Learning about switchboxes

[robert644]

Hello,

I have a 1978 Mercury 900. S/N 5138059. I'm trying to figure out why I have no spark, and I have read the CDI testing proceedure outlines here on the site. I didn't make it very far in testing before it started raining and I had to cover up my motor with the tarp. But here is my questions ...

Posts on the site idicate that the red and white wires leading into the switchbox should be both 12vdc. The white one, only when the ignition is on. The red on all the time. (This is the 2986 box). Does this matter? I have both jumpered together. I don't understand why the switch box would need power when the motor isn't running?

The wire to the ignition coil reads 0 volts when both red and white wires are 12vdc. I'll make a wild guess and say the switch box is bad. Can I remove the box and wire straight to the coil? The hit in performance and fuel economy isn't as much of a hit as the cost of a new box, at this point in time. I read that the timing needs to be adjusted in this case also?

Thanks!

 

[achris]

Re: Learning about switchboxes

...Posts on the site idicate that the red and white wires leading into the switchbox should be both 12vdc. The white one, only when the ignition is on. The red on all the time. (This is the 2986 box). Does this matter? I have both jumpered together. I don't understand why the switch box would need power when the motor isn't running?

You are right.. And you can jump the wires together, but you would have to remove the jumper to shut the engine down.

The wire to the ignition coil reads 0 volts when both red and white wires are 12vdc.

That is also correct.

I'll make a wild guess and say the switch box is bad.

Wild guess, yes. Right? maybe...

Can I remove the box and wire straight to the coil?

Definitely NOT!!! This is not a points ignition system with an electronics box. There is a sensor in the dissy that would not take too kindly to that!

The hit in performance and fuel economy isn't as much of a hit as the cost of a new box, at this point in time. I read that the timing needs to be adjusted in this case also?

Thanks!

I posted a little while back the full test procedure for this ignition system, complete with diagrams. Do a search and if you can't find it I'll re-post...

Chris.....

 

[robert644]

Re: Learning about switchboxes

Achris,

Thanks for the reply. It's raining outside, and will be for the next 20 days or so. I probably won't be doing much electrical work on the engine for that time. I have seen several posts about testing the switchbox, but since I'm inside on a rain day, I'm just trying to understand what's going on here.

With the ignition on, why isn't there voltage going to the coil? In your post you said it was correct that with the white and red wires energized, the voltage to the coil is correct to read '0'. Well, when does it provide electricity? I don't understand

Thanks for your help!

 

[achris]

Re: Learning about switchboxes

The switchbox is powered by the battery and charges an internal capacitor. When the switchbox receives a pulse from the sensor inside the distributor it discharges the capacitor to the coil. Thus creating a spark. Until a pulse is received from the distributor nothing goes to the coil. If the engine isn't turning and there is no pulse and the capacitor is fully charged, then the switchbox just doesn't draw any more current.

Thus ends 'Capacitor discharge ignitions 101'...

Chris........

 

[robert644]

Re: Learning about switchboxes

Achris,

Thanks for the explanation! That clears up that mystery! I'm going to go outside and try out under the tarp I think. Maybe. I don't know about going out in this rain, it'll be like one of those old time photographers who put the hood over the camera when they took pictures.

Tanks, Sincerely Robert.

 

[robert644]

Re: Learning about switchboxes

So, while cranking, the voltmeter shows .1 volts going to the coil.

 

[achris]

Re: Learning about switchboxes

Can't use a standard meter, gotta use a DVA...

Look at THIS thread, and THIS one....