72 Merc 7.5 no spark

[gdc0581]

I have been given a 1972 7.5 horse merc. It looks to be in pretty good condition, but has no spark. I've done a lot of work on Johnson/Evinrude motors, but this is my first experience with a Merc. I've purchased a Seloc manual that covers this motor, but I'm a little disappointed with the section on troubleshooting the ignition (thunderbolt, type II). I've measured the resistance of the excitation coils. red-yellow is 4660 Ohms and blue-yellow is 176 Ohms. The capacitor looks like a good capacitor, showing high Ohms as the capacitor charges and then going to infinity. The points seem to behave like points and pass the test outlined in the Seloc manual. What I'm not sure about is the electronics module under the flywheel. The wiring is in terrible shape, but I do have continuity. It looks like ignition parts for this motor are either very expensive or not available. Before I go any further, I'd like to know what works and what doesn't. Does anybody have a good troubleshooting proceedure for this ignition?

 

[River Boy]

Re: 72 Merc 7.5 no spark

This link might help you out:
http://ocmmarketing.com/onlinemercuryoutboardmanual.com/Mercury_Outboard_Manual_2_Cyl_1971-1975.html

If you're not getting fire to either ignition coil it's under the flywheel somewhere. Thunderbolt Ignition points fire on the 'make'. The points don't open to let a magnetic field collapse like old OMCs but trigger the capacitor's charge into the ignition coils on the close. Hence, 'maker points'. Cap must be in very good shape and make sure the diodes aren't blown. The ignition coils (going to the spark plugs) are easy to check out with an ohm meter. Look for just a few ohms on the primary and 6K or so on the secondary.
You said your you have continuity on th wiring but it is in bad shape. Check with your ohm meter for the wires going to the ignition primary shorting out to ground. Really, you ought to replace any cracked wiring you find, or at the least tape it up.

 

[gdc0581]

Re: 72 Merc 7.5 no spark

Thanks. The link is very helpful. I know I'll need to fix the wires, but I'm trying to avoid it until I can see if I need any parts and determine if those parts are available. When you say "the cap must be in very good shape" are you referring to the red electronics module? My DMM doesn't have a 'diode' scale, so I'm not sure what I should be measuring. It looks like I'm checking forward and reverse resistance, but I'm not sure what the reading should be in Ohms. Assuming my diodes are bad, is there any way to replace them?

 

[River Boy]

Re: 72 Merc 7.5 no spark

Thanks. The link is very helpful. I know I'll need to fix the wires, but I'm trying to avoid it until I can see if I need any parts and determine if those parts are available. When you say "the cap must be in very good shape" are you referring to the red electronics module? My DMM doesn't have a 'diode' scale, so I'm not sure what I should be measuring. It looks like I'm checking forward and reverse resistance, but I'm not sure what the reading should be in Ohms. Assuming my diodes are bad, is there any way to replace them?

What I mean is, on OMC points, if the capacitor is a going down it'll just beat up your points. But, on these Merc Thunderbolt ignitions the capacitor is the vital link between the two little alternator coils and the ignition coil. That make sense? You said it loaded up ok on an ohm meter so it's probably ok.

The orange module is what I sent you the link about, it's contains diodes. That sucks you don't have a meter with diode check. It would be a quick and easy way to check out the module. I coulldn't tell you what the actual resistance across the diodes in the module are, never checked one that way. I guess you could set your meter on a 4k or 5k scale and look for a gate one way and 1or 2k resistance the other way. DVM with diode check would be better though. That way you could check it with the specs.

 

[Laddies]

Re: 72 Merc 7.5 no spark

Here is test for the system from CDI electronics.