1977 Mercury 1500 No Spark

[eggs712]

I have a '77 Mercury 1500, and I have no spark on any cylinder. I used the CDI ignition troubleshooting guide, and I think I may have narrowed down the problem... I removed the mercury tilt switch, I have 12V getting to both the red and white wires on the starboard side of the switch box, and I have 12 volts to the brown trigger wire. However, I get no power at all from the green coil wire that goes from the switch box to the ignition coil at cranking, which should read very high voltage were it properly working.
Does this indicate a bad switch box? Or could something else cause this? Lastly, if my switch box is bad, can I fix it or make a new one myself? Thanks for any help in advance.

 

[wired247]

Re: 1977 Mercury 1500 No Spark

The switch box is a combination relatively high farad capacitor and MOSFET solid state relay. I'm not saying you could not make one yourself because quite frankly I'm sure it's possible but unless you know your way around electronics pretty well I'd go with a new unit from CDI. They combine their switch box functions with a new coil.

If you are electonics capable put a pic of what you come up with. I'd love to see it. Shouldnt be that difficult with off the shelf amplified SCR's like you'd find in some automotive distributors feeding into a capacitor set.

FWIW the stock Mercury distributor uses an annular magnetic reluctor pickup similar to what you'd find in a 70's through 80's era GM self contained HEI distributor. The amplifier module from one could be used to power the capacitor. CDI makes a replacement Mercury distributor that is based on a Hall effect sensor instead of the original self generating SCR reluctor that works well and will work with the stock trigger or their aftermarket unit.

 

[eggs712]

Re: 1977 Mercury 1500 No Spark

Oh wow, I didn't know that it was a complicated part. I'll just buy a used one on ebay to save money or a new one from CDI.
But back to my main question, do you think I have a bad switch box or could something else be wrong?

 

[Chris1956]

Re: 1977 Mercury 1500 No Spark

Eggs, There is a test for the switchbox. It may be found in the stickys on this forum, or on the CDI website. It will test the switchbox and all wiring, and assumes the coil is good (which the vast majority are).

BTW the coil is pulsed with high voltage to fire the spark plugs, so you will not see a steady voltage on the green wire.

 

[eggs712]

Re: 1977 Mercury 1500 No Spark

I've been reading different orders for testing the ignition from different sites and forum posts, so I thought I'd ask for myself.

I understand that there is a trigger/switchbox/coil test via jumping the brown and white trigger wires and touching the black trigger lead from the switch box to ground to see if the coil sparks. There should also be at least 3V from the white and black trigger wires while connected to the switch box. I will perform those additional tests this weekend when I can visit the boat. Is there anything else I should check?

Also, if the jumper test indicates that the switchbox and/or coil are bad, how do I isolate which one is faulty? If only the coil is bad, I don't wanna spend $250 on the new CDI part which replaces both the coil and box if I can replace only the coil for <$50 on ebay.

Thanks again guys. I'm a broke, newbie college student learning as I go on this boat and trying to avoid going to the shop.

 

[Chris1956]

Re: 1977 Mercury 1500 No Spark

Coils are usually tested with an ohmmeter. There are mercury specs for them, but my service manual went out with the tide caused by Sandy. A rule of thumb is to test the coil primary. It should be very close to a dead short, maybe an ohm or two of R. The secondary is tested to ground. It should be a few hundreds of ohms.

Carholme may have a manual for you.

 

[eggs712]

Re: 1977 Mercury 1500 No Spark

Alright, I checked spark on all cylinders and got one spark only on cylinder six, but I couldn't reproduce that spark again. I performed the jumper test and got spark through the coil (I held my finger too close to the wire and got a good wake up call!). I took the voltage of the black and white trigger wires and got no voltage at cranking, so I'm going with a bad trigger. Do y'all agree?

 

[Faztbullet]

Re: 1977 Mercury 1500 No Spark

Replace trigger as its toast....