1974 Mercury 20HP Thunderbolt Ignition Problems

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ShaneC

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I just recently bought this motor. The guy fired it up for me with the muffs on and it started on the 3rd pull. I loaded on the back of my truck, strapped down and drove about 10 miles home. I put is on the stand and put in fresh plugs and new foot oil.

Now I am not getting any fire to the plugs. I have read through several post on troubleshooting the problem and still not luck. I have tested at all the recommended test points and some things don't add up. My Red lead from stator to switch box read correct on resistance(550 ohm) and volts(20V). My blue lead reads a little high on resistance(6400 Ohms) but only getting ~20V. At the coil I am only seeing about 2 volts. Between the brown and white leads I am seeing 660ohms and I see the specs call for more like 150ohms.

I have alread disconnected the kill switch lead and have it taped off during all test. I can't believe something could have went this wrong with just a short ride home.

I really don't want to take this thing to the shop but running out of ideas. Any help appreciated.

Shane
 
7

7920hpMercury

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Re: 1974 Mercury 20HP Thunderbolt Ignition Problems

Bummer indeed!

You might consider checking all of your grounds, but likely you have done this already. Also check for loose or frayed wires. For some reason, Mercury used aluminum coated or aluminum wires in these years which corrodes. Hard to splice but sometimes doable.

Otherwise, I would follow the service manual, pull off and check ignition coils ohms plus clean ground wire (continuity and 900-1200ohms), pull flywheel off and disconnect red/blue(185-205ohms) and blue/white(6000-7000ohms) and check for correct ohm, trigger leads for correct ohms (800-1000ohms). And if all that checks then I would guess likely your switchbox is bad.

Good luck.
 
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ShaneC

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Re: 1974 Mercury 20HP Thunderbolt Ignition Problems

Should I not be getting 150-180V on the Blue Lead that goes from the Stator to the switch box?

Shane
 
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7920hpMercury

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Re: 1974 Mercury 20HP Thunderbolt Ignition Problems

I usually decide the switchbox is bad basis a process of elimination. If all the other stuff is good, then I figure the switchbox is the problem.

I do not have a DVA meter, but check some posts here, Laddies' provides outstanding info. If everything else checks good and your wires are good and if the DVA readings are not right, then I would guess your switchbox is bad. My 2 cents. Good luck and sorry I could not help further.

One other thought, long shot, put the original plugs back in and try it.
 
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ShaneC

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Re: 1974 Mercury 20HP Thunderbolt Ignition Problems

Thanks 7920 for the info. Yea I threw the old plugs back in prior to starting the troubleshooting deep dive with no luck.

Seems if I am not getting that high voltage pulse on the Blue lead from the Stator to the Switchbox then I would never be charging the internal capacitance. Maybe someone can tell me if I am wrong in my logic but the way I thought this system worked is the blue wire sends a high voltage pulse to the switch box where it is stored in a capacitor. When the trigger is flipped then the charge on the switchbox is then sent to the coil. So my thinking was if I am not getting that initial high voltage pulse to the switchbox then my problem exist in the stator area of the system.

I have seen some of Laddies info on this board maybe he will chime in and give me some other diagnostic steps. I haven't pulled the flywheel yet but that is my next step so I can inspect the stator and associated wiring.

Shane
 
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Laddies

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Re: 1974 Mercury 20HP Thunderbolt Ignition Problems

Shane sorry I didn't see your problem earlier but you system is one of the easier to check, here's the test for the system.
scan0003.jpg
 
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ShaneC

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Re: 1974 Mercury 20HP Thunderbolt Ignition Problems

Yep I found this on another thread and used the same guide.. so my Blue Wire that goes from Stator to switch box. When I test the resistance I get a reading of 6400 Ohms but only getting ~20V instead of ~180V. So I am thinking I have some sort of problem in the Stator area of the system.

Shane
 
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merc_man

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Re: 1974 Mercury 20HP Thunderbolt Ignition Problems

Hello I have the same motor as you and had the same problem last year 6 mile out in the bay. any how i noticed the fly wheel had a lot of up and down movement. under the fly wheel and the points and condensor ther was a few bolts that had worked loose on the motor. This made the fly wheel shaft raise up enough that the lobes on the shaft were not opening and closing the points cousing no spark. check this and let me now. thanks and good luck.
 
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Laddies

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Re: 1974 Mercury 20HP Thunderbolt Ignition Problems

Sounds like you lost one of the stator coils
 
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ShaneC

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Re: 1974 Mercury 20HP Thunderbolt Ignition Problems

Ok.. So I went to pull the flywheel to inspect the Stator and the wiring under the wheel.. I didn't have the proper flywheel puller but tapped around on it to see if I could free it.. For what ever reason I manually spun the wheel and heard a couple of pops. ( I had the plugs out just hanging by the wires and touching the block) I looked back and saw both of them popping.

