1975 MERC 850 Switch Box Bad?

I recently bought a 24' playcraft pontoon with this 1975 MERC 850 (I'm assuming) serial # 4296995.

I took it to a recommended merc vet mechanic and he rebuilt carbs, replaced stator wires, coil, rectifier, and got it running great. After about 3 or 4 days out the motor was idling and all of a sudden died. I pulled the cover and noticed the insulationi on one of the connectors for the wires coming from the switch box had vibrated down the wire leaving the connector exposed. Im assuming this shorted out on the case of the motor and caused the issue.

I took the motor to U.S. Boatworks in Kansas City, KS and they said the coil had a weak spark and replaced it. When i got it back the motor would start only with the rev switch all the way forward. It would also rev quickly then immediately die. After this happened for about 5 minutes I was able to get it to idle and run properly. I ran it across the lake about 10 minutes and as soon as reduced to idle speed it died.

Now it will not start. Does this sound like the switch box is bad from shorting out and causes the coil to go bad as well (maybe the new coil bandaded the problem for a very short time?)

If anyone is educated on these motors (MERC850 out there?) please let me know what you think. I am only 26 so I don't know much about these old things and i'm trying to fix this one cheap (I have about a $1000 budget for a motor for this boat)

Re: 1975 MERC 850 Switch Box Bad?

Paul, That motor should have a distributor/battery driven/CDI ignition. The CDIElectronics website will have an ignition test for your motor.

In general the coils almost never go bad. The stator only charges the battery, so it is not relevent.

There is a red and adjacent white wire on the switchbox (non distributor side). The red will always have +12VDC on it and the white wire will have +12VDC on it when the ign key is on. Check those for voltage. If one or both don't have voltage, you will not have spark.

Re: 1975 MERC 850 Switch Box Bad?

The Fri box is part # 114-2986. Coming from it are the following wires:

White with red stripe - reads 12v with key on.
Black - goes strait to ground (cdi box mounting bolt used)
Green - runs to coil.
White only - runs to distributor
Black - runs to distributor
Brown - runs to distributor

The white with red stripe wire does read 12v when the key is turned to the on position.

The brown wire does read above 9.9 volts while cranking.
The white only wire reads no voltage while cranking.

The manual says this means the problem is in the harness, starter, battery, or key switch.

I do remember the large set of wires running from the motor to the key switch and controls got smashed on a trailer and the motor was making the whining sound like the key was on. I repaired the wire by cutting and re soldering all wires and insulated them appropriately.

The motor operated fine on multiple outings before this, and then quit the second time out after this. Could this have shorted something out?? Also why after having the coil and distributor replaced would it run for 1 day and then quit again?

I have not checked for spark to all cylinders yet

Re: 1975 MERC 850 Switch Box Bad?

The brown wire also read above 9.5 volts with the key in the on position AND while cranking

Re: 1975 MERC 850 Switch Box Bad?

The white wire reads about 1.5 volts while cranking and the
brown wire voltage reads approx 12v while key on and 11.6 while cranking.

Sorry for so many replies I am troubleshooting as we speak and just used a digital voltmeter as opposed to analog for more accurate readings.

Re: 1975 MERC 850 Switch Box Bad?

What switchbox terminal is the white/red wire connected to? Is there a wire on the terminal above it?

Re: 1975 MERC 850 Switch Box Bad?

I had a 76 850 thunderbolt and went through what you are going through with the repairs etc. Finally my mechanic after consulting with others in the field said that the engine was worn to the point that it would not suck fuel. It ran great when not under load like out of the water with the muffs on it. But once in the water it did what yours is doing. I don't know if my guy was correct but I sold the boat and engine to a lawyer in Ohio. I charged him for the boat and threw the engine in for free telling him what was wrong with it.

Re: 1975 MERC 850 Switch Box Bad?

Chris1956 said:

What switchbox terminal is the white/red wire connected to? Is there a wire on the terminal above it?

Click to expand...

The cdi box is a sealed unit so there are no terminals. The date on the cdi box shows 2004. Again, the boat ran perfectly fine for weeks before the harness wire got smashed. After I repaired that wire it only worked a few days. Then again after the cook was repaired it worked 1 more day.

