IH 666 Weak Spark When Cranking

Morning all and Happy Sunday!

I've noticed my tractor will start only when I let off the starter button. Gets to be an issue when it's really cold and it doesn't spin over quite as fast. Not all the time, but it's big trouble when the cows are hungry.

It has an on/off key switch and a momentary push button for the starter.

Where and how should I start diagnosing before replacing parts? Check voltages along the line while cranking? What do you think the problem is? Got a good battery and new, big battery cables.

I haven't had much trouble with points or condenser, but is an electronic ignition upgrade justifiable? I want to make this puppy as reliable as I can.

Thank you. I will try to post back. I know I'm not very good at following up.

Well first check to see what the battery voltage is while cranking. If it is low then you could have a bad battery.

I am not sure how a IH 666 gas is wire but it should have resistor in the poser supply to the coil. Usually they also had a bypass circuit to give the coil full voltage while the starter is engages. On Delco starters that is what the "I" terminal is for on the solenoid. You have a wire directly from there to the coil. So when the solenoid is engage there is full battery voltage to the ignition system.

I do not know that tractor as such but it could have a by pass circuit like JD said and if it does that may have gone bad. Simple way to test that is to hot wire it. Simply run a hot wire form the ignition side of the battery to the ignition side of the coil and try to start it. If it starts like it should then the problem is in the by pass if it has one. Or it could also be a starter that is drawing to many amps which will also cause that and when hot wired it should act the same if it is the starter

First of all, in my opinion FWIW, IF IT EVEN HAS ONE ???? I just don't envision this as any "ballast by pass system failure" as discussed below. If it even has a ballast resistor ?? and a by pass when cranking arrangement ?? (aids in cold weather starting) often when those go bad such as the ballast becomes open, THEY WILL START AND PRODUCE FIRE WHEN CRANKING but die when you let off the starter. But you say yours "STARTS" when you let off the starter!!!!!!!!!!!!! which is the OPPOSITE of what happens if a ballast resistor is open on a tractor that uses a ballast by pass starting system.

EXPLANATION OF BALLAST BY PASS: Some 12 volt tractors use a 6 volt coil plus a series ballast resistor. If it has a ballast by pass to aid in cold weather starting, ONLY WHILE CRANKING is the ballast by passed so the coil sees full unballasted voltage. This may be like the "I" terminal on a solenoid or manual start switches with the extra small terminal. If the ballast became open she would fire as long as the start switch is depressed BUT DIE WHEN YOU LET OFF THE STARTER. HOWEVER yours STARTS when you let off the starter.

HERES MY FIRST GUESS. If the starter motor is bad and/or the battery is weak, while cranking the voltage is too low to produce a good spark BUT AS SOON AS THE STARTER IS OUT OF THE CIRCUIT AND BATERY VOLTAGE ISNT REDUCED, SHE CAN FIRE STRONG AND SHE STARTS.

Soooooooo instead of a bad ballast by pass system as discussed below, Ima thinking your battery is weak or you have a bad connection or the starter is drawing excess current thereby dropping voltage.

FIRST THING I would do is remove, clean and wire brush and re attach each and every battery and starter and switch and ground and cable connection,,,,,,,,,,,,,,,SECOND insure battery is good and full charged, a battery shop will load test it for free HAVE THAT DONE.......THIRD measure battery voltage while cranking, if it stays say 11 to 12 or so while cranking the battery is likely okay and the starter IS NOT drawing excess current HOWEVER if voltage drops way down say less then 10.5 the battery may be weak (load test will show) OR the starter may be bad and drawing too much current and dropping voltage causing a weak spark when cranking.

SECOND GUESS: You could have a faulty resistive ignition switch that's dropping voltage so when cranking the spark is too weak but even with the bad switch when the starter isn't drawing current and dropping voltage she can run when you let off the switch..........TO TEST FOR THAT see what the voltage is on the coils input (from switch) with ignition on and points closed??????????? It should be near the same as the battery voltage, if its way less, the ignition switch may have burned loose resistive contacts dropping voltage causing a weaker spark. TO TROUBLESHOOT run a hot wire direct from battery to coil to by pass the switch and hot wire the ignition coil, if it starts and runs fine hot wired but NOT using the ignition switch, the switch may be bad and resistive..........

I just think its either a weak battery or a bad connection or a bad starter or bad ignition switch instead of a bad ballast by pass system!!!!!!!!!!!!!!!! CHECK BATTERY VOLTAGE WHILE CRANKING (for bad battery or bad starter) AND CHECK COIL INPUT VOLTAGE WITH IGNITION ON AND POINTS CLOSED (for bad ignition switch, try to hot wire it)

John T

The 666 uses a resistance wire between the ignition switch and the I terminal on the starter solenoid to serve as a ballast resistor.

Troubleshoot the system by checking the voltage at the switch side of the coil and compare that voltage reading while cranking with the voltage at the battery while cranking. They should be the same within a tenth or so. If not next check the cranking voltage on the I terminal compared to the battery terminal. If there is much difference there the problem is in the solenoid.

What you are describing is a symptom of weak spark. There are basically 2 reasons for this, either there is not enough voltage at the coil during cranking, or there is weak spark due to an ignition problem.

If everything is working correctly, on a low compression, 6 cylinder engine, there will be plenty of power to run both the starter and ignition system. Sometimes several things can add up to cause the problem.

A good place to begin is to check all the electrical connections, battery cables, grounds (especially where the ground cable connects to the engine block), starter cable, even under the dash at the amp meter and back of the ignition switch. A bad connection will show up as being corroded, loose, discolored from heat.

The starter is a common cause of weak spark during cranking. If the bushings are worn, the armature will drag on the field magnets, causing the starter to draw too much current. Typical symptoms are slow cranking when the engine is hot, or cold weather-cold engine start up. Also the battery cables will tend to heat up. The Delco starter is field rebuildable. New brushes, bushings, and starter drive will usually make it good as new for minimal cost.

As for the ignition system, it could simply be time for a tune up. Worn plugs will do exactly what you are describing. Anything that weakens the spark (bad points/condenser, points out of adjustment, low voltage to the coil primary side, weak coil) can contribute. Then there is spark delivery, plug wires, cap, and rotor. Resistor wires can break down internally, wire core wires can leak through old insulation. Look under the distributor cap, be sure the carbon contact is in place where the rotor contacts the cap.

Another clue, a weak ignition system will generally be more troublesome in damp weather, and can cause misfire under load.

As for the electronic conversion kits, I've never had a problem with one. They are a great solution for a distributor with worn bushings, and with the questionable quality of today's points and condensers, the kit is a real problem solver.

My Jubilee has a push button solenoid that picks up the starter solenoid. No place on solenoid to short out ballast, so I connected a wire from starter to coil and installed a diode so when power to starter was removed current from starter would stop flowing through diode.

Shorting out ballast is an old school way of doing business. Most people don't mention that.

I'm not so sure the old updraft zenith carbs weren't designed for hand cranking and work best when cranking slower than 6v starters powered with 12v batteries. Just my opinion.