72 Mercury 500 No Spark

[RRafkin72]

Hey Guys new to the forum, found a lot of good info on here so far. Thank you

Just picked up a 1960, 16ft. Glasspar Citation with a 1972 Merc 500 50hp with no spark. I'm a carreer marine mechanic but strictly inboard diesel so this is all new territory for me.

Here's where I'm at...
• Known good battery
• Engine cranks good
• Feels like good compression. Will test soon
• Zero spark on all 4 cyl.
• Old ass plugs and plug wires
• Previous owner installed a new on engine harness, starter, and fairly sure a new vessel harness as well.

• I've disconnected my stator from the rectifier. removed the rectifier, removed ignition coil, and switch box.

• I tested the coil per Merc.son youtube video and I have 0.1 ohm across my coil green wire and the ground. I have about 330k ohms between the big post and either small post on the coil.

• Stator has low impedance across the 2 output wires and neither wire is shorted to ground.

I am looking for an official wiring diagram for my year if anyone has one that would shed some light forsure.
I have the switch box with 4 posts on one side only. I know the blue and white posts have continuity to ground and I believe that the merc switch when grounded will kill spark correct?
Any help or insight is greatly appreciated thanks guys

 

[merc850]

Clever bit of engineering
I have the 1975 wiring diagram for your reference it might be different than your motor

 

[RRafkin72]

Clever bit of engineeringView attachment 372193
I have the 1975 wiring diagram for your reference it might be different than your motorView attachment 372194

Thanks merc850,

So from what I understand in a normal condition the blue switch box terminal is momentarily shorted to ground to stop the engine.

And what I'm seeing is my blue switch box terminal is permanently shorted to ground (as is the white switch box terminal too) like im holding the stop button and trying to start it.

The diagram does show a cowl frame ground for the switch box but the posts should be normally isolated right?

 

[merc850]

Yes the cowl must be grounded as well as the ignition driver and all are grounded to the neg side of the battery via the starter. None of the posts on the switchbox should be grounded for RUN. Disconnect the ground from the mercury switch until engine is proven to run. This motor uses the standard timing belt check to see if it is still intact. Previous owner might have been drinking at the time he installed the wiring harnesses so follow the wires.

 

[RRafkin72]

Ok so I'm fairly sure my switch box is shorted out because with all wires disconnected the blue and white posts have continuity to ground.

I checked all the wires while disconnected from the switch box and none were grounded.

I've found a NOS switch box online for about 400

I definitely need new plugs and wires just based on visual inspection but I'm hoping my stator, coil, and ignition driver are still good. Does anyone know the correct bench tests for these?

 

[merc850]

All I have is the 1975 year but it should? be the same; did you check the belt?

 

[RRafkin72]

All I have is the 1975 year but it should? be the same; did you check the belt?
View attachment 372205
View attachment 372206

I do believe the 75 is the same because in 76 they changed the coil and switch box and ive seen those diagrams they are certainly different.

The belt is definitely intact maybe a little loose but spins the driver forsure. I don't have a voa meter and or thunderbolt analyzer but at least now I know the ign driver harness should not have continuity to ground which it doesn't.

I think the PO possibly bought an incorrect year harness because I have brown, black(small wire, not starter cable), and white wires coming out of the plug where the harness passes thru the cover on the inside. He had black and white taped off and tucked back, and brown landed on the blue switch box terminal along with the merc switch, the orange wire, and the black ign driver wire.
The diagram you sent only has 3 wires on the blue switchbox terminal and doesn't show a white, small black, or brown coming thru the cover to the engine. But after tracing everything it seems the PO attempted to follow that diagram while most likely drinking

 

[merc850]

The Ignition driver provides the power for the switchbox so it doesn't need to have 12v going in to it; this is an electronic version of a magneto. All you need is a wire to ground it out for stop.
The 500 is a great motor smooth running and powerful and they made them for decades!
Have you checked the impeller and gear lube yet?
Here's a 1962 500 that I owned in the 70's

 

[RRafkin72]

The Ignition driver provides the power for the switchbox so it doesn't need to have 12v going in to it; this is an electronic version of a magneto. All you need is a wire to ground it out for stop.
The 500 is a great motor smooth running and powerful and they made them for decades!
Have you checked the impeller and gear lube yet?
Here's a 1962 500 that I owned in the 70's
View attachment 372268

Thats beautiful, really cool shot. What was that on? Haven't gotten around to the lower unit at all yet.

Any idea what voltage I should see out of the ign driver to to the switch box?

 

[merc850]

It was a 14' runabout that I had for a few years; don't know the voltage but anything is a start.
Here's my current rig .....

 

[RRafkin72]

It was a 14' runabout that I had for a few years; don't know the voltage but anything is a start.
Here's my current rig .....
View attachment 372324

Right on looks clean man

I got around to doin a compression check and I'm reading 95-100 on all cylinders and now im not sure what direction to go. Seems pretty low

 

[racerone]

Test with another gauge ?

 

[RRafkin72]

Test with another gauge ?

Im going to get another to verify seems like the guage may be whack.

I found a 1969 65hp for sale that runs with no lower unit would my 72 lower work to swap over?

 

[racerone]

Will nor swap over !----Not a chance.

 

[RRafkin72]

Will nor swap over !----Not a chance.

I found this engine and lower anyone know if these are compatible or look right? Engine serial is 2713736 not sure on the lower unit numbers

 

[RRafkin72]

Ok so I'm about to pull the trigger on this 65hp and the lower.
The engine serial is 2713736 (1969)
The lower serial is 2804236 (1970 I believe)

Anyone feel free to correct me if I'm off on those serial years. Hopefully it'll work
Thanks again

 

[jimmbo]

The 69s, had the nicest Cowl Decal that Mercury ever had
If the Lower unit came off of a 4 cylinder 65hp, 75hp, 80hp or 85hp, it will have the proper gear ratio for any of those engines. Hopefully the shaft lengths match and are correct for you boat
Myself, I like the post 78 cases better, slightly better Hydrodynamics
A case off of the Inline 6s will also fit, but the gear ratio will be wrong
None of the gearcases above will fit on the 50hp.

If you buy them, take the water pump apart and install a new Impeller. What kind of Compression Numbers does the powerhead have?
While I like these engines, parts are getting harder to find, the last 4 cylinder 65, was a 1972, 50 yrs ago.

Edit: As pointed out below, the last 4 cylinder 65 was in 71. In 72, the oddball 3 cylinder was introduced

 

[racerone]

I think 1971 was the last 4 cylinder 65 HP.----Those early 3 cylinder motors were never in the top 10 on the hit parade.

 

[jimmbo]

I think 1971 was the last 4 cylinder 65 HP.----Those early 3 cylinder motors were never in the top 10 on the hit parade.

You're Correct, the 72 was 3 cylinder. I have the 72 Brochure on the shelf next to my desk. Damn it you caught me again

The 3s never were in the top 10, unless you are referring to Poor Running/Performance.

 

[racerone]

The early 3 cylinder used a less than robust gearcase.----Issues with top cylinder dome cracking.----Flimsy cowling.----Designed in a hurry to compete with popular OMC 3 cylinder motors.