Started looking red was broke that feeds key switch fixed it. I now have fire at all plugs, but it will not run pops and gets real stuborn turning over. What is the firing order maybe they have plug wires on distributer wrong You know what i mean . Where woul i get firing order at and how they hook to cap. Thanks Ken.
 

emckelvy

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Re: mercury 850 wiring diagram

There should be numbers cast into the cap on the end where the spark plug wires go in. Note: #1 cyl at top, #4 at bottom.

BTW the wires screw into the cap so don't try to pull them out as it's possible to pull the wires completely out of the crimped threaded terminals. This includes the coil wire; but at the coil end it just pulls out (with quite a stout pull, usually).

You may also want to pull the clamp off the cap and check inside for debris/trash. Carb cleaner in the spray can works real well for flushing junk/gunk out of the inside of the cap. Just don't spray on the trigger or other parts in the dist, just the cap itself. Also check the center carbon contact, it's spring-loaded and should be free to move in its hole. If not, pull it out and clean. The bottom of the spring is a bit bigger than the hole it goes in, you just twist it into the hole and it'll conform to the smaller diameter.

If you have no luck with the cap, you might also want to check dist. belt alignment, who knows what else they've haywired!

Set the flywheel at TDC per the timing marks, and remove the top guard off the dist. pulley, you'll see an arrow cast-into one of the ribs of the pulley.

When the dist belt is properly aligned, this arrow is supposed to point to the timing marks (typically punch marks or a scribed line) on the flywheel and on thru to the centerline of the crankshaft. It should either be 'dead on', or the arrow should be pointing a bit "to the right" of the mark.

HTH..........ed
 

kenlynn

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Re: mercury 850 wiring diagram

Well plug wires were right. Inside of distributer cap a little dirty, don't have carb cleaner will get some tomarrow let you know how it turns out. Do you think maybe plug wires could be jumping fire got a pretty good jolt when i was checking fire, even had a pair of insulated handled pliers {not very funny ] ha ha thanks again ken Thanks a million Ken
 

kenlynn

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Re: mercury 850 wiring diagram

You were right distributer was 180 degrees out of time. Runs like a top now thinks everone for your help.Now Ihave no water coming out of hole. Does this mean a new water pump and how hard is that to do.
 

emckelvy

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Re: mercury 850 wiring diagram

The telltale could be plugged, blow compressed air up the hose or snake a fine wire up there. If it's clear, likely an impeller change will be in order.

Make sure you've got enough water pressure on the flusher, there should be enough pressure to make a bit of water spill past the 'muffs'. This ensures the suction side of the water pump is completely flooded.

Warm water should flow out the middle of the prop. Then you know this water is circulating thru the motor and picking up heat.

The block should never get more than just barely lukewarm when running on a flusher. The exhaust cover should be stone cold (big cover on the Port side aft part of the block).

Plenty of posts here regarding impeller changeout, use the Search feature. Also check the FAQ at the Forums' main page, there should be something there too.

HTH.......ed
 

kenlynn

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Re: mercury 850 wiring diagram

Yep right again had mudddobber nest in it. Only have a trickle of water at idel but pull fast idel has a pretty good stream of warm water.Faster you run it, water runs out faster so i guess its okay thanks Ken
 

Tommy McMinn

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Oct 23, 2015
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Hi Ed,
i have a Merc 850 (76 i think). The red wire from the key switch goes to the rectifier per the diagram. Odd thing is so does the white wire. It was like this when i purchased the boat. The engine runs but the ignition and gauges are all energized with key off which will eventually run the battery down and likely not good for the ignition. Where does the white wire need to go. the diagram shows a switch box but no terminal indicators. my box has a dozen or so terminals. Not 5 like the diagram shows. Any suggestions?
 

GA_Boater

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May 24, 2011
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Hi Ed,
i have a Merc 850 (76 i think). The red wire from the key switch goes to the rectifier per the diagram. Odd thing is so does the white wire. It was like this when i purchased the boat. The engine runs but the ignition and gauges are all energized with key off which will eventually run the battery down and likely not good for the ignition. Where does the white wire need to go. the diagram shows a switch box but no terminal indicators. my box has a dozen or so terminals. Not 5 like the diagram shows. Any suggestions?

Tommy - This thread is over 7 years old. Read the Help Tip on top of the page and start a new topic. Old threads are seldom looked at and you have a better chance for help with a new thread.

Some advice - When you start the new topic, give the serial number of the Merc 850 so we can find the right wiring diagram for the motor.
 
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