How careful should I be???

1974 Merc 500 50hp (serial starting with 414). Looking for a main (ignition) coil. After some searching, I'm getting the impression that this main ignition coil can be used on a bunch of different motors of this time frame. Of course, there is a specific serial number for this part for my motor, but then I'm finding several seemingly reputable companies on eBay claiming that their coils for a comparable time period are also compatible. How careful should I be?? Does the output really matter that much?

No. Age and quality do.

Jeff

Thanks, Jeff. I rebuilt this sucker and it runs, just not smoothly. I finally decided it must be something in the electrical system but don't have all the equipment to fully test the coil, switchbox and distributor. So, I took it to a marina where the mech specializes in Mercs, especially old ones. Per their policy, after 1 hour in, they called and said it was electrical but it would take quite a bit more time to really determine exactly what or what combination of things it might be. Of course, he suggested: wiring harness (which one, I don't know, but mine are fine), ignition coil, rotor (which I know is fine because I just rebuilt the bottom part of my distributor), and/or switchbox. Not much help, really. I knew all that. Whatever it is, I'm not getting a strong enough spark and it will only jump maybe 1/4" gap. Any more than that and it starts skipping sparks. I figure I will just go ahead and replace the ignition coil first and see if that's it. I am determined to get this thing running. Put in too much time to give up now!

Weak spark is going to be: 1)weak output from distributer to switch box 2) weak output to coil 3) Coil severely breaking down. My bet is the distributer or switchbox output as the swichbox is nothing but a amplifer .....do you have a dva adapter???

faztbullet said:

Weak spark is going to be: 1)weak output from distributer to switch box 2) weak output to coil 3) Coil severely breaking down. My bet is the distributer or switchbox output as the swichbox is nothing but a amplifer .....do you have a dva adapter???

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Thanks for your reply. Yes, I actually made my own DVA adapter (and it does work); however, I was having a bit of a time trying to get readings from the switchbox (i.e., the numbers would jump around all over the place and never build up in the capacitor). Any advice and what I should be looking for as I test (and even what to touch the leads to, to make sure I'm doing this right) would be VERY HELPFUL! I was using the CDI troubleshooting guide to test, but was still getting some crazy numbers.

Thanks so much.

Do I test with the engine running or just cranking?

I would test it running....

Once owned an older merc. There is an ohm test that can be done on the coil with a multi-meter.

The manual for your motor gives clear instructions for doing a non-running ohm test on the switch box. Needs to be done with all connections loose.
Get a repair manual for your year and then check it out.

sixlegfish said:

The manual for your motor gives clear instructions for doing a non-running ohm test on the switch box. Needs to be done with all connections loose.
Get a repair manual for your year and then check it out.

Click to expand...

Yep, already have one. Already checked the resistance for the switchbox and it was fine (as long as I used a digital multimeter and not an analog one -- weird). The red and blue coils in the distributor checked out fine for resistance as well, but the red coil is way too low on voltage output. You can have ohm resistance just perfect but it doesn't mean the coil is good. I'm replacing both for good measure.