Fuel delivery is not a problem. The motor functioned just fine under load at wot when it was running.

Please read the above posts in detail they describe that the problem began shortly after the harness wire was smashed. I'm assuming this shorted something to the point that it is not producing strong spark. Us boatworks diagnosed this and replaced a coil and distributor cap which fixed the boat for 1 day.

Could something be knocking out coils and that is why the fixed coil bandaded the problem?

Re: 1975 MERC 850 Switch Box Bad?

Check again just to be sure that you are getting a hot blue spark at least 7/16 inch to every cyl. If you have good spark then check the belt to ensure it hasn't worn or stretched so much as to allow it to have 'jumped' a tooth and thrown the timing way out.

If these check out then the problem is still in the fuel system, take it back to the guy that did the carbs and see if he is willing to make it right.

These old motors are thirsty and take a LOT of fuel to start, lots of choke, if the carb floats are not set right they can get very tough to start.

Re: 1975 MERC 850 Switch Box Bad?

OK< so you have a CDIElectronics replacement switchbox. You should not be using a stock Merc coil, as a special CDIElectronics coil is mounted adjacent to the CDI Switchbox. Is that what was replaced by your mechanic? Did he replace it with a CDI Coil?

Re: 1975 MERC 850 Switch Box Bad?

The spark seems week and sometimes intermittent at each cylinder. Again, the white wire on the cdi box is checking out according to cdi. If I jumper Tue white and brown wires together then ground the black wire with a spark plug on the main coil wire it sparks every time I ground the black wire.... This is making me think it is tue trigger. Can the trigger be harmed by the harness shorting out? I also called my mechanic who is a month away from getting to it. He said it could be the rectifier and or trigger

Re: 1975 MERC 850 Switch Box Bad?

I meant to say the white wore coming from the switchbox shows 1.5 volts sometimes spiking up to 16 but only every so often

Re: 1975 MERC 850 Switch Box Bad?

Triggers can be intermittent. I do not know if they can be damaged by shorting, but the test you ran is designed to prove the trigger is bad.

Re: 1975 MERC 850 Switch Box Bad?

Retest the White and Brown wires using a volt meter with the key ON, and again with the key OFF.

If the White wire is NOT turning on and off with the key you have a problem with the ignition switch and/or wiring harness.

Triggers in the old distributors do not usually come and go, when they fail they are usually toast and will never work again.

But before even thinking of ordering any NON-RETURNABLE electrical part you need to test, re-test, and test again yet a third time to be sure that you have proved the part bad.

Ignition switches are a very common failure, but again, PROVE IT.

Re: 1975 MERC 850 Switch Box Bad?

Chris1956 said:

OK< so you have a CDIElectronics replacement switchbox. You should not be using a stock Merc coil, as a special CDIElectronics coil is mounted adjacent to the CDI Switchbox. Is that what was replaced by your mechanic? Did he replace it with a CDI Coil?

Click to expand...

this was replaced by a cdi coil

Re: 1975 MERC 850 Switch Box Bad?

Charlie B.

Ok you are referring to the white only wire right? The white/red stripe (main power wire to cdi box) gets voltage at key on, the brown wire also gets voltage at key on. You are saying the white wire should too!? What voltage should it read? Previous tests showed white only wire at 0 volts key on and 1.5 while cranking, sometimes spiking up around 16. I will test again today to make sure. Also, the rectifier tested bad (no resistance between posts at all), but this would only affect charging the battery right not the motor starting?

Re: 1975 MERC 850 Switch Box Bad?

Stator is used only for battery charging.

The switchbox has a constant power supply and a 'switched' power supply from the ignition switch. Both of these two MUST HAVE at least 9 and 1/2 volts cranking or there is a problem in the ignition switch or wiring harness.

Re: 1975 MERC 850 Switch Box Bad?

Charlie, My reading on the CDI Switchbox is that they combined the constant and switched power supplies into a single switched power supply (white/red), and insist on a special CDI Coil to replace stock coil.

I do not know what the brown wire does for that CDI switchbox.