666 Weak Spark Revisit

in-too-deep

Well-known Member
I'm sorry I didn't reply. I did go back and read the replies, and thank you old, JD, John T, Steve, Owen, and Geo.

I finally made time to check things this evening in the twilight's last gleaming. Here's what I found:

Battery voltage 12.34v

Battery voltage with ignition on 12.05v

Battery voltage when CRANKING 10.15v

Voltage at coil with key on 3.9v Really scratching my head there. Why only 3.9? Don't I want all 12 or at least 10.15 at the coil?

I have yet to clean ALL my connections per John T, but I will. The battery cables are brand new. Custom made at the dealership. That 10.15v when CRANKING surely isn't helping.

I need to load test the battery before I start buying battery or rebuilding a starter.

Why only 3.9v at coil though?

Thanks!
 
I went back and re-read. The coil doesn't normally get full voltage unless the engine is being cranked. I understand the bypass system and resistive wire, now. That kinda explains the 3.9v. I didn't have long enough arms to check the coil voltage while cranking, so I will either rig up a remote starter switch or have Mrs help me. Still gotta clean the connections and load test the battery, too.
 
3.9 volts at the coil but are the point open or closed?? Point most the time are closed so you will in fact read way less then battery voltage. Now if the points are open you should read battery voltage at the ignition side of the coil.

The battery voltage when cranking could well be the problem and you may have a starter pulling to many amps to give you a good spark. Check your spark and make sure it is a good blue/white that just a 1/4 inch gap at all the plug wires
 
"I didn't have long enough arms to check the coil voltage while cranking"

All you need is a long jumper wire with alligator clips, or even a simple length of insulated wire. Loosen the nut on the coil's primary terminal that connects to the distributor and slip the stripped end of the wire under it and snug it down, lead the extra wire towards the operator's station and connect your meter to a good ground and to that temporary wire and you can monitor the primary ignition voltage during cranking from the comfort and safety of the seat.
 
Like Old said the 3.9V at the coil could very well be caused by the points being closed. Closed points grounds the system and sucks the voltage out of the wire, if you want to say it that way. Open the points and stick a thin piece of clean plastic between them to insulate them then test the voltage. Remember to remove the plastic insulator as soon as you are done testing so you don't forget and leave it there.
 
As a side note, I wouldn't load test the battery with those voltage readings. It's 50% charged and would fail a carbon pile load test. The best way to charge a lead acid battery is with low amperage for a long time.
 
I would say your ballast has too much resistance. The ballast on my Jubilee uses 6v and coil gets 6v and coil current is 4a the instant I turn the key on.

I took a pic of my solution to by-pass ballast during cranking. I put the cathode end of a diode to the + coil and the anode end is wired to the starter, so when there is +12v on starter current by-passed ballast and full cranking voltage is applied to coil. Like you, I have a starter button and starter solenoid has no terminal I can use to by-pass ballast.
I had the same problem you had too. This fixed my problem. geo.
a238081.jpg
 
Too deep, there are too many unknowns here to give any perfect answer but I will do the best I can under the circumstances to answer your question.

Based on what you say I will assume you have a 12 volt tractor that uses a 6 volt coil with an external ballast resistance (or wire or thermistor) plus a start by pass system that applies unballasted battery voltage to the coil ONLY when cranking THATS HOW I WILL PROCEED IF THATS NOT WHAT YOU HAVE THIS IS DIFFERENT.


YOU STATE:

Battery voltage 12.34v

Battery voltage with ignition on 12.05v

RESPONSE

Since the ignition system ONLY draws around 4 amps with points closed (as normally are) that doesn't sound like any real good strong battery HAVE IT LOAD TESTED AND INSURE ITS WELL CHARGED


YOU STATE

Battery voltage when CRANKING 10.15v

RESPONSE

Unless the starter is bad drawing excess current, again that don't sound like any real good strong or adequate CCA battery

YOU STATE

Voltage at coil with key on 3.9v Really scratching my head there. Why only 3.9? Don't I want all 12 or at least 10.15 at the coil?

