1990 90hp Merc 3 cylinder, weak/no spark

[bigvinnie]

Motor was running fine until the following happened:

Went to start after drifting a few minutes... started fine but my partner didn't realize the key gets stuck in the start position occasionally because of a weak return spring in the switch, and you just have to manually move the key back. So, motor ran fine for about 10 seconds and died, and we could hear the bendix still working... took another 10 seconds to figure out what it was, and then got it turned off... would not start after that. I figure we musta fried something.

Now seeing no spark at no. 2 plug, and very weak spark at no. 1 and 3 plugs... Battery is fresh and charged up. I have a Clymer shop manual for the motor, just looking for advice on possible likely areas to test.

Appreciate any input. Thanks.

 

[Laddies]

Re: 1990 90hp Merc 3 cylinder, weak/no spark

That sounds like the ign. switch might be bad. Go to the switchbox on the motor and disconnct the bkack/yellow wire from the box. It's the ground out wire for the ign. system, the system works like a mad and 12 volt will burn it up.

 

[bigvinnie]

Re: 1990 90hp Merc 3 cylinder, weak/no spark

thanks for the reply...

I agree that the switch needs replacing... are you thinking that replacing it could be a possible fix for the weak spark issue..??

I'll remove the yellow/black wire and see what happens.

thanks again for the info.

 

[bigvinnie]

Re: 1990 90hp Merc 3 cylinder, weak/no spark

Removing the blk/ylw wire at the switch box had no effect, still no spark or very weak spark at the plugs, as indicated earlier.

I'm hoping someone can tell me what needs to be tested next... or point me to the CDI trouble shooting manual that I've seen mentioned here... how do I tell what kinda of ignition I have, or is there just one for this year/model ..???

thanks very much for the input.

 

[Laddies]

Re: 1990 90hp Merc 3 cylinder, weak/no spark

Here's the test from CDI'

 

[bigvinnie]

Re: 1990 90hp Merc 3 cylinder, weak/no spark

thanks for the info, will continue with the testing... I forgot to say that I also tested with the kill switch in both positions just to be sure it wasn't something dumb, and I was surprised that it had no effect - no spark at no. 2 plug, and very weak spark at no. 1 and 3 plugs.

I guess I thought there should be no spark at all with kill switch engaged... can someone explain..??

thanks.

 

[bigvinnie]

Re: 1990 90hp Merc 3 cylinder, weak/no spark

Disconnected the stator wires from the rectifier according to step 2 of the CDI instructions... still no spark so I'm waiting for my DVA adapter to show up so I can continue testing... but I need assistance in re-installing a wire onto the rectifier that came off while testing...

it's a yellow wire that comes from the box in the first pic (regulator?)... there are two yellow wires from this box that connect to the rectifier, and one slipped off one of the rectifier lugs, which is shown in the second pic.

I had both nuts off the top and bottom rectifier lugs that I removed the stator wires from, and I'm not sure which one this wire slipped off of....

Just need to know if both of these yellow wires go on the same lug or different ones... the middle rectifier lug was not touched . I'm guessing they go on different lugs but I didn't want to take the chance of doing something stupid.

I apologize if this is confusing, thanks for the assistance.

 

[Laddies]

Re: 1990 90hp Merc 3 cylinder, weak/no spark

You need a yellow on either end with the red in the middle, the gray tach sender can go with either yellow.

 

[bigvinnie]

Re: 1990 90hp Merc 3 cylinder, weak/no spark

thanks for the reply... I was able to answer my own question after finding the wiring schematic in the back of the clymer manual...

I do have a question about step 4 of the cdi trouble shooting instructions you posted...

what's the difference between gound and engine ground, and where is the best source for both..??

Also, step 4 lists oem and cdi numbers....how do I tell if my setup is oem or cdi..??

thanks for the assistance.

 

[Laddies]

Re: 1990 90hp Merc 3 cylinder, weak/no spark

Many times as I have used the test I really never noticed that but thet are tha same thing as everything is bonded together.
Most of the time the first 3 digits on Mercury ignition number start with a 3// and CDI start with a 1//

 

[bigvinnie]

Re: 1990 90hp Merc 3 cylinder, weak/no spark

Thanks for the info... I have a question about the trigger test...

It says to touch brown to white/black and take some readings... only problem that there is no white/black, but there's a solid white wire in the trigger harness that connects to the switch box with the other trigger wires (right side of switch box)...

I'm hoping someone has seen this before and can verify that the solid white wire is the right one.

Thanks for the assistance.

 

[Laddies]

Re: 1990 90hp Merc 3 cylinder, weak/no spark

There are 3 wires that are sender wires from the trigger to the box they connect to 3 terminals that are all in a row the 4th wire goes to a different row of terminals on the box that is the ground wire you test the winding 3 of them between the sender wire and the ground wire on each winding.

 

[bigvinnie]

Re: 1990 90hp Merc 3 cylinder, weak/no spark

ok, thanks for the explanation, makes the test more clear.... I'll check the trigger harness again and see what color the 4th wire is... I see from the test instructions that the 3 sender wires are brown, white and purple.

Thanks for the assistance.

 

[bigvinnie]

Re: 1990 90hp Merc 3 cylinder, weak/no spark

OK, turns out the blk/wht wire is grey in this case for some reason... but I'm not clear on how to do the trigger test, hoping someone can explain...

for example, it says to touch brown to blk/wht and test for resistance... how would the mulitimeter leads be hooked up for this, and what setting should the mutimeter be on..?? I've included a pic of the multimeter.

And I don't understand where it says "800-1400 4V or more connected"... does it mean resistance should be 800-1400 and dva should be 4V or more connected ..??? and what does "connected" mean in this case..??

sorry for the ignorance, and I appreciate the assistance.

 

[Laddies]

Re: 1990 90hp Merc 3 cylinder, weak/no spark

The OHM test is done disconnected from the box as you are reading the resistance between each trigger winding and the voltage test is done hooked up to load the system on most systems if the voltage is low they will tell you to then disconnect it from the box and recheck, if the voltage increases the box has a internal short. With the DVA I would not even worry about the OHM reading as the voltage readings are much more accurate.

 

[bigvinnie]

Re: 1990 90hp Merc 3 cylinder, weak/no spark

thanks again for the info...

It appears that the low-speed stator voltage output and resistance are not up to spec... high-speed checks out OK for votage and resistance, so I'm confident that my meter settings and methods are correct.

Trigger checks out OK as well, so I'll replace the stator and report my results.

Will also replace the ignition switch while I'm at it.

Thanks for sticking with me on this.

 

[bigvinnie]

Re: 1990 90hp Merc 3 cylinder, weak/no spark

Replaced the stator and she fired right up and ran great... now just need to replace the ignition switch and I'll be in good shape...

I've not been able to find a replacement ignition switch... can someone point me in the right direction for this..??

Laddies, thanks for your assistance.