[dixie_deere]

Ok, my 1986 JD 420 shut down on Saturday after mowing heavy grass for about an hour. It was just like I turned off the key! I asked if anyone else was having this same problem and it sounds like it is pretty common on these tractors. I have checked the ignition switch for continuity and it was good. I have checked the fuses and the 25A breaker and they are also good. I also checked the B+ tab on the voltage regulator with the ignition switch on and I have 12.4 volts at that point. I checked the hot side on the solenoid and found 12.4 volts there too! If I jump the solenoid for the starter, the motor will spin over so I know power is getting to the starter. After checking all those items, what can I check next in the system? Also, what is in that black box behind the dash in the pedestal...should I look in there for any reason?

How can I have nothing at the key switch and no dash lights but still be getting power down at the motor through the system? .... Any ideas anyone?
This has me scratching my head ...???

 

[chuckv]

Lew,

1986 was the year prior to the engine being changed from the points ignition to the electronic style...but your tractor may not have the original engine. Do you know if your Onan is a B-series or a P-series?

If your engine has points, the issue may be a failing condenser as those tend to become temperature sensitive and cause the spark to degrade as the engine warms up. It could be a shorted condenser -- check this by taking the condenser out of the circuit to see if the engine will at least start and idle (it still not run well at speed without a good condenser...)

Condensers should be replaced EVERY time you replace the points. Use only USA made parts of you can get them -- some of the import parts are of questionable quality and are short lived.

Here is the wiring excerpt of the 420 ignition section (note the insert for the points style engines.) Pay careful attention to where the condenser connects, as it is different for the P-series than the insert showing the B-series. P-series condensers go to the + terminal of the coil, whereas B-series condensers go to the - terminal, so be careful!!

Note that the module you mentioned (the TDCM, or time delay control module) does sense the state of the seat switch and inhibit spark if no operator is present. Not shown here is the other end of the #710 purple wire connected at the ignition switch terminal S2 comes from the brake switch on later tractors or the neutral switch on earlier ones -- this allows the engine to be started and run if the ground speed lever is in neutral AND the parking brake is set (later tractors) AND all the PTO switches are OFF independent of the presence of an operator on the seat...

Let us know what you find and if you still have the original B-series engine with the points.

Chuck

 

[chuckv]

OOPs!!

On re-reading your post it may be that you do not even get the engine to crank over with the starter when you turn the key switch -- is that correct?

If that is the case, here is the wiring excerpt that pertains:

If your tractor does not have a rear PTO switch, then there will be a jumper at X5 bypassing that portion of the above. Your tractor is also too old to have the parking brake switch, so the key switch terminal S2 is connected directly to the neutral switch terminal X9. The PTO switch(es) must be off, and the ground speed lever in the neutral slot for the engine to crank with the key switch. When the key switch is turned to the START position the small tab connector "S" on the solenoid (the purple #700 wire) should be hot with 12 volts. If the solenoid does not actuate at that point, it is bad -- but if the voltage is not there with the key in the START position then the issue is further upstream...either a switch not in the correct position, a switch is bad, fuse blown or a bad fuse holder, etc. Sometimes the key switch itself is flat worn out if it original to the tractor, and the connector mating with the back of the key switch can also have corrosion or heat damage... If you have doubts about the age of your key switch just replace it, it is good practice and inexpensive preventative maintenance...

Chuck

 

[dixie_deere]

Chuck,
My tractor has the B series Onan and I believe it is the original motor. I will say that the tractor was good and hot when it shut down on me from mowing very tall grass on Saturday, so the heat issue may be the culprit. I will check the solenoid first and then the condensor next but how would the condensor give me a dead key switch? Also, I did pick up a new ignition switch last night after work to replace the original one even though it tested ok.

Lew

 

[gzec]

I've had the wire break on the coil. Caused an immediate shut down. Think it happened twice in 7 yrs?

 

[dixie_deere]

Chuck,

I was able to check the solenoid after work tonight and I found no 12 volt power at the solenoid purple wire when the key was turned to the start position. I ran out of daylight and was not able to get to the condenser but I wanted to ask what is the best way to check the condenser itself to determine if it is working? You also mentioned that it could be a bad PTO switch(I have rear PTO too)so what is the best way to check those also.... continuity or voltage test?
Do you think I should just replace the starter solenoid anyway because it will have almost 10 years on it now?
Anyway, still have a dead key switch at this point and still searching.....

Lew

 

[chuckv]

Lee,

If the key switch is the original one that came on the tractor in 1986 you should just replace it -- they get worn out over the years. Be sure to inspect the mating connector sockets for signs of overheating and/or corrosion and clean everything up, then use dielectric grease to keep any more corrosion or oxidation from forming on the cleaned contacts.

If after replacing the key switch assembly you still do not have any 12 volts on the purple wire at the solenoid when the key is in the START position, you can do the following test.

Beginning at the key switch S2 with the key in the RUN position you should have 12 volts if the ground speed lever is in neutral and the two PTO switches are OFF. If that voltage is not there, move to the #720 purple wire side of the neutral switch, X10. If you have 12 volts there then the neutral switch is either out of adjustment or it is bad. Here is how to adjust the neutral switch:

Please note: the continuity tests shown in the illustration are made with the key switch in the OFF position...

If you do not have any 12 volts at X10 of the neutral switch with the key switch in the RUN position, then the issue is further 'upstream' and we need to look at the #730 purple wire on the front PTO switch X3 -- 12 volts present there means the rear PTO switch or its connections/wires are at fault. No 12 volts there means that the front PTO switch or its connections/wires are at fault, or the 20 amp fuse is bad. On older tractors such as yours the fuses are of the glass tube type and fit in plastic holders that are notoriously unreliable. You might consider upgrading to the automotive tab type of fuse and holder. Here is an illustration of the differences:

Hope this helps narrow things down, and let us know what you find.

Chuck

 

[dixie_deere]

Chuck,

I thought about what could be the open circuit all day at work Friday and I got up this morning and decided to start at the beginning again. After reading other posts about the key switch and the fuses, I went to the first thing I checked when the tractor shut down....the fuses.
Since the fuses checked out fine, I thought I would try the old aluminum foil trick on the fuse holder itself and sure as shootin... BINGO!
All this week, I was checking every component along the electrical system from the key to the engine and it turned out to be something so simple in the end. If nothing else, I did learn quite a bit about the electrical system of that tractor!
Thanks for all your suggestions, photos, and illustrations which were very helpful to narrow things down through a process of elimination.
It is always a good idea to talk with someone else (who has been there-done that) when trying to figure out a problem you need to solve!

The old 420 is up and running like a champ again with upgraded blade style fuse holders now and no more glass fuses. Bring on the tall grass!!!


Thanks again...

Lew

 

[chuckv]

Lew,

Glad to hear of your success -- Yes those old style fuse holders are *very* unreliable and you will be glad for decades that you upgraded to the tab style...

Chuck