1975 Mercury 500 / no spark help

[Doc279]

New to the forum and need some help.

I have a 1975 Mercury 500 - serial number 3723346 ( confirmed 1975)
I also have the Mercury service manual for the motor. I have no spark to the plugs. The manual describes the ignition system as " Lighning Ignition" - a Thunderbolt series. ( Has a distributor / driver that is belt driven)

To date: I've cleaned the distributor/driver and verified that I have voltage from the distributor/driver coils to the switch box. Initially, the ignition coil failed the ohm check on the seconday side (coil tower to ground) the primary side was good. I purchased another coil off of E-bay after confirming with the seller that the ohm readings are within specification for the primary and the secondary.

I replaced the coil after testing the coil myself - ohm readings ok on the primary and secondary readings - still no spark.

I have no way of checking the switch box - Thunderbolt Ignition analyzer required - so I ordered a new one ($180.00) - still no spark. I've confirmed voltage to the switch box while cranking (electric start) plugs removed. I do not have any voltage coming through the switch box to the green coil + lead.

Mercury switch has been disconnected / boat switch wiring disconnected at the switch box - blue terminal.

What am I missing ??? This old motor is making me look bad !!!!

Suggestions please - I'll buy the beer.

Docboat2@msn.com if you would rather e-mail - Knoxville, TN

 

[Doc279]

Re: 1975 Mercury 500 / no spark help

BUMP

Surely somebody has an idea or thought - this thing is making me grow more grey hair.

help !!!!!

 

[timmathis]

Re: 1975 Mercury 500 / no spark help

let me do some homework and i will get back soon I hope.

 

[Laddies]

Re: 1975 Mercury 500 / no spark help

The blue wire is the high speed stator winding and must be hook up for ign. here's a simple test for the system.

 

[jayard78]

Re: 1975 Mercury 500 / no spark help

I had a similar issue on my Merc 40hp 1980. I turned out the ignition was wired poorly and the cutoff switch wire was causing me an issue. After I got my ignition rewired I then needed to replace my switch box.
BTW a tell tale sign of a switch box is if you are getting spark from some of the starter packs but not all. In my case I had spark on one and not on the other. (2 cyl)

It helped me a lot to trouble shoot with a volt meter starting at the spark plug wires and working my way back to the switch box. I dunno if this will help, but it is just some history of a similar issue.

Check kill switch wire. Mine was black with a stripe.

 

[Doc279]

Re: 1975 Mercury 500 / no spark help

The blue wire is the high speed stator winding and must be hook up for ign. here's a simple test for the system.

Thanks Laddies - that's something that I did not have. The blue wire from the ignition driver is still hooked up - I just removed the mercury switch wire and kill switch wire from that terminal on the switch box.

The manual that I have is a Mercury service manual and only gives me ohm meter checks for the ignition driver wires (disconnected) - it does not contain the DVA meter reading that you posted. I have the spark plugs removed and I've squirted oil in the cylinders so that it will spin pretty fast with the starter motor - I'm sure that it's faster than 250 rpm.

I'll do the DVA meter checks tomorrow. I do have voltage on the red, blue and white wire when cranking - it will light my probe up when cranking. I have nothing on the green wire going to the coil -from the switch box.

I'll keep digging. thanks for the responses !!!!!

 

[timmathis]

Re: 1975 Mercury 500 / no spark help

Sorry i had no time to help. looks like you are in good hands now.Good luck Tim

 

[Doc279]

Re: 1975 Mercury 500 / no spark help

Update: Using Laddies posted information sheet. I do not have a DVA meter and had to use a Mercury Digital Multi-meter DMT 2000. I'll have to order the DVA adapter.

Red to white ohm checks at 401 ohms resistance.
Blue to white ohm checks at 9.75 ohms resistance.

Using the digital multi-meter set on AC voltage - I get 98 volts AC on the Red to white wires check on the switch box. I get 10.60 volts AC on the Blue to White wire - with the motor being cranked - spark plugs removed - motor spinning freely.

On the Green wire to engine ground - there is no voltage present when cranking. (no power going to the coil)

Again, I don't have a DVA meter - just using what I have. It seems like the disributor/driver is working like its supposed to - the new switch box is not allowing any voltage to reach the coil - is it possible that I have a defective new switch box ?

 

[Laddies]

Re: 1975 Mercury 500 / no spark help

Disconnect the coil at the switch box and see if the is voltage on the output terminal.

 

[Doc279]

Re: 1975 Mercury 500 / no spark help

Coil wire disconnected - nothing at the green post on the switch box with the motor cranking.

I am about to give up on this motor - I've checked everything and spent $220.00 on parts with nothing to show for it. What is a 50hp - no spark motor worth for parts ? The motor looks to be in great shape - other than the no spark problem.

I have a running 65hp motor that I plan to swap to this boat.

 

[Laddies]

Re: 1975 Mercury 500 / no spark help

Doc, everything is within specs on the system, if you have both the mercury switch and the orange wire disconnected, I would be suspect of the new switch box although thats not at all normal. Was it in a sealed container or could it be a return that someone has messed with and returned and you have been blessed with?

 

[Doc279]

Re: 1975 Mercury 500 / no spark help

The new switch box was ordered through the local Sea Ray dealer where I have an account. The switch box came in a white carboard box that was not sealed - the switch box was wrapped in a single layer of un-taped bubble wrap. I was concerned because it was not sealed - but the switch box looked new - it was clean and the terminals did not appear to have ever had wires or nuts on them.

As I understand it - the switch box can only be tested using a Thunderbolt Ignition analyzer - I can find nobody that has one.

As you know - electrical parts can not be returned. I am probably screwed in that department.

Everything seems to be working properly - except for no voltage on the green post on the switch box - which equals no voltage to the coil.

I was concerned that my voltage test on the red - white wire and blue white wires was low - but I do not have a DVA meter. Is there a chance that the switch box has to have a certain amount of voltage to turn on internally ?

I have WAY to much time in this - for it to be so non-productive !

I am Mercruiser certified but only work on my outboard engines - I spent 23 years working for the Sea Ray Corporation at the cruiser plant/Corporate office.

 

[Laddies]

Re: 1975 Mercury 500 / no spark help

The system does not require a mininum voltage to turn on but if the there is low voltage going in there will be low out and I don't really know how you reading without the DVA relate to the specs. Here's a breakdown of the system it's really quite simple. The 2 stator winding pruduce AC thats rectified to DC to charge the capacitor that 4 times a revolution and pulses the SCR the fire the system so maybe a weak low speed winding would stop at the pulse at cranking speed and the voltage readings that you have are a little low.

 

[Doc279]

Re: 1975 Mercury 500 / no spark help

I've removed this motor from the boat and placed it on a stand in the corner of my shop. I'll mess with it again when I have the time.

Maybe, I'll find someone with a Quicksilver Ignition analyzer to check this switch box for me. I've done all that I know to do.

Thanks for the insite and the assistance.

Doc Staley