I put the manual starter assembly back on and tested again. I was then only getting a spark on one plug. I swapped the coil wires and then the other plug would fire but not the orginal (Problem followed the wires coming from the switch box). So this proved the coils were not the problem.

So what does it all mean? Could I still have a problem with stator or does this test point more to the switch box? $180-$200 for a new switchbox, ouch. Just want to make sure that is my problem before I drop that much cash.

Shane
 
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ShaneC

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Re: 1974 Mercury 20HP Thunderbolt Ignition Problems

So I pulled the flywheel and took a ohm reading of each of the stator coils.

One of them has a Red and Blue wire coming from it. This one read 173 Ohms.
The other has a Blue and White wire coming from it. This one read 6.37k Ohms.

Each of them is wired a little differently. Both of the blue leads coming from the stator coils are tied to togther on a terminal block (back side) and connect to the red wire (front side) running to the ignition switch. The red wire from the stator coil runs to the terminal block (back side) and is tied to the blue wire (front side) running to the ignition switch. The white wire is tied to the front of the terminal block and appears to be a simple ground connection. Due to the different color wiring for each of the stator coils I assume they have different charcteristics and that might explain the difference in ohm readings. (Maybe?) The wires all seem to be in good shape and no frays. I know the coil that is reading high could have a break or this is how it was made. The one with the lower reading I would assume that is a normal reading for a coil of wire (173 Ohms).

I attached a picture of the stator coils with the wires from each to aid in my description.

Thoughts?

Shane
 

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green4themoney

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Re: 1974 Mercury 20HP Thunderbolt Ignition Problems

merc_man said:
Hello I have the same motor as you and had the same problem last year 6 mile out in the bay. any how i noticed the fly wheel had a lot of up and down movement. under the fly wheel and the points and condensor ther was a few bolts that had worked loose on the motor. This made the fly wheel shaft raise up enough that the lobes on the shaft were not opening and closing the points cousing no spark. check this and let me now. thanks and good luck.
Click to expand...

this ignition system was pointless no?
 
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ShaneC

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Re: 1974 Mercury 20HP Thunderbolt Ignition Problems

That is correct it doesn't have points..

Laddies - you got your ears on? Please see the info I posted on the stator coils.

Shane
 
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ShaneC

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Re: 1974 Mercury 20HP Thunderbolt Ignition Problems

Well I talked to Laddies and he informed me on the Stator Coil measurements

Red to blue should be 180 to 340 ohms at Rx100
White to blue should bs 5.2 to 7.2 at Rx 1000

So looks like I am a little low on the Red to Blue but not by much.
I have cleaned all wiring connections and everything under the flywheel. I am going to put things back together tonight and re-test. Not really expecting much to change but maybe cleaning all the connections helped out.

Update after testing
 
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ShaneC

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Re: 1974 Mercury 20HP Thunderbolt Ignition Problems

Well still no luck..Only one plug still firing. I am going to buy the switch box. Any recommendations on the switch box vendor?

CDI = $200
Sierra = $168
Mallory = $182

Shane
 
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ShaneC

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Re: 1974 Mercury 20HP Thunderbolt Ignition Problems

Well I went with the Mallory. Installed it last night and both plugs fired. Put everything back together and fired it it up on the second pull.

Now on to the the next problem saw some dark black oil coming out around the propeller but that is another thread after I start taking it apart.

Thanks to all the good advice I got from this board. Special thanks to Laddies.

Shane
 
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7920hpMercury

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Re: 1974 Mercury 20HP Thunderbolt Ignition Problems

Glad to hear you got it running.

Couple of thoughts:

you can use a regular auto steering wheel puller with 2 - 1/4"-28 (? think that is right) bolts to get the flywheel off. Tapping or hammering on the flywheel can mess up your magnets.

And I think one of the reasons switchboxes go bad is not having a good ground. So, I would make sure you have a good ground for the switchbox and seal the connection with dielectric grease.

Great motors and good luck !
 
JGra

JGra

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Re: 1974 Mercury 20HP Thunderbolt Ignition Problems

I have a 1968 merc 200 that I have been rebuilding/restoring and have the same problem with a oil leak from the prop. I pulled the prop and found about 10 feet of fishing line wrapped around the shaft. The seal was somewhat melted to the fishing line I quess due to the friction. I have a lower seal kit on the way and hope to tackle it this weekend. I will post any issues or ploblems if that will help you. good luck with yours. I havent had mine in the water yet but it seems to run real well in the bucket and on the muffs. It had not been run in 15 years prior to me starting the repairs. If it was not for this forum I would not have gotten as fas as I have with the repairs. Thanks !!!!! to all of you people out there who offer there help and knowldge.
 
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