oh crap, 1971 500 died

[68C15]

ser. # 3011762, according to research and previous/original owners bill of sale and title it shows as a 1971. I got no spark at plugs. I have no idea on how this thunderbolt system functions.

y'all want the whole story?

last summer it ran PERFECT. always started easy and ran so smooth.

last fall I installed a used mercury tachometer with the correct 3 pin plug that fits into the front of controls. it didn't work but engine still ran same as before.

this winter while doing repairs to the steering helm I managed to pinch a wire that feeds power from the main 20amp fuse up to the junction block at dash. I repaired the wire with solder and heat shrink and replaced fuse. the fuse is in the red wire of a 3 wire harness (red, white, black) leading from engine to junction block.

this morning in preparation for my first outing of the year I removed the inop tach. since it didn't work anyway (it was just laying on floor threatening to get tangled in my feet). I got to the ramp and primed the fresh fuel and it fired right up nicely, same as last year. it ran for 3 or 4 minutes to warm up (38 degrees out and 44 degree water) before I slowly took off from the dock. after a couple minutes I got up on plane and then backed down to a slow cruise. another 2 minutes and it just died like someone turned the key off.

I checked the fuse (good) and grabbed a #2 phillips to ground a plug wire to block. no spark. all the other items on boat work great.

I have no clue how this system works.

help, what do I do?

I have a wiring diagram I found at maxrules.com which appears to match my engine

 

[Maxz695]

Re: oh crap, 1971 500 died

check for spark LOL Just kidding maybe the switchbox stator etc. check engine wires for corrosion does this have a distributer if so check the cap and rotorunhook the harness fron the boat to the engine try to start it jumping the soleniod if it fires then it,s in the boat not the engine No spark in all cylinders or just 1 or 2 ? these are the questions that will get you proper help

 

[68C15]

Re: oh crap, 1971 500 died

I checked when I got it home. no spark on all 4 cylinders. that was with using a remote start switch at the starter solenoid and key on.

 

[Maxz695]

Re: oh crap, 1971 500 died

ok unhook the harness except the battery eliminating all outside interference remove the plugs and see if you get spark. if not i would check for rotted wiring on the stator and any lose connections. mostly i,ve seen that when a switchbox goes some cylinders still fire being that you get NO FIRE shows no power somewhere in the system at that point your gonna need to do some testing on the electrical componants to narrow down the search for the culprit

 

[68C15]

Re: oh crap, 1971 500 died

waiting for kids to go to bed before I continue testing I did some thinking. does the non-functional tach have any insight into my problem?

 

[Maxz695]

Re: oh crap, 1971 500 died

the tach is just a shootoff from the terminal after the switchbox and should not affect spark. I did come across a startling revalation today and could very well be the problem. if you have a saftey switch with a lanyard make sure the lanyard is on and it is not worn mostly ther are located on the shifter and when you messed with the control may have knocked it losse also if theres one check to make sure it isn,t worn out or cracked if the switch (spring loaded) is not engaged (Pulled outwrds by lanyard) You wil get NO IGNITION

 

[68C15]

Re: oh crap, 1971 500 died

I have yet to see any safety switch but I will sure look.

one other thought; I see in the diagram there is a mercury switch. I see it on the port side of the case. are these a problem? I'm going to try your previous suggestions as well as disconnect this and try it.

 

[Maxz695]

Re: oh crap, 1971 500 died

View attachment 90281 they do go bad this will shut the engine down when the engine gets lifted disconnect it from the switchbox if it,s bad you,ll get spark when removed. The Blue switch in the pick

 

[Maxz695]

Re: oh crap, 1971 500 died

if your ignition is in the throttle undernieth it will be a switch (Round) this is where you clip the lanyard so when you fall overboard it cuts the engine but check for this switch could be elswhere

 

[68C15]

Re: oh crap, 1971 500 died

putting kids to bed now. will be back with results in 45 minutes or less

 

[Maxz695]

Re: oh crap, 1971 500 died

pk i,ll be eating dinner by then hope it works

 

[68C15]

Re: oh crap, 1971 500 died

tried all suggestions with nothing but empty results. using DVOM I have 3.5Vac from stator while cranking. red wire terminal on switch box has a pulsing ground (dist. triggering coil?). coil is 0.2 ohms on primary (low?) all other terminals on switch box show ground using logic probe (high impedence and LED illumination)

bad switch box? please say no. I do NOT like the prices and the fact it is a "send me in and wait for me to come back"

 

[Maxz695]

Re: oh crap, 1971 500 died

not much on electrical specs and would like a pic of the stator and the switchbox if possible if mine won,t fit I might be able to find one for you as low as i can find. how many wires are on the stator if 4 two yellow shold peg on ohm meter and red and blue i think shoul be between 0 and 1 the less resistance i believe the better

 

[68C15]

Re: oh crap, 1971 500 died

the fuse I mentioned earlier is purely for boat electronics. it has nothing to do with engine or related wiring. the 3 wire harness is also for boat (red=batt +, white=stern light, black=batt-)

 

[Maxz695]

Re: oh crap, 1971 500 died

with the stator hooked up put the ohm meter on red and blue turn the engine you should get a pulse reading there if you switch it shows nothing. please read previous post as well. try this test hook positive straight to the coil if you get spark ththen we can iliminate coils plugs. does you stator have a tag with the pert # on it?

 

[68C15]

Re: oh crap, 1971 500 died

^^^^^^^
I did this and while turning flywheel by hand I got varying resistance. with cranking I ~13Vac and ~12.5Vdc

2 yellow wires on stator. I'll take some pics and post them here. have a good night and thank you

 

[Maxz695]

Re: oh crap, 1971 500 died

trhis is a good sign the stator is doing at least half the job

 

[68C15]

Re: oh crap, 1971 500 died

half?!

 

[Maxz695]

Re: oh crap, 1971 500 died

didnt get the yelow to yeloow reading begining to look like the swittch box but i don,t as well as you want you to buy something you might not need but just for shts and giggle send a pic of the switchbox so I can look into it for you. 1971 500 50HP right? ser. # 3011762 how many cyl? 3 6 eBay 75.00 Mercury Switchbox 7778A7 V3 and V6 outboard engines
Location: Beecher City, IL
(890 mi from 34207, USA) Watch this item Buy It Now $75.00
+$5.00 shipping Time left:24d 6h 17m

 

[68C15]

Re: oh crap, 1971 500 died

^^^^^ correct
let's see if my new PC will upload pics.