Not getting Spark on MCM170 Alpha 1

[zang8027]

Ok, now that I am not getting spark, I have a few questions. The engine is cranking but not starting. I placed a spark plug out on metal but not getting a spark. I checked the points under the cap but I do not have spark either.

I am getting 4V continually on the coil (+). When I crank it drops to about 3.9v. Shouldn't this be 12? I checked a few tests in a SELOC manual. None worked. I held the high voltage line coming from the coil near ground (1/4") but did not get a spark jump.

So my two questions when relating to the coil + term is:

What voltage should I get coming up with key on, not cranking?
What voltage should I get cranking?

 

[Don S]

Re: Not getting Spark on MCM170 Alpha 1

Are your batteries charged up? Battery connections (Positive and Negative) clean and tight with a wrench, not just your fingers.

Without that to start, it not going to start, and if it's cranking slow................ forget it.

 

[zang8027]

Re: Not getting Spark on MCM170 Alpha 1

Yep, charged yesterday. Granted they could probably use some tonight since we have been failing to start today. I still had 12v on the one as I tested it. I will charge it up again tonight.

The wing nuts seem to be as tight as can be with hand.

 

[Bt Doctur]

Re: Not getting Spark on MCM170 Alpha 1

key on should be around 9 ish volts because you looking thru resistance wire, cranking should be higher because you direct off the solenoid during cranking only.
Check the electric choke , that should have 12v with key on , and there should be a yellow/purple wire (i think) going from the choke to the coil. report findings
Are you positive you have the point gap correct ?You installed a NEW set of points?
Try running a jmper frpm the + battery to the coil , check for spark by cranking the engine

 

[Don S]

Re: Not getting Spark on MCM170 Alpha 1

If you put a multimeter on the battery and it showed only 12V, That is dead.

 

[zang8027]

Re: Not getting Spark on MCM170 Alpha 1

No, old points. I checked they are .20 I think and manual says .22. My dad told me it should be good enough. I will charge that battery.

 

[Bt Doctur]

Re: Not getting Spark on MCM170 Alpha 1

old points get glazed and do not make contact to produce spark. sandpaper or nail file to test, then new points.

 

[stonyloam]

Re: Not getting Spark on MCM170 Alpha 1

There should be a purple/yellow wire running from your slave solenoid to the coil +. That wire provides full battery voltage to the coil during starting. Coil + should read the same as battery during starting. Yeah a fully charged battery should read over 13 volts. Your points could have glazed over during the time the engine was sitting. New points is a good idea.

 

[zang8027]

Re: Not getting Spark on MCM170 Alpha 1

There should be a purple/yellow wire running from your slave solenoid to the coil +. That wire provides full battery voltage to the coil during starting. Coil + should read the same as battery during starting. Yeah a fully charged battery should read over 13 volts. Your points could have glazed over during the time the engine was sitting. New points is a good idea.

This brings up a second issue. That wire gets HOT. Is that normal? the purple with a yellow stripe running to the coil +. I can barely touch that thing.

I charged the batteries up so after work I will see what I can do. I wire brushed the cap but ill take some sand paper to it.

 

[Don S]

Re: Not getting Spark on MCM170 Alpha 1

When does it get hot? Key on engine off ???
Disconnect the purple/yellow wire from the solenoid completely, check the terminal on the solenoid and see if it has voltage with the key off and check again with the key in the run position.

This may be why you had the extra solenoid.

 

[zang8027]

Re: Not getting Spark on MCM170 Alpha 1

When does it get hot? Key on engine off ???
Disconnect the purple/yellow wire from the solenoid completely, check the terminal on the solenoid and see if it has voltage with the key off and check again with the key in the run position.

This may be why you had the extra solenoid.

Ill check tonight to see when it actually gets hot. I would say yes, but it may be hot from cranking. I charged battery up last night, so I will do some testing. I think it was hot when I had that third solenoid too.

1) Ill sand paper the points in the cap
2) Ill check to see when wire gets hot
3) Ill disconnect that p/y wire on the solenoid near the heat exchanger and check.Should I have voltage with off?

 

[Don S]

Re: Not getting Spark on MCM170 Alpha 1

You will have voltage on the wire with the key on, but it shouldn't get hot.