RESPONSE

With key on and points closed (as they are normally when engines not running) IFFFFFFFF its a 12 volt tractor with a 6 volt coil and series ballast resistance (or wire or thermistor) ID EXPECT MORE LIKE 5 TO 7 VOLTS so that 3.9 sounds like the ballast resistance is too high or the resistor or thermistor or resistive wire is bad or one of the connections is bad and/or resistive dropping voltage. THERE MAY BE A BAD OR LOOSE OR RESISTIVE CONNECTION IN THE IGNITION AND BALLAST CIRCUIT THATS DROPPING VOLTAGE!!!!!!!!!

ALSO a bad resistive ignition switch can be dropping voltage with ign on and points closed, hot wire the switch to by pass any resistive contacts and see what happens

NOTE to test the ballast by pass system (If it has one) see what the coils input voltage is WHEN CRANKING !!! When cranking it should rise way above that 3.9 volts you measure when its just setting there with ignition on. Again it may be a bad resistive ignition switch.

NOTE I cant say from here if you have a ballast or 6 volt coil orrrrrrrr some have a thermistor resistive ballast versus a regular discrete ballast and some have a resistive wire that's temperature compensated that starts out at low but then higher resistance once warmed up.

HOWEVER 3.9 volts on the coils input with points closed and ignition ON SOUNDS TOO LOW FOR A 6 VOLT COIL (if that's what you have??) and I suspect a bad ballast or bad thermistor or resistor wire or a bad connection or bad ignition switch causing that such low 3.9 volts.

Try to hot wire voltage to the coils input to eliminate any bad switch or bad connection or bad ballast problem to get coil voltage up and see how she starts????????????????????? Don't let it run too long if its a 6 volt coil !!!!!!!!

HAVE BATTERY LOAD TESTED,,,,,,,,,,CHECK ALL CONNECTIONS,,,,,,,,,,MEASURE COIL VOLTAGE INPUT WHILE CRANKING,,,,,,,,,,,,,,TRY TO HOT WIRE COIL DIRECT TO ELIMINATE ANY SWITCH OR BALLAST PROBLEMS

YOU STATE:

Why only 3.9v at coil though?

RESPONSE

Bad ballast or bad thermistor or resistor wire or bad resistive ignition switch (try hot wire)

John T Long retired Electrical Engineer and rusty so no warranty
 
John, if the points are in perfect condition and closed, creating a 100 percent dead short wouldn't that create a zero voltage reading in the primary wire? I am not questioning anyone else's theories, just curious.
 
YES perfect closed non resistive point closure would yield ZERO volts BUT THATS ON THE COILS LV PRIMARY OUTPUT SIDE (wires to distributors points). The coils input voltage is still battery volts MINUS any ballast or switch I x R Voltage drops when ign is on and points are closed.

Got it???????? Good question

John T
 
We're seeing all these "cranking voltages" and assuming a lot of faith in their accuracy.

Unless the voltage is being checked with a scope, how could you ever get an accurate reading?

With points opening and closing, the starter cranking through each compression cycle, the best you're going to get is an average reading on an analog meter. And who knows what a digital would show, other than total confusion!
 
I agree, that's why I warned there were too many unknowns in my post, were not even sure of his ballast and by pass system or exactly what in the world he has or what true voltages are, so were in the dark but doing our best under the circumstances.

Fun chattin with ya

John T
 
JohnT,
I R don't gots it?? Isn't a vote-meter what pollsters use to determine who is going to win before the election occurs? So what's points got to do with it? geo
 
A "vote meter" and "points" is probably as reliable as the info we are working with lol but that 3.9 volts even if off some is troublesome, maybe a bad resistor (or wire or thermistor) or resistive ignition switch or faulty primary ignition circuit connection is the problem?? or a weak battery?? DARN IF I KNOW we all tried our best.

Have a good day Hoosier Neighbor

John T
 
The IH 666 used a resistor wire in the harness between the starter solenoid and a connector behind the fuel tank. Since it is a fairly brittle wire the strands of the wire can be broken and upset the total resistance of the wire. The one on my 2444 had enough broken strands that the remaining strands got hot enough to start the harness on fire.
 

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