JD 620 start/stop

History: tractor ran perfect. Put in new radiator and power-steering pump guts, and related gaskets/etc. NEVER touched the carb. Had to remove several wiring connections, but THINK I have them back correctly. Removed/emptied gas tank. Problem: tractor fires up on first rotation, runs for 2-3 seconds, shuts off. I can choke it, or not: same problem. I cleaned out carb side drain plug, hole behind it, top strainer/plug, bowl drain, and fuel lines cleaned out. With carb-bowl off, open fuel-valve, gas runs out (all along the top). Push up on float, gas shuts off. Seems there is ample gas to draw on. Then I heard of the 12v/6v/key issue. I turned key on, and it reflects a steady 7-volts at the coil terminal. WHAT am I missing????

This post was edited by Farmer MM on 04/17/2023 at 04:47 pm.

change the condenser.

Ok. Is 7-volts at coil too high, indicating bad condensor?

With a 6 volt coil and working ballast resistor system ignition ON AND POINTS CLOSED Id expect 6+ to 7+
volts on the coils input and of course near zero on the output to distributor. With a good ballast by pass
circuit WHEN CRANKING Id expect near battery voltage on the coils input since the ballast is then by
passed..

John T

Ok, will check what points at. Plan to test resistor's resistance, as it's original. A perfectly running tractor prior to the repair, and now won't run...SAD/Frustrating! Some how I MUST have missed a wire in the panel, or damaged resistor or condenser. It faithfully starts, but immediately shuts off, which sounds like the 12v - 6v switching process.

Dont sound like your issue but in cases where it starts and runs but ONLY so long as shes cranking but
stop s IMMEDIATELY after you let off starter, thats a bad ballast resistor issue. Sounds like yours runs a
short bit AFTER you stop cranking and thats a different problem. The sediment bowl auto shut off on low oil
pressure problem would let it run a fair time before it ran out of gas. I have seen ignition switches
develop bad/resistive contacts that can cause ignition failure. True condensors can go bad especially AFTER
warm up but yours sounds so much quicker. Another possible problem is coils can go bad. You have to see if
it stops because she stops sparking orrrrrrrrr she starves out of gas, but once the carbs bowl is full it
would take a fair amount of time before she stops NOT right away which sounds more like a spark problem.

Condensor, Coil, Ign switch, ballast resistor, or ballast by pass circuitry comes to mind but which ???
Theres 2 wires on each side of coil if wired correct.

John T

As SOON as I release starter, it quits. I happened to keep my foot on starter pedal a couple seconds longer, and it kept running. Released it, and immediately dies. Black smoke coming out, tells me ample fuel supply.

If everything John T says checks out I'd be looking at the starter switch on top of the starter. Sucks to get to because the starter has to come out. Mine would do the same thing and I replaced the switch and it fixed it. Do an Ohm check on the wire coming from the side terminal on that starter switch. See if it's shorted to ground. The starter switch on mine got hot and deformed the copper part of a switch which probably created a path to ground.

As SOON as I release starter, it quits.

THAT SOUNDS LIKE the ballast resistor orrrrrrrrrr its wiring and connection to coil is BAD/OPEN. As long
as the starter is depressed EVEN IF THE BALLAST IS BAD/OPEN it has ignition because the ballast is by
passed and the coil gets voltage up from the little side stud on the starter switch..

HOWEVER !!!!!!!!!! If the ballast was open then you would NOT get that 6/7 volts on the coils input with
ign on and points closed SO THOSE TWO IF TRUE ARENT COMPATIBLE

You need to get your voltmeter and insure theres battery voltage out of ign switch when ON,,,,,,,,To and
through the ballast,,,,,,,,,to the coils input,,,,,,and with ign on and points closed its 6/7 volts IS
IT??? If so the ballast is okay

Then WHEN CRANKING the coils input voltage SHOULD RISE to 10 to 12 or so volts AS LONG AS STARTER IS
ENGAGED........Then when you stop cranking coils input voltage should drop down to the 6/7 range and you
have spark and she keeps running. If theres NO coil voltage after not cranking NO SPARK Well DUH

That little side stud on starter switch should connect the big battery input cable voltage to the little
side stud and thats how the ballast gets by passed cuz the little wire goes up to the coils input

I wonder if the coil is wired right (2 wires on both sides),,,,,,,,the starter switch has a short or
problem especially the little side terminal function (hot when depressed open otherwise) ,,,,,,,the ballast
resistor and/or its wiring or connections are bad ????????

John T

Good point if theres a short there (small switch stud to ground) since that wires up to the coils input
that would kill the spark......

John T

I have a couple of those in line spark testers, the ones that are put between the plug and the wire and shows light when the plug fires. Doesn't really diagnose a problem but does show if you are getting spark with the plug in place. In this instance, it would show if the plug is still firing as the engine coasts after disengaging the starter. As I said, it won't diagnose the problem, but would tell you, if it sparks when coasting, that you should looking at fuel rather than ignition.

Good idea! Thanks.

Properly setting the stage: I am HORRIBLE at electrical issues!!!!! My dad was an electrical-geek, and I flunked ALL the classes! I checked the ohms on the ballast resistor (one side to other), and it never moved the gauge. I "assume" it should have some measurable resistance. Thoughts?

(quoted from post at 15:01:10 04/19/23) Properly setting the stage: I am HORRIBLE at electrical issues!!!!! My dad was an electrical-geek, and I flunked ALL the classes! I checked the ohms on the ballast resistor (one side to other), and it never moved the gauge. I "assume" it should have some measurable resistance. Thoughts?

Click to expand...

I would GUESS the resistance across the ballast resistor should be in the 1.5 - 2 ohms range.

Makes sense. Thanks.

There's a reason these Forums work!!!!! I replaced the Ballast Resistor (somebody forgot to tell me they cost ~$60!!!! ), and she fired up like a NEW TRACTOR! Runs perfect!

THANK YOU all for your input!!!!

DLT