'88 Merc Inline 6 115hp w/ no fire on 4 and 6

[bmbecker]

I have a '88 Merc Inline 6 115hp that I am having some ignition issues with. Seems as though the motor will only rev up to a certain RPM when in gear and running. Suddenly it will kick right in and take off and run like it should. I took the hood off and check the plugs...all looked good but changed then anyway. Then checked fire from my coils. No. 4 and 6 have no spark. I swapped coil No. 5 with No. 6 hoping it would work and I could replace the 2 coils and be on my way. But this didn't fix the prob. My book talks about testing the Stator, Trigger, and Rectifier but I dont know if it says to much about testing the switch box. I would just replace it except that...knowing the switch box controls 2, 4, and 6 and 2 has spark...im not real sure as to how to approach this. I am hoping someone out there has some idea of what step to take next. I appreciate all help. Thanks...

 

[CharlieB]

Re: '88 Merc Inline 6 115hp w/ no fire on 4 and 6

More likely you have a broken wire from the trigger, sometimes making contact as the trigger is rotated to advance with throttle opening.

With all plugs out and wires connected to a spark tester.

Disconnect the throttle cable

Crank and watch for spark while advancing and retarding the timing/throttle linkage. Or use a DVA Meter on the suspect trigger leads and watch for an intermittant reading as the timing is advanced/retarded, moving the trigger wires.

PROVE the fault before ordering any electrical parts.

 

[bmbecker]

Re: '88 Merc Inline 6 115hp w/ no fire on 4 and 6

Im thinking about switching my switch boxes around and seeing if the prob persists on the same cyclinders or if it moves with the control box. I already did a similar test with the coils and proved the coils be good. I would guess if it doesnt fix things that my triggers is the next thing. Not really sure about testing for a broken wire. I dont have a DVA meter, just a multi-meter from walmart...been around the block a couple times and works great but dont know that it does all the special things as a DVA meter. At least I wouldnt think so, considering the cheapest DVA meter I have seen online is about $400 or so and mine cost me about $20 or so. I will look into that though.

 

[emckelvy]

Re: '88 Merc Inline 6 115hp w/ no fire on 4 and 6

Very simple to put your meter on the ohms scale and check the trigger wires for proper resistance reading. Wiggle and pull on the trigger wire harness and a break should be readily apparent.

What happens is, the wires flex every time the throttle is cycled. This can eventually lead to a break in the wire. If you give a good straight tug on each trigger wire after you disconnect them from the CDI boxes, and one of the wires stretches, you've got a bad one.

If it's not all corroded you can repair it by cleaning up the wires, twisting them neatly together, and soldering. Don't use a splice connector, too bulky and it'll make a stress point that'll bust sooner or later. Place one layer of electrical tape around the soldered union and a length of heat shrink tubing over that, be sure you overlap the joint with the heat shrink about 3/8" on either side.

HTH & let us know what you find..........ed