77 CDI Wiring

[lagunaman]

I am helping a friend with a JD Liquifier. The sled came without a cdi box so we are replacing that with a used one before we get into checking all of the system. He called me tonight and told me the CDI box has two plugs (type A), a ground wire for the head, two wires for the coils and then another black wire that was snipped. Does this make sense to have two black wires? If so, where does the other go?

[Sprailfire340]

Here is the wiring diagram for the 77's.
http://www.vintagesnow.com/John_Deere_f ... wiring.jpg

[lagunaman]

I have checked that out. From what he is saying, there is one more wire than what it showns on the diagram. Does that make sense?

[Sprailfire340]

IF I follow correctly one black is the ground, the other connects to "key switch A" on the wiring diagram. The ground wire does not appear to be shown on the wiring diagram.

[harleysportster]

lagunaman/Mitch,
You are correct, the ground wire is not on the wiring diagram. It would be the thicker wire of the 2 blacks and should have a stud-terminal crimped on it. The thinner black wire would hook up to the white wire of the ignition circuit. The original wiring from 76 had this wire in the rectangle engine connector but was removed and connected directly due to an update service memo.
The white wire hooks up to the MAG side coil, red wire to the PTO/CLUTCH side coil.

[lagunaman]

Thank you everyone. That is what I suspected but was not sure. Will be off to testing here this afternoon to see if we can bring spark back into its life! I am guessing that key switch wire is needed in the grand scheme of things.

[harleysportster]

I am guessing that key switch wire is needed in the grand scheme of things.

Only if you plan on turning off the engine. Seriously, the engine will run without the switch plugged in. When the switch is turned to the" OFF" position, it grounds out the ignition.

[JDT]

Yep. Nothing runs like a Deere... especially if you can't turn it off.

[lagunaman]

Checked things over this afternoon. Low speed coil tested good at 2.65 and the high speed was at 77. Everything appears to be ok with the stator. Hooked up used CDI box and absolutely nothing. Am I correct in suspecting a bad CDI? Any way to test the actual CDI?
I tested the red and white wires at the coils and got absolutely nothing when turning it over. Shouldn't I have voltage there if the CDI is good?

[harleysportster]

There is no way of checking the CDI without the proper tester, JDM-74 or it's Mercury equivalent. More than likely the CDI went bad with the previous owner.

[400brian]

I can't speak to this system specifically, but many CDI systems will not make firing voltage until the flywheel is turning 300 rpms or so.

[lwb140]

have you checked out the trigger coil ? wade

[lagunaman]

Gotta help me on the trigger coil Wade..........Decent wrench but not an electrician!!!

[lwb140]

somewhere on here it talks about checking the "timing ring" as the wiring diagram calls it . resistance measured between the common wire and the 2 "hots " ill look around and see if i can find it wade

[lwb140]

somewhere on here it talks about checking the "timing ring" as the wiring diagram calls it . resistance measured between the common wire and the 2 "hots " ill look around and see if i can find it wade

i think i would check with an ohm meter between the red/green white/green id check to see that they arent open ,make sure there is some resistance and that the 2 measurements are similar in resisitance to each other i dont find a spec on it but i can measure an nos one if youd like wade