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1979 mercury 40hp 402 very weak spark advice needed

N

nath-rszt

New member
Hi all i'm new to the site and sorry my 1st post is a problem.

I have a 1979 mercury 402 40hp and when i dug it out the shed i put some fuel in it and it fired straight up (it had been sat a few years). i order new impeller and new plugs and installed them and everything seemed ok. I then tried to start it again but nothing. i sometimes have a very weak spark and sometimes nothing at all. I'm trying to do some test on the outputs but i dont have a manual to refer to and am having trouble locating on the internet.

I read somewhere that i should be seeing 180v coming from my stator. I have 4 wires coming from my stator, a red a blue and 2 yellows. I have 0.5v from the red, 7.0v from the blue and 1.7v coming from the yellow wires.

I dont have a DVA just a multimeter but i also read that you can read the values in AC the multiply the reading by 1.41 to give DVA readings.

Any help would be appreciated.

Regards

Nathan
 
Sounds like a bad stator or switchbox. You can test the stator with an ohm mete, but not the switch box.

Jeff
 
inspect your wiring closely....mercury had a problem with wiring going to pot for a long time....look for insulation cracked or peeling and take care of any green corrosion including the terminal strips or screw connections...
 
papyson said:
inspect your wiring closely....mercury had a problem with wiring going to pot for a long time....look for insulation cracked or peeling and take care of any green corrosion including the terminal strips or screw connections...
Click to expand...

Thanks for your reply, I see that page yesterday but have just worked out how to read it. from what i gather its says on the stator the blue to engine earth should be 180+v (i'm only getting 7.0v) and the red to engine earth should be 20+v (i'm only getting 0.5v).

Your rite my wiring was shot in some places it litterally fell to pieces when moved so i have cut back and replaced the main offending wires and have cleaned up every spade and ring terminal.

I'm getting correct readings from the trigger but i'm not sure if i'm testing the stator right. I have the plugs out of the engine so when i'm turning it over its spining really fast yet i'm still getting really low readings from the stator.

I dont mind buying new parts but i'd like to try and pin point exactly whats faulty before spending out.

Thanks for help guys
 
sine wave.gifthis is a sine wave for 60 cycle ac..your meter probably reads average..to use a regular ac meter to try and compute peak voltage can and will be misleading on an outboard because of the properties of the sine wave itself...so will a true rms meter..you need a dva adapter or meter to properly read the peak voltage..
 
papyson said:
View attachment 6995this is a sine wave for 60 cycle ac..your meter probably reads average..to use a regular ac meter to try and compute peak voltage can and will be misleading on an outboard because of the properties of the sine wave itself...so will a true rms meter..you need a dva adapter or meter to properly read the peak voltage..
Click to expand...

ye i dont have one :(. even thow i need a correct meter surely my realy low voltage readings would be a little higher if my stator was working correctly.

I just done some more tests my stator ohm reading are

high side, red to blue 1237 (states on the link above they should be 5000-7000

low side yellow to yellow 002 (should be 180-220

My trigger seems ok, the ohm reading is 820 and should be 750-1400

Voltage should be 0.5v and i'm getting 0.5v

I know i need a DVA adaptor but do my readings sound like i'm doing it right? as i'm a novice with the sparky side of things.

Either i'm doing it wrong or my stator is well shagged.

thanks
 
you probably have a bad stator....i would check compression etc before i spent any more money on this motor.....one option is to take it to a good mech and verify stator is bad..should cost one hours labor...stator will probably run you 250 bucks plus or minus 50...
 
Well i have an update. i've been tinkering and head scratching, now she runs like a dream. fixed , solved

I had random voltages coming out of the stator so i was sure there was something shorting the power, I disconnected the 2 yellow charging wires to the rectifiyer and spun it over by hand and sure enough. CRACK.... CRACK...CRACK... FROM BOTH PLUGS. a massive spark. Put the plugs in and it fired straight up, idles on the nose and revs up nicely.

So I'm pretty sure something has shorted in the rectifiyer and somehow killing the power and spark.

I'll buy another part and fit it but when playing with a dead engine for hours on end then hearing it run again its a real good feeling and music to my ears.
 
go back and take a resistance reading on yellow to yellow coming from stator?also take a reading from each yellow to ground...have the wires disconnected when you do this...
 
papyson said:
go back and take a resistance reading on yellow to yellow coming from stator?also take a reading from each yellow to ground...have the wires disconnected when you do this...
Click to expand...

sorry for the delay i have been waiting for my new meter to turn up.

I set my meter to 200ohm setting and yellow to yellow is saying 02.0 and yellow to earth both read nothing, am i doing something wrong?

Thanks for help
 
sorry for delay...been out of touch at my camp...i think you are reading the right wires...just dont like the reading..are you sure the meter is not set to the wrong scale and your 2.0 is not really 200?...please let us know what happens when you put a new rectifier on the motor...
 
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