1975 Mercury 850 no spark...how to isolate culprit?

[malverts]

Hello,

I am new to the Mercury family and have enjoyed trying to get this motor serviceable again. My budget is quite tight due to having a young family but here is what I know about my problem. Your help and advice is very much appreciated. I bought an older boat with the 850 on it. It did not run at the time and last Sunday I got it running for 1 minute! Then it cut out and there is now no spark.

The manual I have (SELOC) said to remove the switchbox and the trigger and have them tested to save money.. I removed the two parts and called a local marine repair center. They informed me that they cannot test those items off of the boat. Is this true?

Secondly, there is a coil as well. How do I test this part?

Lastly, the distributor cap seems serviceable but I noticed that there was some broken off debris from the coil terminal plunger (don't know the technical term). I am not sure if it is making contact with the distributor shaft or not. It seems the plug wires are not removable from the distributor, is this true?

Thanks for all the help!

Mike

 

[Laddies]

Re: 1975 Mercury 850 no spark...how to isolate culprit?

Try this

 

[Chris1956]

Re: 1975 Mercury 850 no spark...how to isolate culprit?

Mike, There is a simple troubleshooting procedure for the Merc Distributor models. I do not have it at my fingertips now, but I believe it may be posted in the FAQ section of this forum. If not, a search on "No spark" should turn it up. BTW, the distributor wires unscrew (CCW) from the distributor cap. The rotor (under the cap) is not replaceable except as a rotor and shaft assembly ($240). The Dissy cap is >$100 to replace.

On that motor to have spark, you need to have +12V on the red switchbox wire (starboard side of motor) at all times and +12V volts on the white wire below it when the ignition switch is turned on. Check this first. If that checks out, you need to check the coil and switchbox. Remove the quick electrical plug on the outside of the motor. Now remove the three wires from the distributor to the switchbox. Now jumper the white and brown terminals on the switchbox. These are the ones used by the distributor wires. Now reconnect the electric to the motor and hold the coil wire 3/16 from the block. now use a jumper on the black switchbox terminal. Each time you touch the black terminal to ground the coil should fire. If it fires, the coil and switchbox are good, as is the balance of the ignition wiring. The distributor trigger >$250, may be bad......

 

[malverts]

Re: 1975 Mercury 850 no spark...how to isolate culprit?

thank you for your quick and helpful replies. It will take me a few days before I have time to try this but I will let you know how it goes.

I did perform some bonehead moves that may need some further advice. please see next post!

thanks!

Mike