TOPIC: 1970s? merc 850

OK dumb stupid move on my part I had the battery cables backwards on the battery. Explains everything other then why it was still cranking shouldn't it not crank if they are backwards. Next time I go to my parents ill have to check it out and see.

Well, you definitely blew the rectifier. And hard to say about the switch box. Might or might not be ok.

I suspect your root cause is a failed distributor trigger. Unfortunately the trigger housing itself is about $500 for a CDI replacement. Then you have to completely disassemble the distributor to replace the housing, along with installing new bearings.

Maybe your parts motor has a good distributor. You can check it by disconnecting the 3 wires from your existing distributor, then hooking up the parts distributor (obviously, that one has to be removed from the parts motor). Make sure the "test" distributor is grounded (with the normal dist ground wire, a hunk of wire, or whatever, to the block), and with the key "ON" turn the distributor shaft clockwise. You should get spark with a good distributor.

This guy on YouTube has the ign box test down pretty good, just jumper brown and white terminals as he describes. then momentarily ground the black terminal, and you should get spark out of the coil wire, if the switch box is good.



Anyway, there's enough info for you to test the critical parts of this ign system, the distributor and switchbox. I'd wouldn't worry about the coil, it's pretty tough and unless you see cracks in the laminations or anywhere else, it's very likely to be good.

I'll have to check that as well next time I go over to my parents. Is the rectifier what is connected to the stater if so good news there is its still good as I haven't connected it and have not hooked up my new stater yet.

I'll bring the extra distributor with me as well next time and test it. If not 500 dollars is still cheaper then a new motor. Just a matter of getting it all apart and back together and running which hopefully I can do. The engine did run though hopefully it was just a dumb moment for me.

After doing this test today I think that the switchbox and or ignition are bad.

I might just bite the bullet and get the cdi switchbox does anyone have any helpful info on getting these or how to install them?

I work on older Mercs and have repaired or restored a number of them. The switch boxes are readily available used from a number of sources including Ebay and your local outboard bone yard. Triggers are rarely the problem unless the wiring has failed. usually caused by mice chwewing wires or the old bad wiring originally used on Merc's before 1990.

The used switch boxes I've installed function fine but in most cases you will need to do some re-wiring and cleaning of a all terminals. Coils in these old motor are usually fine but easily checked. Also you should check plug wires for corrosion or at least for continuity. I budget about $200 for a used switchbox and trigger assembly if required.

When you do that test, you also want to make sure there's 12V to ground at the Red terminal on the opposite side of the ign box. That's the power for this ignition. If you had that, and jumpered the brown & white terminals on the other side where the distributor wiring connects, and there were no sparks out of the coil when you momentarily grounded the black terminal, then the box is probably bad.

BTW for this test you did disconnect the distributor, right? Were you able to test the other distributor with the suspected-bad switchbox?

How about the ign box on the spare motor? The switchbox is one of the most reliable parts in this ign system besides the coil. Unless, of course, it's been damaged by a polarity reverse. The trigger in the distributor is the least reliable.

Here are some switch boxes on eBay. NOT CHEAP! The least expensive new one is $110 shipped. There are some CDI switch boxes with a combined smaller ign coil on a nice aluminum mounting bracket, for under $300. These work very well.

Heck, you might be able to find another parts motor with an intact ignition for cheaper!

You can use a 6-cylinder box to run the 4-cyl ign, it'll make sparks just the same. But the "tach" terminal on the box won't drive an old Airguide tach or "SS" tach accurately. You can still drive a "modern" tach off of the rectifier, so this is no big deal.

Here's the ebay link. This site still parses, for whatever reason, any links to eBay. So I have to put spaces in the link, as a workaround. Copy, paste into browser window, and remove the spaces. Or search on "(Mercury, Mariner) 332-2986"

https:// www. ebay. com/sch/i.html?_nkw=%28Mercury%2C+Mariner%29+332-2986+%28module%2C+switchbox%2C+%22switch+box%22%2C+%22ignition+box%22%29&_sacat=0&_from=R40&_trksid=m570.l1313&LH_TitleDesc=1&_odkw=%28Mercury%2C+Mariner%29+2986+%28module%2C+switchbox%2C+%22switch+box%22%2C+%22ignition+box%22%29&_osacat=0

Hopefully that works right! Good Luck!

One thing I forgot on the switch box test, it's been so long since I've done one - On the side of the box that has the big red wire coming from the rectifier (this is opposite the side that the distributor connects to), you'll also see a white wire, connected to the white terminal. Well, the box has to have power not only on the Red wire, but the white wire too.

The power from the white wire comes from the ign switch when it's in the "ON" or "Start" position. Basically it's what turns the switch box power on and off.

So check with a meter, with the ign switch "ON", and see if you're getting +12VDC to ground from the red wire, and +12VDC to ground from the white wire. If so, then you've got the correct power inputs to allow the box to make sparkles!

An excellent source of wiring diagrams for Old Mercs here:

maxrules.com/mercwireindex.php

And they have a ton of other helpful info on the site. Including a link to the Very Aggravating Online CDI Troubleshooting Manual!

I actually did this test with both switchboxes so my assumption is that I've blown up one by reversing the polarity. And the other one just wasnt good to start with. However I did check for voltage before running the test and I did have the 12v that are nessasary on both the starter side and on the distributor side of the motor.