 

[zang8027]

Re: Not getting Spark on MCM170 Alpha 1

key on should be around 9 ish volts because you looking thru resistance wire, cranking should be higher because you direct off the solenoid during cranking only.
Check the electric choke , that should have 12v with key on , and there should be a yellow/purple wire (i think) going from the choke to the coil. report findings
Are you positive you have the point gap correct ?You installed a NEW set of points?
Try running a jmper frpm the + battery to the coil , check for spark by cranking the engine

We have 10v at the electric choke. We have 8 on the solenoid when cranking. We have a purple(brownish) coming from the choke to the coil +. This is the one that is HOT. This is the resistance wire?

 

[Don S]

Re: Not getting Spark on MCM170 Alpha 1

Yes, the wire from the coil to the distributor is a resistor wire, and it will get hot when the ignition switch is on. for a while. It's a resistor and it creates heat. That's what they do.
Did you look at the wiring diagram I posted in your other thread. Here it is again.

View attachment 170 wiring.pdf

You are asking questions, but you never answer the questions asked of you, and those are the ones that will help you pinpoint your problem.

 

[zang8027]

Re: Not getting Spark on MCM170 Alpha 1

So bought a new cap, roter, and capacitor today and will install tomorrow. Looks like it has new points in the kit too. Can't hurt to get that up to date.

Anyway, I remove the p/y wire at front right of the slave and got 0 on that empty terminal with key on? I am getting 12.2 on the left side though which I think is red /yellow. I believe that was more than I got last night. I tested the purple/yellow at the end of the coil and got 12. I don't have the coil in at the moment. I wonder where I am getting 12v there from? Has to be that resistance wire.. its the only thing that was hooked up to that, as i removed the other end at the slave. The resistance wire and p/y wire share a connector at the coil.

 

[stonyloam]

Re: Not getting Spark on MCM170 Alpha 1

Anyway, I remove the p/y wire at front right of the slave and got 0 on that empty terminal with key on? I am getting 12.2 on the left side though which I think is red /yellow. I believe that was more than I got last night. I tested the purple/yellow at the end of the coil and got 12. I don't have the coil in at the moment. I wonder where I am getting 12v there from? Has to be that resistance wire.. its the only thing that was hooked up to that, as i removed the other end at the slave. The resistance wire and p/y wire share a connector at the coil.

That terminal only has voltage when the key is turned to start. The 470 slave is a little different than some other mercruiser slaves. You have 12V on the large r/p wire all the time. When you turn the key to START voltage is applied through the y/r wire which activates the solenoid and closes two sets of points (both fed by the r/p terminal. One set feeds current through the r/y wire to the starter solenoid, and the other feeds battery voltage through the p/y wire to the coil +. With the key ON you will read coil + voltage at the p/y terminal.

 

[zang8027]

Re: Not getting Spark on MCM170 Alpha 1

Resolved - Gray Tact wire on the coil (-) was shorting it out. Removed it and it started right up. Will re run the new wire to get the tact to work.

 

[Don S]

Re: Not getting Spark on MCM170 Alpha 1

Probably not the wire, more likely the tach itself. Hook the wire back up to the coil, then remove the gray wire from the tach. If it starts, the tach is shorted to ground internally.

It's odd that none of us even gave the tach a mention. It's usually one of the first things we should suggest you check with a no spark condition. (along with the shift interrupt switch, also not mentioned)

 

[zang8027]

Re: Not getting Spark on MCM170 Alpha 1

It was the wire. Even disconnected from the tact, it grounded out. We re ran a new wire and it works now. Still got a problem with the temp guage and tilt guage. May check wires there too.

 

[Scott06]

I know this is an older post, but can you discuss the possible shift interrupt issue. I have a similar situation, but I DID remove the tach wire from the coil to see if that would work and it didn't. Mine is a 1986 3.0 140 HP Block Tag is B484261. As the original poster did, I ran a hot wire direct to the + coil terminal and it fired right up. That being the case we can't live with it this way.

Best to start your own post so we can get details of what's going on.
your examples sounds like you aren’t getting 12 v to coil either bad ignition switch, man overboard switch is tripped , or shift interrupt is tripped . Disconnect shift interrupt at shift plate to test .